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rfc:rfc2642

Network Working Group L. Kane Request for Comments: 2642 Cabletron Systems Incorporated Category: Informational August 1999

               Cabletron's VLS Protocol Specification

Status of this Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard of any kind.  Distribution of this
 memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (1999).  All Rights Reserved.

Abstract

 The Virtual LAN Link State Protocol (VLSP) is part of the InterSwitch
 Message Protocol (ISMP) which provides interswitch communication
 between switches running Cabletron's SecureFast VLAN (SFVLAN)
 product.  VLSP is used to determine and maintain a fully connected
 mesh topology graph of the switch fabric.  Each switch maintains an
 identical database describing the topology. Call-originating switches
 use the topology database to determine the path over which to route a
 call connection.
 VLSP provides support for equal-cost multipath routing, and
 recalculates routes quickly in the face of topological changes,
 utilizing a minimum of routing protocol traffic.

Table of Contents

  1. Introduction............................................  3
     1.1 Acknowledgments.....................................  3
     1.2 Data Conventions....................................  3
     1.3 ISMP Overview.......................................  4
  2. VLS Protocol Overview...................................  5
     2.1 Definitions of Commonly Used Terms..................  6
     2.2 Differences Between VLSP and OSPF...................  7
         2.2.1 Operation at the Physical Layer...............  8
         2.2.2 All Links Treated as Point-to-Point...........  8
         2.2.3 Routing Path Information......................  9
         2.2.4 Configurable Parameters.......................  9
         2.2.5 Features Not supported........................  9
     2.3 Functional Summary.................................. 10
     2.4 Protocol Packets.................................... 11

Kane Informational [Page 1] RFC 2642 Cabletron's VLS Protocol Specification August 1999

     2.5 Protocol Data Structures............................ 12
     2.6 Basic Implementation Requirements................... 12
     2.7 Organization of the Remainder of This Document...... 13
  3. Interface Data Structure................................ 14
     3.1 Interface States.................................... 16
     3.2 Events Causing Interface State Changes.............. 18
     3.3 Interface State Machine............................. 21
  4. Neighbor Data Structure................................. 23
     4.1 Neighbor States..................................... 25
     4.2 Events Causing Neighbor State Changes............... 27
     4.3 Neighbor State Machine.............................. 29
  5. Area Data Structure..................................... 33
     5.1 Adding and Deleting Link State Advertisements....... 34
     5.2 Accessing Link State Advertisements................. 35
     5.3 Best Path Lookup.................................... 35
  6. Discovery Process....................................... 35
     6.1 Neighbor Discovery.................................. 36
     6.2 Bidirectional Communication......................... 37
     6.3 Designated Switch................................... 38
         6.3.1 Selecting the Designated Switch............... 39
     6.4 Adjacencies......................................... 41
  7. Synchronizing the Databases............................. 42
     7.1 Link State Advertisements........................... 43
         7.1.1 Determining Which
               Link State Advertisement Is Newer............. 44
     7.2 Database Exchange Process........................... 44
         7.2.1 Database Description Packets.................. 44
         7.2.2 Negotiating the Master/Slave Relationship..... 45
         7.2.3 Exchanging Database Description Packets....... 46
     7.3 Updating the Database............................... 48
     7.4 An Example.......................................... 49
  8. Maintaining the Databases............................... 51
     8.1 Originating Link State Advertisements............... 52
         8.1.1 Switch Link Advertisements.................... 52
         8.1.2 Network Link Advertisements................... 55
     8.2 Distributing Link State Advertisements.............. 56
         8.2.1 Overview...................................... 57
         8.2.2 Processing an
               Incoming Link State Update Packet............. 58
         8.2.3 Forwarding Link State Advertisements.......... 60
         8.2.4 Installing Link
               State Advertisements in the Database.......... 62
         8.2.5 Retransmitting Link State Advertisements...... 63
         8.2.6 Acknowledging Link State Advertisements....... 64
     8.3 Aging the Link State Database....................... 66
         8.3.1 Premature Aging of Advertisements............. 66
  9. Calculating the Best Paths.............................. 67
 10. Protocol Packets........................................ 67

Kane Informational [Page 2] RFC 2642 Cabletron's VLS Protocol Specification August 1999

     10.1 ISMP Packet Format................................. 68
          10.1.1 Frame Header................................ 69
          10.1.2 ISMP Packet Header.......................... 70
          10.1.3 ISMP Message Body........................... 71
     10.2 VLSP Packet Processing............................. 71
     10.3 Network Layer Address Information.................. 72
     10.4 VLSP Packet Header................................. 73
     10.5 Options Field...................................... 75
     10.6 Packet Formats..................................... 76
          10.6.1 Hello Packets............................... 76
          10.6.2 Database Description Packets................ 78
          10.6.3 Link State Request Packets.................. 80
          10.6.4 Link State Update Packets................... 82
          10.6.5 Link State Acknowledgment Packets........... 83
 11. Link State Advertisement Formats........................ 84
     11.1 Link State Advertisement Headers................... 84
     11.2 Switch Link Advertisements......................... 86
     11.3 Network Link Advertisements........................ 89
 12. Protocol Parameters..................................... 89
     12.1 Architectural Constants............................ 90
     12.2 Configurable Parameters............................ 91
 13. End Notes............................................... 93
 14. Security Considerations................................. 94
 15. References.............................................. 94
 16. Author's Address........................................ 94
 17. Full Copyright Statement................................ 95

1. Introduction

 This memo is being distributed to members of the Internet community
 in order to solicit reactions to the proposals contained herein.
 While the specification discussed here may not be directly relevant
 to the research problems of the Internet, it may be of interest to
 researchers and implementers.

1.1 Acknowledgments

 VLSP is derived from the OSPF link-state routing protocol described
 in [RFC2328], written by John Moy, formerly of Proteon, Inc.,
 Westborough, Massachusetts.  Much of the current memo has been drawn
 from [RFC2328].  Therefore, this author wishes to acknowledge the
 contribution Mr. Moy has (unknowingly) made to this document.

1.2 Data Conventions

 The methods used in this memo to describe and picture data adhere to
 the standards of Internet Protocol documentation [RFC1700].  In
 particular:

Kane Informational [Page 3] RFC 2642 Cabletron's VLS Protocol Specification August 1999

    The convention in the documentation of Internet Protocols is to
    express numbers in decimal and to picture data in "big-endian"
    order.  That is, fields are described left to right, with the most
    significant octet on the left and the least significant octet on
    the right.  The order of transmission of the header and data
    described in this document is resolved to the octet level.
    Whenever a diagram shows a group of octets, the order of
    transmission of those octets is the normal order in which they are
    read in English.
    Whenever an octet represents a numeric quantity the left most bit
    in the diagram is the high order or most significant bit.  That
    is, the bit labeled 0 is the most significant bit.
    Similarly, whenever a multi-octet field represents a numeric
    quantity the left most bit of the whole field is the most
    significant bit.  When a multi-octet quantity is transmitted the
    most significant octet is transmitted first.

1.3 ISMP Overview

 The InterSwitch Message Protocol (ISMP) provides a consistent method
 of encapsulating and transmitting control messages exchanged between
 switches running Cabletron's SecureFast VLAN (SFVLAN) product, as
 described in [IDsfvlan].  ISMP provides the following services:
 o  Topology services.  Each switch maintains a distributed topology
    of the switch fabric by exchanging the following interswitch
    control messages with other switches:
 o  Interswitch Keepalive messages are sent by each switch to announce
    its existence to its neighboring switches and to establish the
    topology of the switch fabric.  (Interswitch Keepalive messages
    are exchanged in accordance with Cabletron's VlanHello protocol,
    described in [IDhello].)
 o  Interswitch Spanning Tree BPDU messages and Interswitch Remote
    Blocking messages are used to determine and maintain a loop-free
    flood path between all network switches in the fabric.  This flood
    path is used for all undirected interswitch messages -- that is,
    messages that are (potentially) sent to all switches in the switch
    fabric.
 o  Interswitch Link State messages (VLS protocol) are used to
    determine and maintain a fully connected mesh topology graph of
    the switch fabric.  Call-originating switches use the topology
    graph to determine the path over which to route a call connection.

Kane Informational [Page 4] RFC 2642 Cabletron's VLS Protocol Specification August 1999

 o  Address resolution services.  Interswitch Resolve messages are
    used to resolve a packet destination address when the packet
    source and destination pair does not match a known connection.
    Interswitch New User messages are used to provide end-station
    address mobility between switches.
 o  Tag-based flooding.  A tag-based broadcast method is used to
    restrict the broadcast of unresolved packets to only those ports
    within the fabric that belong to the same VLAN as the source.
 o  Call tapping services.  Interswitch Tap messages are used to
    monitor traffic moving between two end stations.  Traffic can be
    monitored in one or both directions along the connection path.
 Note:  Previous versions of VLSP treated all links as if they were
 broadcast (multi-access).  Thus, if VLSP determines that a neighbor
 switch is running an older version of the protocol software (see
 Section 6.1), it will change the interface type to broadcast and
 begin exchanging Hello packets with the single neighbor switch.

2. VLS Protocol Overview

 VLSP is a dynamic routing protocol.  It quickly detects topological
 changes in the switch fabric (such as, switch interface failures) and
 calculates new loop-free routes after a period of convergence.  This
 period of convergence is short and involves a minimum of routing
 traffic.
 All switches in the fabric run the same algorithm and maintain
 identical databases describing the switch fabric topology.  This
 database contains each switch's local state, including its usable
 interfaces and reachable neighbors.  Each switch distributes its
 local state throughout the switch fabric by flooding.  From the
 topological database, each switch constructs a set of best path trees
 (using itself as the root) that specify routes to all other switches
 in the fabric.

Kane Informational [Page 5] RFC 2642 Cabletron's VLS Protocol Specification August 1999

2.1 Definitions of Commonly Used Terms

 This section contains a collection of definitions for terms that have
 a specific meaning to the protocol and that are used throughout the
 text.
 Switch ID
    A 10-octet value that uniquely identifies the switch within the
    switch fabric.  The value consists of the 6-octet base MAC address
    of the switch, followed by 4 octets of zeroes.
 Network link
    The physical connection between two switches.  A link is
    associated with a switch interface.
    There are two physical types of network links supported by VLSP:
    o  Point-to-point links that join a single pair of switches.  A
       serial line is an example of a point-to-point network link.
    o  Multi-access broadcast links that support the attachment of
       multiple switches, along with the capability to address a
       single message to all the attached switches.  An attached
       ethernet is an example of a multi-access broadcast network
       link.
       A single topology can contain both types of links.  At startup,
       all links are assumed to be point-to-point.  A link is
       determined to be multi-access when more than one neighboring
       switch is discovered on the link.
 Interface
    The port over which a switch accesses one of its links.
    Interfaces are identified by their interface ID, a 10-octet value
    consisting of the 6-octet base MAC address of the switch, followed
    by the 4-octet local port number of the interface.
 Neighboring switches
    Two switches attached to a common link.

Kane Informational [Page 6] RFC 2642 Cabletron's VLS Protocol Specification August 1999

 Adjacency
    A relationship formed between selected neighboring switches for
    the purpose of exchanging routing information.  Not every pair of
    neighboring switches become adjacent.
 Link state advertisement
    Describes the local state of a switch or a link.  Each link state
    advertisement is flooded throughout the switch fabric.  The
    collected link state advertisements of all switches and links form
    the protocol's topological database.
 Designated switch
    Each multi-access network link has a designated switch.  The
    designated switch generates a link state advertisement for the
    link and has other special responsibilities in the running of the
    protocol.
    The use of a designated switch permits a reduction in the number
    of adjacencies required on multi-access links.  This in turn
    reduces the amount of routing protocol traffic and the size of the
    topological database.
    The designated switch is selected during the discovery process.  A
    designated switch is not selected for a point-to-point network
    link.
 Backup designated switch
    Each multi-access network link has a backup designated switch.
    The backup designated switch maintains adjacencies with the same
    switches on the link as the designated switch.  This optimizes the
    failover time when the backup designated switch must take over for
    the (failed) designated switch.
    The backup designated switch is selected during the Discovery
    process.  A backup designated switch is not selected for a point-
    to-point network link.

2.2 Differences Between VLSP and OSPF

 The VLS protocol is derived from the OSPF link-state routing protocol
 described in [RFC2328].

Kane Informational [Page 7] RFC 2642 Cabletron's VLS Protocol Specification August 1999

2.2.1 Operation at the Physical Layer

 The primary differences between the VLS and OSPF protocols stem from
 the fact that OSPF runs over the IP layer, while VLSP runs at the
 physical MAC layer.  This difference has the following repercussions:
 o  VLSP does not support features (such as fragmentation) that are
    typically provided by network layer service providers.
 o  Due to the unrelated nature of MAC address assignments, VLSP
    provides no summarization of the address space (such as, classical
    IP subnet information) or level 2 routing (such as,
    IS-IS Phase V DECnet).  Thus, VLSP does not support grouping
    switches into areas.  All switches exist in a single area.  Since
    a single domain exists within any switch fabric, there is no need
    for VLSP to provide interdomain reachability.
 o  As mentioned in Section 10.1.1, ISMP uses a single well-known
    multicast address for all packets.  However, parts of the VLS
    protocol (as derived from OSPF) are dependent on certain network
    layer addresses -- in particular, the AllSPFSwitches and
    AllDSwitches multicast addresses that drive the distribution of
    link state advertisements throughout the switch fabric.  In order
    to facilitate the implementation of the protocol at the physical
    MAC layer, network layer address information is encapsulated in
    the protocol packets (see Section 10.3).  This information is
    unbundled and packets are then processed as if they had been sent
    or received on that multicast address.

2.2.2 All Links Treated as Point-to-Point

 When the switch first comes on line, VLSP assumes all network links
 are point-to-point and no more than one neighboring switch will be
 discovered on any one port.  Therefore, at startup, VLSP does not
 send its own Hello packets over its network ports, but instead,
 relies on the VlanHello protocol [IDhello] for the discovery of its
 neighbor switches.  If a second neighbor is detected on a link, the
 link is then deemed multi-access and the interface type is changed to
 broadcast.  At that point, VLSP exchanges its own Hello packets with
 the switches on the link in order to select a designated switch and
 designated backup switch for the link.
 This method eliminates unnecessary duplication of message traffic and
 processing, thereby increasing the overall efficiency of the switch
 fabric.

Kane Informational [Page 8] RFC 2642 Cabletron's VLS Protocol Specification August 1999

 Note:  Previous versions of VLSP treated all links as if they were
 broadcast (multi-access).  Thus, if VLSP determines that a neighbor
 switch is running an older version of the protocol software (see
 Section 6.1), it will change the interface type to broadcast and
 begin exchanging Hello packets with the single neighbor switch.

2.2.3 Routing Path Information

 Instead of providing the next hop to a destination, VLSP calculates
 and maintains complete end-to-end path information. On request, a
 list of individual port identifiers is generated describing a
 complete path from the source switch to the destination switch.  If
 multiple equal-cost routes exist to a destination switch, up to three
 paths are calculated and returned.

2.2.4 Configurable Parameters

 OSPF supports (and requires) configurable parameters.  In fact, even
 the default OSPF configuration requires that IP address assignments
 be specified.  On the other hand, no configuration information is
 ever required for the VLS protocol.  Switches are uniquely identified
 by their base MAC addresses and ports are uniquely identified by the
 base MAC address of the switch and a port number.
 While a developer is free to implement configurable parameters for
 the VLS protocol, the current version of VLSP supports configurable
 path metrics only.  Note that this has the following repercussions:
 o  All switches are assigned a switch priority of 1.  This forces the
    selection of the designated switch to be based solely on base MAC
    address.
 o  Authentication is not supported.

2.2.5 Features Not supported

 In addition to those features mentioned in the previous sections, the
 following OSPF features are not supported by the current version of
 VLSP:
 o  Periodic refresh of link state advertisements.  (This optimizes
    performance by eliminating unnecessary traffic between the
    switches.)
 o  Routing based on non-zero type of service (TOS).
 o  Use of external routing information for destinations outside the
    switch fabric.

Kane Informational [Page 9] RFC 2642 Cabletron's VLS Protocol Specification August 1999

2.3 Functional Summary

 There are essentially four operational stages of the VLS protocol.
 o  Discovery Process The discovery process involves two steps:
    o  Neighboring switches are detected by the VlanHello protocol
       [IDhello] which then notifies VLSP of the neighbor.
    o  If more than one neighbor switch is detected on a single port,
       the link is determined to be multi-access.  VLSP then sends its
       own Hello packets over the link in order to discover the full
       set of neighbors on the link and select a designated switch and
       designated backup switch for the link.  Note that this
       selection process is unnecessary on point-to-point links.
    The discovery process is described in more detail in Section 6.
 o  Synchronizing the Databases
    Adjacencies are used to simplify and speed up the process of
    synchronizing the topological database (also known as the link
    state database) maintained by each switch in the fabric.  Each
    switch is only required to synchronize its database with those
    neighbors to which it is adjacent. This reduces the amount of
    routing protocol traffic across the fabric, particularly for
    multi-access links with multiple switches.
    The process of synchronizing the databases is described in more
    detail in Section 7.
 o  Maintaining the Databases
    Each switch advertises its state (also known as its link state)
    any time its link state changes.  Link state advertisements are
    distributed throughout the switch fabric using a reliable flooding
    algorithm that ensures that all switches in the fabric are
    notified of any link state changes.
    The process of maintaining the databases is described in more
    detail in Section 8.

Kane Informational [Page 10] RFC 2642 Cabletron's VLS Protocol Specification August 1999

 o  Calculating the Best Paths
    The link state database consists of the collection of link state
    advertisements received from each switch.  Each switch uses its
    link state database to calculate a set of best paths, using itself
    as root, to all other switches in the fabric.
    The process of recalculating the set of best paths is described in
    more detail in Section 9.

2.4 Protocol Packets

 In addition to the frame header and the ISMP packet header described
 in Section 10.1, all VLS protocol packets share a common protocol
 header, described in Section 10.4.
 The VLSP packet types are listed below in Table 1.  Their formats are
 described in Section 10.6.
    Type   Packet Name            Protocol Function
    1      Hello                  Select DS and Backup DS
    2      Database Description   Summarize database contents
    3      Link State Request     Database download
    4      Link State Update      Database update
    5      Link State Ack         Flooding acknowledgment
                Table 1: VLSP Packet Types
 The Hello packets are used to select the designated switch and the
 backup designated switch on multi-access links.  The Database
 Description and Link State Request packets are used to form
 adjacencies.  Link State Update and Link State Acknowledgment packets
 are used to update the topological database.
 Each Link State Update packet carries a set of link state
 advertisements.  A single Link State Update packet may contain the
 link state advertisements of several switches.  There are two
 different types of link state advertisement, as shown below in Table
 2.

Kane Informational [Page 11] RFC 2642 Cabletron's VLS Protocol Specification August 1999

       LS     Advertisement    Advertisement Description
       Type   Name
       1      Switch link      Originated by all switches. This
              advertisements   advertisement describes the collected
                               states of the switch's interfaces.
       2      Network link     Originated by the designated switch.
              advertisements   This advertisement contains the list
                               of switches connected to the network
                               link.
                Table 2: VLSP Link State Advertisements

2.5 Protocol Data Structures

 The VLS protocol is described in this specification in terms of its
 operation on various protocol data structures.  Table 3 lists the
 primary VLSP data structures, along with the section in which they
 are described in detail.
       Structure Name                        Description
       Interface Data Structure              Section 3
       Neighbor Data Structure               Section 4
       Area Data Structure                   Section 5
                   Table 3: VLSP Data Structures

2.6 Basic Implementation Requirements

 An implementation of the VLS protocol requires the following pieces
 of system support:
 Timers
    Two types of timer are required.  The first type, known as a one-
    shot timer, expires once and triggers an event.  The second type,
    known as an interval timer, expires at preset intervals.  Interval
    timers are used to trigger events at periodic intervals.  The
    granularity of both types of timers is one second.
    Interval timers should be implemented in such a way as to avoid
    drift.  In some switch implementations, packet processing can
    affect timer execution.  For example, on a multi-access link with
    multiple switches, regular broadcasts can lead to undesirable
    synchronization of routing packets unless the interval timers have
    been implemented to avoid drift.  If it is not possible to

Kane Informational [Page 12] RFC 2642 Cabletron's VLS Protocol Specification August 1999

    implement drift-free timers, small random amounts of time should
    be added to or subtracted from the timer interval at each firing.
 List manipulation primitives
    Much of the functionality of VLSP is described here in terms of
    its operation on lists of link state advertisements.  Any
    particular advertisement may be on many such lists. Implementation
    of VLSP must be able to manipulate these lists, adding and
    deleting constituent advertisements as necessary.
 Tasking support
    Certain procedures described in this specification invoke other
    procedures.  At times, these other procedures should be executed
    in-line -- that is, before the current procedure has finished.
    This is indicated in the text by instructions to "execute" a
    procedure.  At other times, the other procedures are to be
    executed only when the current procedure has finished.  This is
    indicated by instructions to "schedule" a task.  Implementation of
    VLSP must provide these two types of tasking support.

2.7 Organization of the Remainder of This Document

 The remainder of this document is organized as follows:
 o  Section 3 through Section 5 describe the primary data structures
    used by the protocol.  Note that this specification is presented
    in terms of these data structures in order to make explanations
    more precise.  Implementations of the protocol must support the
    functionality described, but need not use the exact data
    structures that appear in this specification.
 o  Section 6 through Section 9 describe the four operational stages
    of the protocol:  the discovery process, synchronizing the
    databases, maintaining the databases, and calculating the set of
    best paths.
 o  Section 10 describes the processing of VLSP packets and presents
    detailed descriptions of their formats.
 o  Section 11 presents detailed descriptions of link state
    advertisements.
 o  Section 12 summarizes the protocol parameters.

Kane Informational [Page 13] RFC 2642 Cabletron's VLS Protocol Specification August 1999

3. Interface Data Structure

 The port over which a switch accesses a network link is known as the
 link interface.  Each switch maintains a separate interface data
 structure for each network link.
 The following data items are associated with each interface:
 Type
    The type of network to which the interface is attached -- point-
    to-point or broadcast (multi-access).  This data item is
    initialized to point-to-point when the interface becomes
    operational.  If a second neighbor is detected on the link after
    the first neighbor has been discovered, the link interface type is
    changed to broadcast.  The type remains as broadcast until the
    interface is declared down, at which time the type reverts to
    point-to-point.
 Note:  Previous versions of VLSP treated all links as if they were
 multi-access.  Thus, if VLSP determines that a neighbor switch is
 running an older version of the protocol software (see Section 6.1),
 it will change the interface type to broadcast.
 State
    The functional level of the interface.  The state of the interface
    is included in all switch link advertisements generated by the
    switch, and is also used to determine whether full adjacencies are
    allowed on the interface.  See Section 3.1 for a complete
    description of interface states.
 Interface identifier
    A 10-octet value that uniquely identifies the interface. This
    value consists of the 6-octet base MAC address of the neighbor
    switch, followed by the 4-octet local port number of the
    interface.
 Area ID
    A 4-octet value identifying the area.  Since VLSP does not support
    multiple areas, the value here is always zero.

Kane Informational [Page 14] RFC 2642 Cabletron's VLS Protocol Specification August 1999

 HelloInterval
    The interval, in seconds, at which the switch sends VLSP Hello
    packets over the interface.  This parameter is not used on point-
    to-point links.
 SwitchDeadInterval
    The length of time, in seconds, that neighboring switches will
    wait before declaring the local switch dNeighboring switches
    A list of the neighboring switches attached to this network link.
    This list is created during the discovery process. Adjacencies are
    formed to one or more of these neighbors. The set of adjacent
    neighbors can be determined by examining the states of the
    neighboring switches as shown in their link state advertisements.
 Designated switch
    The designated switch selected for the multi-access network link.
    (A designated switch is not selected for a point-to-point link.)
    This data item is initialized to zero when the switch comes on-
    line, indicating that no designated switch has been chosen for the
    link.
 Backup designated switch
    The backup designated switch selected for the multi-access network
    link.  (A backup designated switch is not selected for a point-
    to-point link.)  This data item is initialized to zero when the
    switch comes on-line, indicating that no backup designated switch
    has been chosen for the link.
 Interface output cost(s)
    The cost of sending a packet over the interface.  The link cost is
    expressed in the link state metric and must be greater than zero.
 RxmtInterval
    The number of seconds between link state advertisement
    retransmissions, for adjacencies belonging to this interface. This
    value is also used to time the retransmission of Database
    Description and Link State Request packets.

Kane Informational [Page 15] RFC 2642 Cabletron's VLS Protocol Specification August 1999

3.1 Interface States

 This section describes the various states of a switch interface. The
 states are listed in order of progressing functionality. For example,
 the inoperative state is listed first, followed by a list of the
 intermediate states through which the interface passes before
 attaining the final, fully functional state.  The specification makes
 use of this ordering by references such as "those interfaces in state
 greater than X".
 Figure 1 represents the interface state machine, showing the
 progression of interface state changes.  The arrows on the graph
 represent the events causing each state change.  These events are
 described in Section 3.2.  The interface state machine is described
 in detail in Section 3.3.
 Down
    This is the initial state of the interface.  In this state, the
    interface is unusable, and no protocol traffic is sent or received
    on the interface.  In this state, interface parameters are set to
    their initial values, all interface timers are disabled, and no
    adjacencies are associated with the interface.

Kane Informational [Page 16] RFC 2642 Cabletron's VLS Protocol Specification August 1999

     +-------+
     |  any  |  Interface   +----------+  Unloop Ind  +----------+
     | state | -----------> |   Down   | <----------- | Loopback |
     +-------+    Down      +----------+              +----------+
                                 |                         ^
                                 | Interface Up            |
         +-------+  [pt-to-pt]   |                         |
         | Point |<------------type?              Loop Ind |
         |  to   |               |                         |
         | Point |               | [broadcast]             |
         +-------+               V                     +-------+
                           +-----------+               |  any  |
                           |  Waiting  |               | state |
                           +-----------+               +-------+
                                 |
                     Backup Seen |
                                 | Wait Timer
                                 |
                                 |
    +----------+    Neighbor     V     Neighbor    +----------+
    |    DS    | <------------> [ ] <------------> | DS Other |
    +----------+     Change      ^      Change     +----------+
                                 |
                                 |
                 Neighbor Change |
                                 |
                                 V
                            +----------+
                            |  Backup  |
                            +----------+
                 Figure 1:  Interface State Machine
 Loopback
    In this state, the switch interface is looped back, either in
    hardware or in software.  The interface is unavailable for regular
    data traffic.
 Point-to-Point
    In this state, the interface is operational and is connected to a
    physical point-to-point link.  On entering this state, the switch
    attempts to form an adjacency with the neighboring switch.

Kane Informational [Page 17] RFC 2642 Cabletron's VLS Protocol Specification August 1999

 Waiting
    In this state, the switch is attempting to identify the backup
    designated switch for the link by monitoring the Hello packets it
    receives.  The switch does not attempt to select a designated
    switch or a backup designated switch until it changes out of this
    state, thereby preventing unnecessary changes of the designated
    switch and its backup.
 DS Other
    In this state, the interface is operational and is connected to a
    multi-access broadcast link on which other switches have been
    selected as the designated switch and the backup designated
    switch.   On entering this state, the switch attempts to form
    adjacencies with both the designated switch and the backup
    designated switch.
 Backup
    In this state, the switch itself is the backup designated switch
    on the attached multi-access broadcast link.  It will be promoted
    to designated switch if the current designated switch fails.  The
    switch establishes adjacencies with all other switches attached to
    the link.  (See Section 6.3 for more information on the functions
    performed by the backup designated switch.)
 DS
    In this state, this switch itself is the designated switch on the
    attached multi-access broadcast link.  The switch establishes
    adjacencies with all other switches attached to the link.  The
    switch is responsible for originating network link advertisements
    for the link, containing link information for all switches
    attached to the link, including the designated switch itself.
    (See Section 6.3 for more information on the functions performed
    by the designated switch.)

3.2 Events Causing Interface State Changes

 The state of an interface changes due to an interface event.  This
 section describes these events.
 Interface events are shown as arrows in Figure 1, the graphic
 representation of the interface state machine.  For more information
 on the interface state machine, see Section 3.3.

Kane Informational [Page 18] RFC 2642 Cabletron's VLS Protocol Specification August 1999

 Interface Up
    This event is generated by the VlanHello protocol [IDhello] when
    it discovers a neighbor switch on the interface.  The interface is
    now operational.  This event causes the interface to change out of
    the Down state.  The state it enters is determined by the
    interface type.  If the interface type is broadcast (multi-
    access), this event also causes the switch to begin sending
    periodic Hello packets out over the interface.
 Wait Timer
    This event is generated when the one-shot Wait timer expires,
    triggering the end of the required waiting period before the
    switch can begin the process of selecting a designated switch and
    a backup designated switch on a multi-access link.
 Backup Seen
    This event is generated when the switch has detected the existence
    or non-existence of a backup designated switch for the link, as
    determined in one of the following two ways:
    o  A Hello packet has been received from a neighbor that claims to
       be the backup designated switch.
    o  A Hello packet has been received from a neighbor that claims to
       be the designated switch.  In addition, the packet indicated
       that there is no backup.
 In either case, the interface must have bidirectional communication
 with its neighbor -- that is, the local switch must be listed in the
 neighbor's Hello packet.
 This event signals the end of the Waiting state.
 Neighbor change
    This event is generated when there has been one of the following
    changes in the set of bidirectional neighbors associated with the
    interface.  (See Section 4.1 for information on neighbor states.)
    o  Bidirectional communication has been established with a
       neighbor -- the state of the neighbor has changed to 2-Way or
       higher.
    o  Bidirectional communication with a neighbor has been lost --
       the state of the neighbor has changed to Init or lower.

Kane Informational [Page 19] RFC 2642 Cabletron's VLS Protocol Specification August 1999

    o  A bidirectional neighbor has just declared itself to be either
       the designated switch or the backup designated switch, as
       detected by examination of that neighbor's Hello packets.
    o  A bidirectional neighbor is no longer declaring itself to be
       either the designated switch or the backup designated switch,
       as detected by examination of that neighbor's Hello packets.
    o  The advertised switch priority of a bidirectional neighbor has
       changed, as detected by examination of that neighbor's Hello
       packets.
    When this event occurs, the designated switch and the backup
    designated switch must be reselected.
    Loop Ind
       This event is generated when an interface enters the Loopback
       state.  This event can be generated by either the network
       management service or by the lower-level protocols.
    Unloop Ind
       This event is generated when an interface leaves the Loopback
       state.  This event can be generated by either the network
       management service or by the lower-level protocols.
    Interface Down
       This event is generated under the following two circumstances:
       o  The VlanHello [IDhello] protocol has determined that the
          interface is no longer functional.
       o  The neighbor state machine has detected a second neighboring
          switch on a link presumed to be of type point-to-point. In
          addition to generating the Interface Down event, the
          neighbor state machine changes the interface type to
          broadcast.
    In both instances, this event forces the interface state to Down.
    However, when the event is generated by the neighbor state
    machine, it is immediately followed by an Interface Up event.
    (See Section 4.3.)

Kane Informational [Page 20] RFC 2642 Cabletron's VLS Protocol Specification August 1999

3.3 Interface State Machine

 This section presents a detailed description of the interface state
 machine.
 Interface states (see Section 3.1) change as the result of various
 events (see Section 3.2).  However, the effect of each event can
 vary, depending on the current state of the interface. For this
 reason, the state machine described in this section is organized
 according to the current interface state and the occurring event.
 For each state/event pair, the new interface state is listed, along
 with a description of the required processing.
 Note that when the state of an interface changes, it may be necessary
 to originate a new switch link advertisement.  See Section 8.1 for
 more information.
 Some of the processing described here includes generating events for
 the neighbor state machine.  For example, when an interface becomes
 inoperative, all neighbor connections associated with the interface
 must be destroyed.  For more information on the neighbor state
 machine, see Section 4.3.
 State(s):  Down
 Event:  Interface Up
 New state:  Depends on action routine
 Action:
    If the interface is a point-to-point link, set the interface state
    to Point-to-Point.  Otherwise, start the Hello interval timer,
    enabling the periodic sending of Hello packets over the interface.
    If the switch is not eligible to become the designated switch,
    change the interface state to DS Other. Otherwise, set the
    interface state to Waiting and start the one-shot wait timer.
    Create a new neighbor data structure for the neighbor switch,
    initialize all neighbor parameters and set the stateof the
    neighbor to Down.
 State(s):  Waiting
 Event:  Backup Seen
 New state:  Depends on action routine
 Action:
    Select the designated switch and backup designated switch for the
    attached link, as described in Section 6.3.1.  As a result of this
    selection, set the new state of the interface to either DS Other,
    Backup or DS.

Kane Informational [Page 21] RFC 2642 Cabletron's VLS Protocol Specification August 1999

 State(s):  Waiting
 Event:  Wait Timer
 New state:  Depends on action routine
 Action:
    Select the designated switch and backup designated switch for the
    attached link, as described in Section 6.3.1.  As a result of this
    selection, set the new state of the interface to either DS Other,
    Backup or DS.
 State(s):  DS Other, Backup or DS
 Event:  Neighbor Change
 New state:  Depends on action routine
 Action:
    Reselect the designated switch and backup designated switch for
    the attached link, as described in Section 6.3.1.  As a result of
    this selection, set the new state of the interface to either DS
    Other, Backup or DS.
 State(s):  Any State
 Event:  Interface Down
 New state:  Down
 Action:
    Reset all variables in the interface data structure and disable
    all timers.  In addition, destroy all neighbor connections
    associated with the interface by generating the KillNbr event on
    all neighbors listed in the interface data structure.
 State(s):  Any State
 Event:  Loop Ind
 New state:  Loopback
 Action:
    Reset all variables in the interface data structure and disable
    all timers.  In addition, destroy all neighbor connections
    associated with the interface by generating the KillNbr event on
    all neighbors listed in the interface data structure.
 State(s):  Loopback
 Event:  Unloop Ind
 New state:  Down
 Action:
    No action is necessary beyond changing the interface state to Down
    because the interface was reset on entering the Loopback state.

Kane Informational [Page 22] RFC 2642 Cabletron's VLS Protocol Specification August 1999

4. Neighbor Data Structure

 Each switch conducts a conversation with its neighboring switches and
 each conversation is described by a neighbor data structure.  A
 conversation is associated with a switch interface, and is identified
 by the neighboring switch ID.
 Note that if two switches have multiple attached links in common,
 multiple conversations ensue, each described by a unique neighbor
 data structure.  Each separate conversation is treated as a separate
 neighbor.
 The neighbor data structure contains all information relevant to any
 adjacency formed between the two neighbors.  Remember, however, that
 not all neighbors become adjacent.  An adjacency can be thought of as
 a highly developed conversation between two switches.
 State
    The functional level of the neighbor conversation.  See Section
    4.1 for a complete description of neighbor states.
 Inactivity timer
    A one-shot timer used to determine when to declare the neighbor
    down if no Hello packet is received from this (multi-access)
    neighbor.  The length of the timer is SwitchDeadInterval seconds,
    as contained in the neighbor's Hello packet.  This timer is not
    used on point-to-point links.
 Master/slave flag
    A flag indicating whether the local switch is to act as the master
    or the slave in the database exchange process (see Section 7.2).
    The master/slave relationship is negotiated when the conversation
    changes to the ExStart state.
 Sequence number
    A 4-octet number identifying individual Database Description
    packets. When the neighbor state ExStart is entered and the
    database exchange process is started, the sequence number is set
    to a value not previously seen by the neighboring switch. (One
    possible scheme is to use the switch's time of day counter.)  The
    sequence number is then incremented by the master with each new
    Database Description packet sent.  See Section 7.2 for more
    information on the database exchange process.

Kane Informational [Page 23] RFC 2642 Cabletron's VLS Protocol Specification August 1999

 Neighbor ID
    The switch ID of the neighboring switch, as discovered by the
    VlanHello protocol [IDhello] or contained in the neighbor's Hello
    packets.
 Neighbor priority
    The switch priority of the neighboring switch, as contained in the
    neighbor's Hello packets.  Switch priorities are used when
    selecting the designated switch for the attached multi-access
    link.  Priority is not used on point-to-point links.
 Interface identifier
    A 10-octet value that uniquely identifies the interface over which
    this conversation is being held.  This value consists of the 6-
    octet base MAC address of the neighbor switch, followed by the 4-
    octet local port number of the interface.
 Neighbor's designated switch
    The switch ID identifying the neighbor's idea of the designated
    switch, as contained in the neighbor's Hello packets.  This value
    is used in the local selection of the designated switch.  It is
    not used on point-to-point links.
 Neighbor's backup designated switch
    The switch ID identifying the neighbor's idea of the backup
    designated switch, as contained in the neighbor's Hello packets.
    This value is used in the local selection of the backup designated
    switch.  It is not used on point-to-point links.
 Link state retransmission list
    The list of link state advertisements that have been forwarded
    over but not acknowledged on this adjacency.  The local switch
    retransmits these link state advertisements at periodic intervals
    until they are acknowledged or until the adjacency is destroyed.
    (For more information on retransmitting link state advertisements,
    see Section 8.2.5.)

Kane Informational [Page 24] RFC 2642 Cabletron's VLS Protocol Specification August 1999

 Database summary list
    The set of link state advertisement headers that summarize the
    local link state database.  When the conversation changes to the
    Exchange state, this list is sent to the neighbor via Database
    Description packets.  (For more information on the synchronization
    of databases, see Section 7.)
 Link state request list
    The list of link state advertisements that must be received in
    order to synchronize with the neighbor switch's link state
    database.  This list is created as Database Description packets
    are received, and is then sent to the neighbor in Link State
    Request packets.  (For more information on the synchronization of
    databases, see Section 7.)

4.1 Neighbor States

 This section describes the various states of a conversation with a
 neighbor switch.  The states are listed in order of progressing
 functionality.  For example, the inoperative state is listed first,
 followed by a list of the intermediate states through which the
 conversation passes before attaining the final, fully functional
 state.  The specification makes use of this ordering by references
 such as "those neighbors/adjacencies in state greater than X".
 Figure 2 represents the neighbor state machine.  The arrows on the
 graph represent the events causing each state change.  These events
 are described in Section 4.2.  The neighbor state machine is
 described in detail in Section 4.3.
 Down
    This is the initial state of a neighbor conversation.
 Init
    In this state, the neighbor has been discovered, but bidirectional
    communication has not yet been established. All neighbors in this
    state or higher are listed in the VLS Hello packets sent by the
    local switch over the associated (multi-access) interface.

Kane Informational [Page 25] RFC 2642 Cabletron's VLS Protocol Specification August 1999

        +----------+     KillNbr, LLDown,   +-----------+
        |   Down   | <--------------------- | any state |
        +----------+   or Inactivity Timer  +-----------+
             |
       Hello |
        Rcvd |
             |
             V
 +-----< [pt-to-pt?]
 | yes       |
 |           | no
 |           V
 |      +----------+   1-Way   +----------+
 |      |   Init   | <-------- | >= 2-way |
 |      +----------+           +----------+
 |           |
 |     2-Way |
 |      Rcvd |                  +-------+   AdjOK? +------------+
 |           +----------------> | 2-Way | <------- | >= ExStart |
 |           | (no adjacency)   +-------+     no   +------------+
 |           |
 |           V
 |      +---------+   Seq Number Mismatch  +-------------+
 +----> | ExStart | <--------------------- | >= Exchange |
        +---------+       or BadLSReq      +-------------+
             |
 Negotiation |
     Done    |
             V
        +----------+
        | Exchange |
        +----------+
             |
    Exchange |                        +--------+
      Done   +----------------------> |  Full  |
             | (request list empty)   +--------+
             |                             ^
             V                             |
        +---------+      Loading Done      |
        | Loading | ----------------------->
        +---------+
                Figure 2: Neighbor State Machine

Kane Informational [Page 26] RFC 2642 Cabletron's VLS Protocol Specification August 1999

 2-Way
    In this state, communication between the two switches is
    bidirectional.  This is the most advanced state short of beginning
    to establish an adjacency.  On a multi-access link, the designated
    switch and the backup designated switch are selected from the set
    of neighbors in state 2-Way or greater.
 ExStart
    This state indicates that the two switches have begun to establish
    an adjacency by determining which switch is the master, as well as
    the initial sequence number for Database Descriptor packets.
    Neighbor conversations in this state or greater are called
    adjacencies.
 Exchange
    In this state, the switches are exchanging Database Description
    packets.  (See Section 7.2 for a complete description of this
    process.)  All adjacencies in the Exchange state or greater are
    used by the distribution procedure (see Section 8.2), and are
    capable of transmitting and receiving all types of VLSP routing
    packets.
 Loading
    In this state, the local switch is sending Link State Request
    packets to the neighbor asking for the more recent advertisements
    that were discovered in the Exchange state.
 Full
    In this state, the two switches are fully adjacent.  These
    adjacencies will now appear in switch link and network link
    advertisements generated for the link.

4.2 Events Causing Neighbor State Changes

 The state of a neighbor conversation changes due to neighbor events.
 This section describes these events.
 Neighbor events are shown as arrows in Figure 2, the graphic
 representation of the neighbor state machine.  For more information
 on the neighbor state machine, see Section 4.3.

Kane Informational [Page 27] RFC 2642 Cabletron's VLS Protocol Specification August 1999

 Hello Received
    This event is generated when a Hello packet has been received from
    a neighbor.
 2-Way Received
    This event is generated when the local switch sees its own switch
    ID listed in the neighbor's Hello packet, indicating that
    bidirectional communication has been established between the two
    switches.
 Negotiation Done
    This event is generated when the master/slave relationship has
    been successfully negotiated and initial packet sequence numbers
    have been exchanged.  This event signals the start of the database
    exchange process (see Section 7.2).
 Exchange Done
    This event is generated when the database exchange process is
    complete and both switches have successfully transmitted a full
    sequence of Database Description packets.  (For more information
    on the database exchange process, see Section 7.2.)
 BadLSReq
    This event is generated when a Link State Request has been
    received for a link state advertisement that is not contained in
    the database.  This event indicates an error in the
    synchronization process.
 Loading Done
    This event is generated when all Link State Updates have been
    received for all out-of-date portions of the database.  (See
    Section 7.3.)
 AdjOK?
    This event is generated when a decision must be made as to whether
    an adjacency will be established or maintained with the neighbor.
    This event will initiate some adjacencies and destroy others.

Kane Informational [Page 28] RFC 2642 Cabletron's VLS Protocol Specification August 1999

 Seq Number Mismatch
    This event is generated when a Database Description packet has
    been received with any of the following conditions:
    o  The packet contains an unexpected sequence number.
    o  The packet (unexpectedly) has the Init bit set.
    o  The packet has a different Options field than was
       previously seen.
    These conditions all indicate that an error has occurred during
    the establishment of the adjacency.
 1-Way
    This event is generated when bidirectional communication with the
    neighbor has been lost.  That is, a Hello packet has been received
    from the neighbor in which the local switch is not listed.
 KillNbr
    This event is generated when further communication with the
    neighbor is impossible.
 Inactivity Timer
    This event is generated when the inactivity timer has expired,
    indicating that no Hello packets have been received from the
    neighbor in SwitchDeadInterval seconds.  This timer is used only
    on broadcast (multi-access) links.
 LLDown
    This event is generated by the lower-level switch discovery
    protocols and indicates that the neighbor is now unreachable.

4.3 Neighbor State Machine

 This section presents a detailed description of the neighbor state
 machine.
 Neighbor states (see Section 4.1) change as the result of various
 events (see Section 4.2).  However, the effect of each event can
 vary, depending on the current state of the conversation with the
 neighbor.  For this reason, the state machine described in this
 section is organized according to the current neighbor state and the
 occurring event.  For each state/event pair, the new neighbor state
 is listed, along with a description of the required processing.

Kane Informational [Page 29] RFC 2642 Cabletron's VLS Protocol Specification August 1999

 Note that when the neighbor state changes as a result of an interface
 Neighbor Change event (see Section 3.2), it may be necessary to rerun
 the designated switch selection algorithm. In addition, if the
 interface associated with the neighbor conversation is in the DS
 state (that is, the local switch is the designated switch), changes
 in the neighbor state may cause a new network link advertisement to
 be originated (see Section 8.1).
 When the neighbor state machine must invoke the interface state
 machine, it is invoked as a scheduled task.  This simplifies
 processing, by ensuring that neither state machine executes
 recursively.
 State(s):  Down
 Event:  Hello Received
 New state:  Depends on the interface type
 Action:
    If the interface type of the associated link is point-to-point,
    change the neighbor state to ExStart.  Otherwise, change the
    neighbor state to Init and start the inactivity timer for the
    neighbor.  If the timer expires before another Hello packet is
    received, the neighbor switch is declared dead.
 State(s):  Init or greater
 Event:  Hello Received
 New state:  No state change
 Action:
    If the interface type of the associated link is point-to-point,
    determine whether this notification is for a different neighbor
    than the one previously seen. If so, generate an Interface Down
    event for the associated interface, reset the interface type to
    broadcast and generate an Interface Up event.
 Note:  This procedure of generating an Interface Down event and
 changing the interface type to broadcast is also executed if the
 neighbor for whom the notification was received is running an older
 version of the protocol software (see Section 6.1).  In previous
 versions of the protocol, all interfaces were treated as if they were
 broadcast.
    If the interface type is broadcast, reset the inactivity timer for
    the neighbor.

Kane Informational [Page 30] RFC 2642 Cabletron's VLS Protocol Specification August 1999

 State(s):  Init
 Event:  2-Way Received
 New state:  Depends on action routine
 Action:
    Determine whether an adjacency will be formed with the neighbor
    (see Section 6.4).  If no adjacency is to be formed, change the
    neighbor state to 2-Way.
    Otherwise, change the neighbor state to ExStart.  Initialize the
    sequence number for this neighbor and declare the local switch to
    be master for the database exchange process.  (See Section 7.2.)
 State(s):  ExStart
 Event:  Negotiation Done
 New state:  Exchange
 Action:
    The Negotiation Done event signals the start of the database
    exchange process.  See Section 7.2 for a detailed description of
    this process.
 State(s):  Exchange
 Event:  Exchange Done
 New state:  Depends on action routine
 Action:
    If the neighbor Link state request list is empty, change the
    neighbor state to Full.  This is the adjacency's final state.
    Otherwise, change the neighbor state to Loading.  Begin sending
    Link State Request packets to the neighbor requesting the most
    recent link state advertisements, as discovered during the
    database exchange process.  (See Section 7.2.) These
    advertisements are listed in the link state request list
    associated with the neighbor.
 State(s):  Loading
 Event:  Loading Done
 New state:  Full
 Action:
    No action is required beyond changing the neighbor state to Full.
    This is the adjacency's final state.

Kane Informational [Page 31] RFC 2642 Cabletron's VLS Protocol Specification August 1999

 State(s):  2-Way
 Event:  AdjOK?
 New state:  Depends on action routine
 Action:
    If no adjacency is to be formed with the neighboring switch (see
    Section 6.4), retain the neighbor state at 2-Way. Otherwise,
    change the neighbor state to ExStart.  Initialize the sequence
    number for this neighbor and declare the local switch to be master
    for the database exchange process.  (See Section 7.2.)
 State(s):  ExStart or greater
 Event:  AdjOK?
 New state:  Depends on action routine
 Action:
    If an adjacency should still be formed with the neighboring switch
    (see Section 6.4), no state change and no further action is
    necessary.  Otherwise, tear down the (possibly partially formed)
    adjacency.  Clear the link state retransmission list, database
    summary list and link state request list and change the neighbor
    state to 2-Way.
 State(s):  Exchange or greater
 Event:  Seq Number Mismatch
 New state:  ExStart
 Action:
    Tear down the (possibly partially formed) adjacency.  Clear the
    link state retransmission list, database summary list and link
    state request list.  Change the neighbor state to ExStart and make
    another attempt to establish the adjacency.
 State(s):  Exchange or greater
 Event:  BadLSReq
 New state:  ExStart
 Action:
    Tear down the (possibly partially formed) adjacency.  Clear the
    link state retransmission list, database summary list and link
    state request list.  Change the neighbor state to ExStart and make
    another attempt to establish the adjacency.
 State(s):  Any state
 Event:  KillNbr
 New state:  Down
 Action:
    Terminate the neighbor conversation.  Disable the inactivity timer
    and clear the link state retransmission list, database summary
    list and link state request list.

Kane Informational [Page 32] RFC 2642 Cabletron's VLS Protocol Specification August 1999

 State(s):  Any state
 Event:  LLDown
 New state:  Down
 Action:
    Terminate the neighbor conversation.  Disable the inactivity timer
    and clear the link state retransmission list, database summary
    list and link state request list.
 State(s):  Any state
 Event:  Inactivity Timer
 New state:  Down
 Action:
    Terminate the neighbor conversation.  Disable the inactivity timer
    and clear the link state retransmission list, database summary
    list and link state request list.
 State(s):  2-Way or greater
 Event:  1-Way Received
 New state:  Init
 Action:
    Tear down the adjacency between the switches, if any.  Clear the
    link state retransmission list, database summary list and link
    state request list.
 State(s):  2-Way or greater
 Event:  2-Way received
 New state:  No state change
 Action:
    No action required.
 State(s):  Init
 Event:  1-Way received
 New state:  No state change
 Action:
          No action required.

5. Area Data Structure

 The area data structure contains all the information needed to run
 the basic routing algorithm.  One of its components is the link state
 database -- the collection of all switch link and network link
 advertisements generated by the switches.
 The area data structure contains the following items:

Kane Informational [Page 33] RFC 2642 Cabletron's VLS Protocol Specification August 1999

 Area ID
    A 4-octet value identifying the area.  Since VLSP does not support
    multiple areas, the value here is always zero.
 Associated switch interfaces
    A list of interface IDs of the local switch interfaces connected
    to network links.
 Link state database
    The collection of all current link state advertisements for the
    switch fabric.  This collection consists of the following:
 Switch link advertisements
    A list of the switch link advertisements for all switches in the
    fabric.  Switch link advertisements describe the state of each
    switch's interfaces.
 Network link advertisements
    A list of the network link advertisements for all multi-access
    network links in the switch fabric.  Network link advertisements
    describe the set of switches currently connected to each link.
 Best path(s)
    A set of end-to-end hop descriptions for all equal-cost best paths
    from the local switch to every other switch in the fabric.  Each
    hop is specified by the interface ID of the next link in the path.
    Best paths are derived from the collected switch link and network
    link advertisements using the Dijkstra algorithm. [Perlman]

5.1 Adding and Deleting Link State Advertisements

 The link state database within the area data structure must contain,
 at most, a single instance of each link state advertisement.  To keep
 the database current, a switch adds link state advertisements to the
 database under the following conditions:
 o  When a link state advertisement is received during the
    distribution process
    o  When the switch itself generates a link state advertisement

Kane Informational [Page 34] RFC 2642 Cabletron's VLS Protocol Specification August 1999

 (See Section 8.2.4 for information on installing link state
 advertisements.)
 Likewise, a switch deletes link state advertisements from the
 database under the following conditions:
 o  When a link state advertisement has been superseded by a newer
    instance during the flooding process
 o  When the switch generates a newer instance of one of its self-
    originated advertisements
 Note that when an advertisement is deleted from the link state
 database, it must also be removed from the link state retransmission
 list of all neighboring switches.

5.2 Accessing Link State Advertisements

 An implementation of the VLS protocol must provide access to
 individual link state advertisements, based on the advertisement's
 type, link state identifier, and advertising switch [1].  This lookup
 function is invoked during the link state distribution procedure and
 during calculation of the set of best paths.  In addition, a switch
 can use the function to determine whether it has originated a
 particular link state advertisement, and if so, with what sequence
 number.

5.3 Best Path Lookup

 An implementation of the VLS protocol must provide access to multiple
 equal-cost best paths, based on the base MAC addresses of the source
 and destination switches.  This lookup function should return up to
 three equal-cost paths.  Paths should be returned as lists of end-
 to-end hop information, with each hop specified as a interface ID of
 the next link in the path -- the 6-octet base MAC address of the next
 switch and the 4-octet local port number of the link interface.

6. Discovery Process

 The first operational stage of the VLS protocol is the discovery
 process.  During this stage, each switch dynamically detects its
 neighboring switches and establishes a relationship with each of
 these neighbors.  This process has the following component steps:

Kane Informational [Page 35] RFC 2642 Cabletron's VLS Protocol Specification August 1999

 o  Neighboring switches are detected on each functioning network
    interface.
 o  Bidirectional communication is established with each neighbor
    switch.
 o  A designated switch and backup designated switch are selected for
    each multi-access network link.
 o  An adjacent relationship is established with selected neighbors on
    each link.

6.1 Neighbor Discovery

 When the switch first comes on line, VLSP assumes all network links
 are point-to-point and no more than one neighboring switch will be
 discovered on any one port.  Therefore, at startup, VLSP relies on
 the VlanHello protocol [IDhello] for the discovery of its neighbor
 switches.
 As each neighbor is detected, VlanHello triggers a Found Neighbor
 event, notifying VLSP that a new neighbor has been discovered.  (See
 [IDhello] for a description of the Found Neighbor event and the
 information passed.)  VLSP enters the neighbor switch ID in the list
 of known neighbors and creates a new neighbor data structure with a
 neighbor status of Down.  A Hello Received neighbor event is then
 generated, which changes the neighbor state to ExStart.
 There are two circumstances under which VLSP will change the type of
 an interface to broadcast:
 o  If VLSP receives a subsequent notification from VlanHello,
    specifying a second (different) neighbor switch on the port., the
    interface is then known to be multi-access.  VLSP generates an
    Interface Down event for the interface, resets the interface type
    to broadcast, and then generates an Interface Up event.
 o  If the functional level of the neighbor switch is less than 2, the
    neighbor is running a previous version of the VLSP software in
    which all links were treated as broadcast links. VLSP immediately
    changes the interface type to broadcast and generates an Interface
    Up event.
    In both cases, VLSP assumes control of communication over the
    interface by exchanging its own VLSP Hello packets with the
    neighbors on the link.

Kane Informational [Page 36] RFC 2642 Cabletron's VLS Protocol Specification August 1999

 Note:  These Hello packets are in addition to the Interswitch
 Keepalive messages sent by VlanHello.  VlanHello still continues to
 monitor the condition of the interface and notifies VLSP of any
 change.
 Each Hello packet contains the following data used during the
 discovery process on multi-access links:
 o  The switch ID and priority of the sending switch
 o  Values specifying the interval timers to be used for sending Hello
    packets and deciding whether to declare a neighbor switch Down.
 o  The switch ID of the designated switch and the backup designated
    switch for the link, as understood by the sending switch
 o  A list of switch IDs of all neighboring switches seen so far on
    the link
 For a detailed description of the Hello packet format, see Section
 10.6.1.
 When VLSP receives a Hello packet (on a broadcast link), it first
 attempts to identify the sending switch by matching its switch ID to
 one of the known neighbors listed in the interface data structure.
 If this is the first Hello packet received from the switch, the
 switch ID is entered in the list of known neighbors and a new
 neighbor data structure is created with a neighbor status of Down.
 At this point, the remainder of the Hello packet is examined and the
 appropriate interface and neighbor events are generated.  In all
 cases, a neighbor Hello Received event is generated.  Other events
 may also be generated, triggering further steps in the discovery
 process or other actions, as appropriate.
 For a detailed description of the interface state machine, see
 Section 3.3.  For a detailed description of the neighbor state
 machine, see Section 4.3.

6.2 Bidirectional Communication

 Before a conversation can proceed with a neighbor switch,
 bidirectional communication must be established with that neighbor.
 Bidirectional communication is detected in one of two ways:

Kane Informational [Page 37] RFC 2642 Cabletron's VLS Protocol Specification August 1999

 o  On a point-to-point link, the VlanHello protocol sees its own
    switch ID listed in an Interswitch Keepalive message it has
    received from the neighbor.
 o  On a multi-access link, VLSP sees its own switch ID listed in a
    VLSP Hello packet it has received from the neighbor.
 In either case, a neighbor 2-Way Received neighbor event is
 generated.
 Once bidirectional communication has been established with a
 neighbor, the local switch determines whether an adjacency will be
 formed with the neighbor.  However, if the link is a multi-access
 link, a designated switch and a backup designated switch must first
 be selected for the link.  The next section contains a description of
 the designated switch, the backup designated switch, and the
 selection process.

6.3 Designated Switch

 Every multi-access network link has a designated switch.  The
 designated switch performs the following functions for the routing
 protocol:
 o  The designated switch originates a network link advertisement on
    behalf of the link, listing the set of switches (including the
    designated switch itself) currently attached to the link. For a
    detailed description of network link advertisements, see Section
    11.3.
 o  The designated switch becomes adjacent to all other switches on
    the link.  Since the link state databases are synchronized across
    adjacencies, the designated switch plays a central part in the
    synchronization process.  For a description of the synchronization
    process, see Section 7.
 Each multi-access network link also has a backup designated switch.
 The primary function of the backup designated switch is to act as a
 standby for the designated switch.  If the current designated switch
 fails, the backup designated switch becomes the designated switch.
 To facilitate this transition, the backup designated switch forms an
 adjacency with every other switch on the link.  Thus, when the backup
 designated switch must take over for the designated switch, its link
 state database is already synchronized with the databases of all
 other switches on the link.

Kane Informational [Page 38] RFC 2642 Cabletron's VLS Protocol Specification August 1999

 Note:  Point-to-point network links have neither a designated switch
 or a backup designated switch.

6.3.1 Selecting the Designated Switch

 When a multi-access link interface first becomes functional, the
 switch sets a one-shot Wait timer (with a value of SwitchDeadInterval
 seconds) for the interface.  The purpose of this timer is to ensure
 that all switches attached to the link have a chance to establish
 bidirectional communication before the designated switch and backup
 designated switch are selected for the link.
 When the Wait timer is set, the interface enters the Waiting state.
 During this state, the switch exchanges Hello packets with its
 neighbors attempting to establish bidirectional communication.  The
 interface leaves the Waiting state under one of the following
 conditions:
 o  The Wait timer expires.
 o  A Hello packet is received indicating that a designated switch or
    a backup designated switch has already been specified for the
    interface.
 At this point, if the switch sees that a designated switch has
 already been selected for the link, the switch accepts that
 designated switch, regardless of its own switch priority and MAC
 address.  This situation typically means the switch has come up late
 on a fully functioning link.  Although this makes it harder to
 predict the identity of the designated switch on a particular link,
 it ensures that the designated switch does not change needlessly,
 necessitating a resynchronization of the databases.
 If no designated switch is currently specified for the link, the
 switch begins the actual selection process.  Note that this selection
 algorithm operates only on a list of neighbor switches that are
 eligible to become the designated switch.  A neighbor is eligible to
 be the designated switch if it has a switch priority greater than
 zero and its neighbor state is 2-Way or greater.  The local switch
 includes itself on the list of eligible switches as long as it has a
 switch priority greater than zero.
 The selection process includes the following steps:
 1. The current values of the link's designated switch and backup
    designated switch are saved for use in step 6.
 2. The new backup designated switch is selected as follows:

Kane Informational [Page 39] RFC 2642 Cabletron's VLS Protocol Specification August 1999

    a) Eliminate from consideration those switches that have declared
       themselves to be the designated switch.
    b) If one or more of the remaining switches have declared
       themselves to be the backup designated switch, eliminate from
       consideration all other switches.
    c) From the remaining list of eligible switches, select the switch
       having the highest switch priority as the backup designated
       switch.  If multiple switches have the same (highest) priority,
       select the switch with the highest switch ID as the backup
       designated switch.
 3. The new designated switch is selected as follows:
    a) If one or more of the switches have declared themselves to be
       the designated switch, eliminate from consideration all other
       switches.
    b) From the remaining list of eligible switches, select the switch
       having the highest switch priority as the designated switch.
       If multiple switches have the same (highest) priority, select
       the switch with the highest switch ID as the designated switch.
 4. If the local switch has been newly selected as either the
    designated switch or the backup designated switch, or is now no
    longer the designated switch or the backup designated switch,
    repeat steps 2 and 3, above, and then proceed to step 5.
    If the local switch is now the designated switch, it will
    eliminate itself from consideration at step 2a when the selection
    of the backup designated switch is repeated. Likewise, if the
    local switch is now the backup designated switch, it will
    eliminate itself from consideration at step 3a when the selection
    of the designated switch is repeated. This ensures that no switch
    will select itself as both backup designated switch and designated
    switch [2].
 5. Set the interface state to the appropriate value, as follows:
 o  If the local switch is now the designated switch, set the
    interface state to DS.
 o  If the local switch is now the backup designated switch, set the
    interface state to Backup.
 o  Otherwise, set the interface state to DS Other.

Kane Informational [Page 40] RFC 2642 Cabletron's VLS Protocol Specification August 1999

 6. If either the designated switch or backup designated switch has
    now changed, the set of adjacencies associated with this link must
    be modified.  Some adjacencies may need to be formed, while others
    may need to be broken.  Generate the neighbor AdjOK? event for all
    neighbors with a state of 2-Way or higher to trigger a
    reexamination of adjacency eligibility.
 Caution:  If VLSP is implemented with configurable parameters, care
 must be exercised in specifying the switch priorities.  Note that if
 the local switch is not itself eligible to become the designated
 switch (i.e., it has a switch priority of 0), it is possible that
 neither a backup designated switch nor a designated switch will be
 selected by the above procedure.  Note also that if the local switch
 is the only attached switch that is eligible to become the designated
 switch, it will select itself as designated switch and there will be
 no backup designated switch for the link.  For this reason, it is
 advisable to specify a default switch priority of 1 for all switches.

6.4 Adjacencies

 VLSP creates adjacencies between neighboring switches for the purpose
 of exchanging routing information.  Not every two neighboring
 switches will become adjacent.  On a multi-access link, an adjacency
 is only formed between two switches if one of them is either the
 designated switch or the backup designated switch.
 Note that an adjacency is bound to the network link that the two
 switches have in common.  Therefore, if two switches have multiple
 links in common, they may also have multiple adjacencies between
 them.
 The decision to form an adjacency occurs in two places in the
 neighbor state machine:
 o  When bidirectional communication is initially established with the
    neighbor.
 o  When the designated switch  or backup designated switch on the
    attached link changes.
 The rules for establishing an adjacency between two neighboring
 switches are as follows:
 o  On a point-to-point link, the two neighboring switches always
    establish an adjacency.
 o  On a multi-access link, an adjacency is established with the
    neighboring switch under one of the following conditions:

Kane Informational [Page 41] RFC 2642 Cabletron's VLS Protocol Specification August 1999

    o  The local switch itself is the designated switch.
    o  The local switch itself is the backup designated switch.
    o  The neighboring switch is the designated switch.
    o  The neighboring switch is the backup designated switch.
 If no adjacency is formed between two neighboring switches, the state
 of the neighbor conversation remains set to 2-Way.

7. Synchronizing the Databases

 In an SPF-based routing algorithm, it is important for the link state
 databases of all switches to stay synchronized.  VLSP simplifies this
 process by requiring only adjacent switches to remain synchronized.
 The synchronization process begins when the switches attempt to bring
 up the adjacency.  Each switch in the adjacency describes its
 database by sending a sequence of Database Description packets to its
 neighbor.  Each Database Description packet describes a set of link
 state advertisements belonging to the database.  When the neighbor
 sees a link state advertisement that is more recent than its own
 database copy, it makes a note to request this newer advertisement.
 During this exchange of Database Description packets (known as the
 database exchange process), the two switches form a master/slave
 relationship.  Database Description packets sent by the master are
 known as polls, and each poll contains a sequence number.  Polls are
 acknowledged by the slave by echoing the sequence number in the
 Database Description response packet.
 When all Database Description packets have been sent and
 acknowledged, the database exchange process is completed.  At this
 point, each switch in the exchange has a list of link state
 advertisements for which its neighbor has more recent instances.
 These advertisements are requested using Link State Request packets.
 Once the database exchange process has completed and all Link State
 Requests have been satisfied, the databases are deemed synchronized
 and the neighbor states of the two switches are set to Full,
 indicating that the adjacency is fully functional. Fully functional
 adjacencies are advertised in the link state advertisements of the
 two switches [3].

Kane Informational [Page 42] RFC 2642 Cabletron's VLS Protocol Specification August 1999

7.1 Link State Advertisements

 Link state advertisements form the core of the database from which a
 switch calculates the set of best paths to the other switches in the
 fabric.
 Each link state advertisement begins with a standard header. This
 header contains three data items that uniquely identify the link
 state advertisement.
 o  The link state type.  Possible values are as follows:
    1   Switch link advertisement -- describes the collected states of
       the switch's interfaces.
    2   Network link advertisement -- describes the set of switches
       attached to the network link.
 o  The link state ID, defined as follows:
    o  For a switch link advertisement -- the switch ID of the
       originating switch
    o  For a network link advertisement -- the switch ID of the
       designated switch for the link
 o  The switch ID of the advertising switch -- the switch that
    generated the advertisement
 The link state advertisement header also contains three data items
 that are used to determine which instance of a particular link state
 advertisement is the most current.  (See Section 7.1.1 for a
 description of how to determine which instance of a link state
 advertisement is the most current.)
 o  The link state sequence number
 o  The link state age, stored in seconds
 o  The link state checksum, a 16-bit unsigned value calculated for
    the entire contents of the link state advertisement, with the
    exception of the age field
 The remainder of each link state advertisement contains data specific
 to the type of the advertisement.  See Section 11 for a detailed
 description of the link state header, as well as the format of a
 switch link or network link advertisement.

Kane Informational [Page 43] RFC 2642 Cabletron's VLS Protocol Specification August 1999

7.1.1 Determining Which Link State Advertisement Is Newer

 At various times while synchronizing or updating the link state
 database, a switch must determine which instance of a particular link
 state advertisement is the most current.  This decision is made as
 follows:
 o  The advertisement having the greater sequence number is the most
    current.
 o  If both instances have the same sequence number, then:
    o  If the two instances have different checksum values, then the
       instance having the larger checksum is considered the most
       current [4].
 o  If both instances have the same sequence number and the same
    checksum value, then:
    o  If one (and only one) of the instances is of age MaxAge, then
       the instance of age MaxAge is considered the most current [5].
    o  Else, if the ages of the two instances differ by more than
       MaxAgeDiff, the instance having the smaller (younger) age is
       considered the most current [6].
    o  Else, the two instances are considered identical.

7.2 Database Exchange Process

 There are two stages to the database exchange process:
 o  Negotiating the master/slave relationship
 o  Exchanging database summary information

7.2.1 Database Description Packets

 Database Description packets are used to describe a switch's link
 state database during the database exchange process.  Each Database
 Description packet contains a list of headers of the link state
 advertisements currently stored in the sending switch's database.
 (See Section 11.1 for a description of a link state advertisement
 header.)
 In addition to the link state headers, each Database Description
 packet contains the following data items:

Kane Informational [Page 44] RFC 2642 Cabletron's VLS Protocol Specification August 1999

 o  A flag (the M-bit) indicating whether or not more packets are to
    follow.  Depending on the size of the local database and the
    maximum size of the packet, the list of headers in any particular
    Database Description packet may be only a partial list of the
    total database.  When the M-bit is set, the list of headers is
    only a partial list and more headers are to follow in subsequent
    packets.
 o  A flag (the I-bit) indicating whether or not this is the first
    Database Description packet sent for this execution of the
    database exchange process.
 o  A flag (the MS-bit) indicating whether the sending switch thinks
    it is the master or the slave in the database exchange process.
    If the flag is set, the switch thinks it is the master.
 o  A 4-octet sequence number for the packet.
 While the switches are negotiating the master/slave relationship,
 they exchange "empty" Database Description packets.  That is, packets
 that contain no link summary information.  Instead, the flags and
 sequence number constitute the information required for the
 negotiation process.
 See Section 10.6.2 for a more detailed description of a Database
 Description packet.

7.2.2 Negotiating the Master/Slave Relationship

 Before two switches can begin the actual exchange of database
 information, they must decide between themselves who will be the
 master in the exchange process and who will be the slave.  They must
 also agree on the starting sequence number for the Database
 Description packets.
 Once a switch has decided to form an adjacency with a neighboring
 switch, it sets the neighbor state to ExStart and begins sending
 empty Database Description packets to its neighbor.  These packets
 contain the starting sequence number the switch plans to use in the
 exchange process.  Also, the I-bit and M-bit flags are set, as well
 as the MS-bit.  Thus, each switch in the exchange begins by believing
 it will be the master.
 Empty Database Description packets are retransmitted every
 RxmtInterval seconds until the neighbor responds.

Kane Informational [Page 45] RFC 2642 Cabletron's VLS Protocol Specification August 1999

 When a switch receives an empty Database Description packet from its
 neighbor, it determines which switch will be the master by comparing
 the switch IDs.  The switch with the highest switch ID becomes the
 master of the exchange.  Based on this determination, the switch
 proceeds as follows:
 o  If the switch is to be the slave of the database exchange process,
    it acknowledges that it is the slave by sending another empty
    Database Description packet to the master. This packet contains
    the master's sequence number and has the MS-bit and the I-bit
    cleared.
 o  The switch then generates a neighbor event of Negotiation Done to
    change its neighbor state to Exchange and waits for the first
    non-empty Database Description packet from the master.
 o  If the switch is to be the master of the database exchange, it
    waits to receive an acknowledgment from its neighbor -- that is,
    an empty Database Description packet with the MS-bit and I-bit
    cleared and containing the sequence number it (the master)
    previously sent.
 o  When it receives the acknowledgment, it generates a neighbor event
    of Negotiation Done to change its neighbor state to Exchange and
    begin the actual exchange of Database Description packets.
 Note that during the negotiation process, the receipt of an
 inconsistent packet will result in a neighbor event of Seq Number
 Mismatch, terminating the process.  See Section 4.3 for more
 information.

7.2.3 Exchanging Database Description Packets

 Once the neighbor state changes to Exchange, the switches begin the
 exchange of Database Description packets containing link state
 summary data.  The process proceeds as follows:
 1. The master sends a packet containing a list of link state headers.
    If the packet contains only a portion of the unexchanged database
    -- that is, more Database Description packets are to follow -- the
    packet has the M-bit set.  The MS-bit is set and the I-bit is
    clear.
    If the slave does not acknowledge the packet within RxmtInterval
    seconds, the master retransmits the packet.

Kane Informational [Page 46] RFC 2642 Cabletron's VLS Protocol Specification August 1999

 2. When the slave receives a packet, it first checks the sequence
    number to see if the packet is a duplicate.  If so, it simply
    acknowledges the packet by clearing the MS-bit and returning the
    packet to the master.  (Note that the slave acknowledges all
    Database Description packets that it receives, even those that are
    duplicates.)
    Otherwise, the slave processes the packet by doing the following:
    o  For each link state header listed in the packet, the slave
       searches its own link state database to determine whether it
       has an instance of the advertisement.
    o  If the slave does not have an instance of the link state
       advertisement, or if the instance it does have is older than
       the instance listed in the packet, it creates an entry in its
       link state request list in the neighbor data structure.  See
       Section 7.1.1 for a description of how to determine which
       instance of a link state advertisement is the newest.
    o  When the slave has examined all headers, it acknowledges the
       packet by turning the MS-bit off and returning the packet to
       the master.
 3. When the master receives the first acknowledgment for a particular
    Database Description packet, it processes the acknowledgment as
    follows:
    o  For each link state header listed in the packet, the master
       checks to see if the slave has indicated it has an instance of
       the link state advertisement that is newer than the instance
       the master has in its own database.  If so, the master creates
       an entry in its link state request list in the neighbor data
       structure.
    o  The master then increments the sequence number and sends
       another packet containing the next set of link state summary
       information, if any.
    Subsequent acknowledgments for the Database Description packet
    (those with the same sequence number) are discarded.
    When the master sends the last portion of its database summary
    information, it clears the M-bit in the packet to indicate that no
    more packets are to be sent.

Kane Informational [Page 47] RFC 2642 Cabletron's VLS Protocol Specification August 1999

 4. When the slave receives a Database Description packet with the M-
    bit clear, it processes the packet, as described above in step 2.
    After it has completed processing and has acknowledged the packet
    to the master, it generates an Exchange Done neighbor event and
    its neighbor state changes to Loading.
    The database exchange process is now complete for the slave, and
    it begins the process of requesting those link state
    advertisements for which the master has more current instances
    (see Section 7.3).
 5. When the master receives an acknowledgment for the final Database
    Description packet, it processes the acknowledgment as described
    above in step 3.  Then it generates an Exchange Done neighbor
    event and its neighbor state changes to Loading.
    The database exchange process is now complete for the master, and
    it begins the process of requesting those link state
    advertisements for which the slave has more current instances (see
    Section 7.3).
 Note that during this exchange, the receipt of an inconsistent packet
 will result in a neighbor event of Seq Number Mismatch, terminating
 the process.  See Section 4.3 for more information.

7.3 Updating the Database

 When either switch completes the database exchange process and its
 neighbor state changes to Loading, it has a list of link state
 advertisements for which the neighboring switch has a more recent
 instance.  This list is stored in the neighbor data structure as the
 link state request list.
 To complete the synchronization of its database with that of its
 neighbor, the switch must obtain the most current instances of those
 link state advertisements.
 The switch requests these advertisements by sending its neighbor a
 Link State Request packet containing the description of one or more
 link state advertisement, as defined by the advertisement's type,
 link state ID, and advertising switch.  (For a detailed description
 of the Link State Request packet, see Section 10.6.3.)  The switch
 continues to retransmit this packet every RxmtInterval seconds until
 it receives a reply from the neighbor.

Kane Informational [Page 48] RFC 2642 Cabletron's VLS Protocol Specification August 1999

 When the neighbor switch receives the Link State Request packet, it
 responds with a Link State Update packet containing its most current
 instance of each of the requested advertisements.  (Note that the
 neighboring switch can be in any of the Exchange, Loading or Full
 neighbor states when it responds to a Link State Request packet.)
 If the neighbor cannot locate a particular link state advertisement
 in its database, something has gone wrong with the synchronization
 process.  The switch generates a BadLSReq neighbor event and the
 partially formed adjacency is torn down. See Section 4.3 for more
 information.
 Depending on the size of the link state request list, it may take
 more than one Link State Request packet to obtain all the necessary
 advertisements.  Note, however, that there must at most one Link
 State Request packet outstanding at any one time.

7.4 An Example

 Figure 3 shows an example of an adjacency being formed between two
 switches -- S1 and S2 -- connected to a network link.  S2 is the
 designated switch for the link and has a higher switch ID than S1.
 The neighbor state changes that each switch goes through are listed
 on the sides of the figure.

Kane Informational [Page 49] RFC 2642 Cabletron's VLS Protocol Specification August 1999

 +--------+                                     +--------+
 | Switch |                                     | Switch |
 |   S1   |                                     |   S2   |
 +--------+                                     +--------+
    Down                                           Down
                   Hello (DS=0, seen=0)
          ------------------------------------->
                                                   Init
                Hello (DS=S2, seen=...,S1)
          <-------------------------------------
 ExStart
           DB Description (Seq=x, I, M, Master)
          ------------------------------------->
                                                   ExStart
           DB Description (Seq=y, I, M, Master)
          <-------------------------------------
 xchange
             DB Description (Seq=y, M, Slave)
          ------------------------------------->
                                                   Exchange
           DB Description (Seq=y+1, M, Master)
          <-------------------------------------
            DB Description (Seq=y+1, M, Slave)
          ------------------------------------->
                            .
                            .
                            .
             DB Description (Seq=y+n, Master)
          <-------------------------------------
              DB Description (Seq=y+n, Slave)
          ------------------------------------->
 Loading                                           Full
                     Link State Request
          <-------------------------------------
                     Link State Update
          ------------------------------------->
                            .
                            .
                            .
                     Link State Request
          <-------------------------------------
                     Link State Update
          ------------------------------------->
  Full
       Figure 3: An Example of Bringing Up an Adjacency

Kane Informational [Page 50] RFC 2642 Cabletron's VLS Protocol Specification August 1999

 At the top of Figure 3, S1's interface to the link becomes
 operational, and S1 begins sending Hello packets over the interface.
 At this point, S1 does not yet know the identity of the designated
 switch or of any other neighboring switches.  S2 receives the Hello
 packet from S1 and changes its neighbor state to Init.  In its next
 Hello packet, S2 indicates that it is itself the designated switch
 and that it has received a Hello packet from S1.  S1 receives the
 Hello packet and changes its state to ExStart, starting the process
 of bringing up the adjacency.
 S1 begins by asserting itself as the master.  When it sees that S2 is
 indeed the master (because of S2's higher switch ID), S1 changes to
 slave and adopts S2's sequence number.  Database Description packets
 are then exchanged, with polls coming from the master (S2) and
 acknowledgments from the slave (S1).  This sequence of Database
 Description packets ends when both the poll and associated
 acknowledgment have the M-bit off.
 In this example, it is assumed that S2 has a completely up-to-date
 database and immediately changes to the Full state. S1 will change to
 the Full state after updating its database by sending Link State
 Request packets and receiving Link State Update packets in response.
 Note that in this example, S1 has waited until all Database
 Description packets have been received from S2 before sending any
 Link State Request packets.  However, this need not be the case.  S1
 could interleave the sending of Link State Request packets with the
 reception of Database Description packets.

8. Maintaining the Databases

 Each switch advertises its state (also known as its link state) by
 originating switch link advertisements.  In addition, the designated
 switch on each network link advertises the state of the link by
 originating network link advertisements.
 As described in Section 7.1, link state advertisements are uniquely
 identified by their type, link state ID, and advertising switch.
 Link state advertisements are distributed throughout the switch
 fabric using a reliable flooding algorithm that ensures that all
 switches in the fabric are notified of any link state changes.

Kane Informational [Page 51] RFC 2642 Cabletron's VLS Protocol Specification August 1999

8.1 Originating Link State Advertisements

 A new instance of each link state advertisement is originated any
 time the state of the switch or link changes.  When a new instance of
 a link state advertisement is originated, its sequence number is
 incremented, its age is set to zero, and its checksum is calculated.
 The advertisement is then installed into the local link state
 database and forwarded out all fully operational interfaces (that is,
 those interfaces with a state greater than Waiting) for distribution
 throughout the switch fabric.  See Section 8.2.4 for a description of
 the installation of the advertisement into the link state database
 and Section 8.2.5 for a description of how advertisements are
 forwarded.
 A switch originates a new instance of a link state advertisement as a
 result of the following events:
 o  The state of one of the switch's interfaces changes such that the
    contents of the associated switch link advertisement changes.
 o  The designated switch on any of the switch's attached network
    links changes.  The switch originates a new switch link
    advertisement.  Also, if the switch itself is now the designated
    switch, it originates a new network link advertisement for the
    link.
 o  One of the switch's neighbor states changes to or from Full. If
    this changes the contents of the associated switch link
    advertisement, a new instance is generated.  Also, if the switch
    is the designated switch for the attached network link, it
    originates a new network link advertisement for the link.
 Two instances of the same link state advertisement must not be
 originated within the time period MinLSInterval.  Note that this may
 require that the generation of the second instance to be delayed up
 to MinLSInterval seconds.

8.1.1 Switch Link Advertisements

 A switch link advertisement describes the collected states of all
 functioning links attached to the originating switch -- that is, all
 attached links with an interface state greater than Down.  A switch
 originates an empty switch link advertisement when it first becomes
 functional.  It then generates a new instance of the advertisement
 each time one of its interfaces reaches a fully functioning state
 (Point-to-Point or better).

Kane Informational [Page 52] RFC 2642 Cabletron's VLS Protocol Specification August 1999

 Each link in the advertisement is assigned a type, based on the state
 of interface, as shown in Table 4.
          Interface state     Link type     Description
          Point-to-Point      1             Point-to-point link
          DS Other*           2             Multi-access link
          Backup*             2             Multi-access link
          DS**                2             Multi-access link
  • If a full adjacency has been formed with the designated

switch.

  • *If a full adjacency has been formed with at least one

other switch on the link.

             Table 4: Link Types in a Switch Link Advertisement
 Each link in the advertisement is also assigned a link identifier
 based on its link type.  In general, this value identifies another
 switch that also originates advertisements for the link, thereby
 providing a key for accessing other link state advertisements for the
 link.  The relationship between link type and ID is shown in Table 5.
           Type  Description           Link ID
          1     Point-to-point link   Switch ID of neighbor switch
          2     Multi-access link     Switch ID of designated switch
             Table 5: Link IDs in a Switch Link Advertisement
 In addition to a type and an identifier, the description of each link
 specifies the interface ID of the associated network link.
 Finally, each link description includes the cost of sending a packet
 over the link.  This output cost is expressed in the link state
 metric and must be greater than zero.
 To illustrate the format of a switch link advertisement, consider the
 switch fabric shown in Figure 4.
 In this example, switch SW1 has 5 neighboring switches (shown as
 boxes) distributed over 3 network links (shown as lines).  The base
 MAC address of each switch is also shown adjacent to each box.  On
 switch SW1, ports 01 and 02 attach to point-to-point network links,

Kane Informational [Page 53] RFC 2642 Cabletron's VLS Protocol Specification August 1999

 while port 03 attaches to a multi-access network link with three
 attached switches.  The interface state of each port is shown next to
 the line representing the corresponding link.
                          00-00-1d-22-23-c5
                              +-------+
                              |  SW2  |
                              +-------+
                                  |
                                  | Point-to-Point
                                  |
                                  | 01
     +-------+    Loopback    +-------+
     |  SW3  |----------------|  SW1  | 00-00-1d-1f-05-81
     +-------+             02 +-------+
 00-00-1d-17-35-a4                | 03
                                  |
                                  | DS Other
                                  |
             +--------------------+--------------------+
             |                    |                    |
             | DS Other           | Backup             | DS
             |                    |                    |
         +-------+            +-------+            +-------+
         |  SW4  |            |  SW5  |            |  SW6  |
         +-------+            +-------+            +-------+
      00-00-1d-4a-26-b3    00-00-1d-4a-27-1c    00-00-1d-7e-84-2e
                  Figure 4: Sample Switch Fabric
 The switch link advertisement generated by switch SW1 would contain
 the following data items:
    ; switch link advertisement for switch SW1
    LS age = 0               ; always true on origination
    Options = (T-bit|E-bit)  ; options
    LS type = 1              ; this is a switch link advert

Kane Informational [Page 54] RFC 2642 Cabletron's VLS Protocol Specification August 1999

                             ; SW1's switch ID
    Link State ID = 00-00-1d-1f-05-81-00-00-00-00
    Advertising switch = 00-00-1d-1f-05-81-00-00-00-00
    # links = 2
       ; link on interface port 1
       Link ID = 00-00-1d-22-23-c5-00-00-00-00    ; switch ID
       Link Data = 00-00-1d-1f-05-81-00-00-00-01  ; interface ID
       Type = 1                                   ; pt-to-pt link
       # other metrics = 0                        ; TOS 0 only
       TOS 0 metric = 1
       ; link on interface port 2 is not fully functional
       ; link on interface port 3
       Link ID = 00-00-1d-7e-84-2e-00-00-00-00    ; switch ID of DS
       Link Data = 00-00-1d-1f-05-81-00-00-00-03  ; interface ID
       Type = 2                                   ; multi-access
       # other metrics = 0                        ; TOS 0 only
       TOS 0 metric = 2
 (See Section 11.2 for a detailed description of the format of a
 switch link advertisement.)

8.1.2 Network Link Advertisements

 Network link advertisements are used to describe the switches
 attached to each multi-access network link.
 Note:  Network link advertisements are not generated for point-to-
 point links.
 A network link advertisement is originated by the designated switch
 for the associated multi-access link once the switch has established
 a full adjacency with at least one other switch on the link.  Each
 advertisement lists the switch IDs of those switches that are fully
 adjacent to the designated switch.  The designated switch includes
 itself in this list.
 To illustrate the format of a network link advertisement, consider
 again the switch fabric shown in Figure 4.  In this example, network
 link advertisements will be generated only by switch SW6, the
 designated switch of the multi-access network link between switches
 SW1 and switches SW4, SW5, and SW6.
 The network link advertisement generated by switch SW6 would contain
 the following data items:

Kane Informational [Page 55] RFC 2642 Cabletron's VLS Protocol Specification August 1999

    ; network link advertisement for switch SW6
    LS age = 0               ; always true on origination
    Options = (T-bit|E-bit)  ; options
    LS type = 2              ; this is a network link advert
                               ; SW6's switch ID
    Link State ID = 00-00-1d-73-84-2e-00-00-00-00
    Advertising switch = 00-00-1d-73-84-2e-00-00-00-00
       Attached switch = 00-00-1d-7e-84-2e-00-00-00-00
       Attached switch = 00-00-1d-4a-26-b3-00-00-00-00
       Attached switch = 00-00-1d-1f-05-81-00-00-00-00
       Attached switch = 00-00-1d-4a-27-1c-00-00-00-00
    (See Section 11.3 for a detailed description of the format of a
    network link advertisement.)

8.2 Distributing Link State Advertisements

 Link state advertisements are distributed throughout the switch
 fabric encapsulated within Link State Update packets.  A single Link
 State Update packet may contain several distinct advertisements.
 To make the distribution process reliable, each advertisement must be
 explicitly acknowledged in a Link State Acknowledgment packet.  Note,
 however, that multiple acknowledgments can be grouped together into a
 single Link State Acknowledgment packet. A sending switch retransmits
 unacknowledged Link State Update packets at regular intervals until
 they are acknowledged.
 The remainder of this section is structured as follows:
 o  Section 8.2.1 presents an overview of the distribution process.
 o  Section 8.2.2 describes how an incoming Link State Update packet
    is processed.
 o  Section 8.2.3 describes how a Link State Packet is forwarded --
    both by the originating switch and an intermediate receiving
    switch.
 o  Section 8.2.4 describes how advertisements are installed into the
    local database.
 o  Section 8.2.5 describes the retransmission of unacknowledged
    advertisements.

Kane Informational [Page 56] RFC 2642 Cabletron's VLS Protocol Specification August 1999

  o  Section 8.2.6 describes how advertisements are acknowledged.

8.2.1 Overview

 The philosophy behind the distribution of link state advertisements
 is based on the concept of adjacencies -- that is, each switch is
 only required to remain synchronized with its adjacent neighbors.
 When a switch originates a new instance of a link state
 advertisement, it formats the advertisement into a Link State Update
 packet and floods the packet out each fully operational interface --
 that is, each interface with a state greater than Waiting.  However,
 only those neighbors that are adjacent to the sending switch need to
 process the packet.
 The sending switch indicates which of its neighbor switches should
 process the advertisement by specifying a particular multicast
 destination in the network layer address information (see Section
 10.3).  The sending switch sets the value of the network layer
 destination switch ID field according to the state of the interface
 over which the packet is sent:
 o  If the interface state is Point-to-Point, DS, or Backup, the
    switch is adjacent to all other switches on the link and all
    neighboring switches must process the packet.  Therefore, the
    destination field is set to the multicast switch ID
    AllSPFSwitches.
 o  If the interface state is DS Other, the switch is only adjacent to
    the designated switch and the backup designated switch and only
    those two neighboring switches must process the packet.
    Therefore, the destination field is set to the multicast switch ID
    AllDSwitches.
 A similar logic is used when a switch receives a Link State Update
 packet containing a new instance of a link state advertisement.
 After processing and acknowledging the packet, the receiving switch
 forwards the Link State Update packet as
 o  On the interface over which the original Link State Update packet
    was received:

Kane Informational [Page 57] RFC 2642 Cabletron's VLS Protocol Specification August 1999

    o  If the receiving switch is the designated switch for the
       attached network link, the packet is forwarded to all other
       switches on the link.  (The destination field is set to
       AllSPFSwitches.)  The originating switch will recognize that it
       was the advertisement originator and discard the packet.
    o  If the receiving switch is not the designated switch for the
       attached network link, the packet is not sent back out the
       interface over which it was received.
 o  On all other interfaces:
    o  If the receiving switch is the designated switch for the
       attached network link, the packet is forwarded to all switches
       on the link.  (The destination field is set to AllSPFSwitches.)
    o  If the receiving switch is neither the designated switch or the
       backup designated switch for the attached network link, the
       packet is forwarded only to the designated switch and the
       backup designated switch.  (The destination field is set to
       AllDSwitches.)
 Each Link State Update packet is forwarded and processed in this
 fashion until all switches in the fabric have received notification
 of the new instance of the link state advertisement.

8.2.2 Processing an Incoming Link State Update Packet

 When the a Link State Update packet is received, it is first
 subjected to a number of consistency checks.  In particular, the Link
 State Update packet is associated with a specific neighbor. If the
 state of that neighbor is less than Exchange, the entire Link State
 Update packet is discarded.
 Each link state advertisement contained in the packet is processed as
 follows:
 1. Validate the advertisement's link state checksum and type. If the
    checksum is invalid or the type is unknown, discard the
    advertisement without acknowledging it.
 2. If the advertisement's age is equal to MaxAge and there is
    currently no instance of the advertisement in the local link state
    database, then do the following:

Kane Informational [Page 58] RFC 2642 Cabletron's VLS Protocol Specification August 1999

    a) Acknowledge the advertisement by sending a Link State
       Acknowledgment packet to the sending neighbor (see Section
       8.2.6).
    b) Purge all outstanding requests for equal or previous instances
       of the advertisement from the sending neighbor's Link State
       Request list.
    c) If the neighbor is Exchange or Loading, install the
       advertisement in the link state database (see Section 8.2.4).
       Otherwise, discard the advertisement.
 3. If the advertisement's age is equal to MaxAge and there is an
    instance of the advertisement in the local link state database,
    then do the following:
    a) If the advertisement is listed in the link state retransmission
       list of any neighbor, remove the advertisement from the
       retransmission list(s) and delete the database copy of the
       advertisement.
    b) Discard the received (MaxAge) advertisement without
       acknowledging it.
 4. If the advertisement's age is less than MaxAge, attempt to locate
    an instance of the advertisement in the local link state database.
    If there is no database copy of this advertisement, or the
    received advertisement is more recent than the database copy (see
    Section 7.1.1), do the following:
    a) If there is already a database copy, and if the database copy
       was installed less than MinLSInterval seconds ago, discard the
       new advertisement without acknowledging it.
    b) Otherwise, forward the new advertisement out some subset of the
       local interfaces (see Section 8.2.3).  Note whether the
       advertisement was sent back out the receiving interface for
       later use by the acknowledgment process.
    c) Remove the current database copy from the Link state
       retransmission lists of all neighbors.
    d) Install the new advertisement in the link state database,
       replacing the current database copy.  (Note that this may cause
       the calculation of the set of best paths to be scheduled.  See
       Section 9.)  Timestamp the new advertisement with the time that
       it was received to prevent installation of another instance
       within MinLSInterval seconds.

Kane Informational [Page 59] RFC 2642 Cabletron's VLS Protocol Specification August 1999

    e) Acknowledge the advertisement, if necessary, by sending a Link
       State Acknowledgment packet back out the receiving interface.
       (See Section 8.2.6.)
    f) If the link state advertisement was initially advertised by the
       local switch itself, advance the advertisement sequence number
       and issue a new instance of the advertisement. (Receipt of a
       newer instance of an advertisement means that the local copy of
       the advertisement is left over from before the last time the
       switch was restarted.)
 5. If the received advertisement is the same instance as the database
    copy (as determined by the algorithm described in Section 7.1.1),
    do the following:
    a) If the advertisement is listed in the neighbor's link state
       retransmission list, the local switch is expecting an
       acknowledgment for this advertisement.  Treat the received
       advertisement as an implied acknowledgment, and remove the
       advertisement from the link state retransmission list. Note
       this implied acknowledgment for later use by the acknowledgment
       process (Section 8.2.6).
    b) Acknowledge the advertisement, if necessary, by sending a Link
       State Acknowledgment packet back out the receiving interface.
       (See Section 8.2.6.)
 If the database copy of the advertisement is more recent than the
    instance just received, do the following:
    a) Determine whether the instance is listed in the neighbor link
       state request list.  If so, an error has occurred in the
       database exchange process.  Restart the database exchange
       process by generating a neighbor BadLSReq event for the sending
       neighbor and terminate processing of the Link State Update
       packet.
    b) Otherwise, generate an unusual event to network management and
       discard the advertisement.

8.2.3 Forwarding Link State Advertisements

 When a new instance of an advertisement is originated or after an
 incoming advertisement has been processed, the switch must decide
 over which interfaces and to which neighbors the advertisement will
 be forwarded.  In some instances, the switch may decide not to
 forward the advertisement over a particular interface because it is
 able to determine that the neighbors on that attached link have or

Kane Informational [Page 60] RFC 2642 Cabletron's VLS Protocol Specification August 1999

 will receive the advertisement from another switch on the link.
 The decision of whether to forward an advertisement over each of the
 switch's interfaces is made as follows:
 1. Each neighboring switch attached to the interface is examined to
    determine whether it should receive and process the new
    advertisement.  For each neighbor, the following steps are
    executed:
    a) If the neighbor state is less than Exchange, the neighbor need
       not receive or process the new advertisement.
    b) If the neighbor state is Exchange or Loading, examine the link
       state request list associated with the neighbor.  If an
       instance of the new advertisement is on the list, the
       neighboring switch already has an instance of the
       advertisement.  Compare the new advertisement to the neighbor's
       copy:
       o  If the new advertisement is less recent, the neighbor need
          not receive or process the new advertisement.
       o  If the two copies are the same instance, delete the
          advertisement from the link state request list.  The
          neighbor need not receive or process the new advertisement
          [7].
       o  Otherwise, the new advertisement is more recent.  Delete the
          advertisement from the link state request list.  The
          neighbor may need to receive and process the new
          advertisement.
    c) If the new advertisement was received from this neighbor, the
       neighbor need not receive or process the advertisement.
    d) Add the new advertisement to the link state retransmission list
       for the neighbor.
 2. The switch must now decide whether to forward the new
    advertisement out the interface.
    a) If the link state advertisement was not added to any of the
       link state retransmission lists for neighbors attached to the
       interface, there is no need to forward the advertisement out
       the interface.

Kane Informational [Page 61] RFC 2642 Cabletron's VLS Protocol Specification August 1999

    b) If the new advertisement was received on this interface, and it
       was received from either the designated switch or the backup
       designated switch, there is no need to forward the
       advertisement out the interface.  Chances are all neighbors on
       the attached network link have also received the advertisement
       already.
    c) If the new advertisement was received on this interface and the
       state of the interface is Point-to-Point, there is no need to
       forward the advertisement since the received advertisement was
       originated by the neighbor switch.
    d) If the new advertisement was received on this interface, and
       the interface state is Backup -- that is, the switch itself is
       the backup designated switch -- there is no need to forward the
       advertisement out the interface.  The designated switch will
       distribute advertisements on the attached network link.
    e) Otherwise, the advertisement must be forwarded out the
       interface.
    To forward a link state advertisement, the switch first increments
    the advertisement's age by InfTransDelay seconds to account for
    the transmission time over the link.  The switch then copies the
    advertisement into a Link State Update packet
    Forwarded advertisements are sent to all adjacent switches
    associated with the interface.  If the interface state is Point-
    to-Point, DS, or Backup, the destination switch ID field of the
    network layer address information is set to the multicast switch
    ID AllSPFSwitches.  If the interface state is DS Other, the
    destination switch ID field is set to the multicast switch ID
    AllDSwitches.

8.2.4 Installing Link State Advertisements in the Database

 When a new link state advertisement is installed into the link state
 database, as the result of either originating or receiving a new
 instance of an advertisement, the switch must determine whether the
 best paths need to be recalculated.  To make this determination, do
 the following:
 1. Compare the contents of the new instance with the contents of the
    old instance (assuming the older instance is available). Note that
    this comparison does not include any data from the link state
    header.  Differences in fields within the header (such as the
    sequence number and checksum, which are guaranteed to be different
    in different instances of an advertisement) are of no consequence

Kane Informational [Page 62] RFC 2642 Cabletron's VLS Protocol Specification August 1999

    when deciding whether or not to recalculate the set of best paths.
 2. If there are no differences in the contents of the two
    advertisement instances, there is no need to recalculate the set
    of best paths.
 3. Otherwise, the set of best paths must be recalculated.
 Note also that the older instance of the advertisement must be
 removed from the link state database when the new advertisement is
 installed.  The older instance must also be removed from the link
 state retransmission lists of all neighbors.

8.2.5 Retransmitting Link State Advertisements

 When a switch sends a link state advertisement to an adjacent
 neighbor, it records the advertisement in the neighbor's link state
 retransmission list.  To ensure the reliability of the distribution
 process, the switch continues to periodically retransmit the
 advertisements specified in the list until they are acknowledged.
 The interval timer used to trigger retransmission of the
 advertisements is set to RxmtInterval seconds, as found in the
 interface data structure. Note that if this value is too low,
 needless retransmissions will ensue.  If the value is too high, the
 speed with which the databases synchronize across adjacencies may be
 affected if there are lost packets.
 When the interval timer expires, entries in the retransmission list
 are formatted into one or more Link State Update packets. (Remember
 that multiple advertisements can fit into a single Link State Update
 packet.)  The age field of each advertisement is incremented by
 InfTransDelay, as found in the interface data structure, before the
 advertisement is copied into the outgoing packet.
 Link State Update packets containing retransmitted advertisements are
 always sent directly to the adjacent switch. That is, the destination
 field of the network layer addressing information is set to the
 switch ID of the neighboring switch.
 If the adjacent switch goes down, retransmissions will continue until
 the switch failure is detected and the adjacency is torn down by the
 VLSP discovery process.  When the adjacency is torn down, the link
 state retransmission list is cleared.

Kane Informational [Page 63] RFC 2642 Cabletron's VLS Protocol Specification August 1999

8.2.6 Acknowledging Link State Advertisements

 Each link state advertisement received by a switch must be
 acknowledged.  In most cases, this is done by sending a Link State
 Acknowledgment packet.  However, acknowledgments can also be done
 implicitly by sending Link State Update packets (see step 4a of
 Section 8.2.2).
 Multiple acknowledgments can be grouped together into a single Link
 State Acknowledgment packet.
 Sending an acknowledgment
    Link State Acknowledgment packets are sent back out the interface
    over which the advertisement was received.  The packet can be sent
    immediately to the sending neighbor, or it can be delayed and sent
    when an interval timer expires.
    o  Sending delayed acknowledgments facilitates the formatting of
       multiple acknowledgments into a single packet.  This enables a
       single packet to send acknowledgments to several neighbors at
       once by using a multicast switch ID in the destination field of
       the network layer addressing information (see below).  Delaying
       acknowledgments also randomizes the acknowledgment packets sent
       by the multiple switches attached to a multi-access network
       link.
       Note that the interval used to time delayed acknowledgments
       must be short (less than RxmtInterval) or needless
       retransmissions will ensue.
       Delayed acknowledgments are sent to all adjacent switches
       associated with the interface.  If the interface state is
       Point-to-Point, DS, or Backup, the destination field of the
       network layer addressing information is set to the multicast
       switch ID AllSPFSwitches.  If the interface state is DS Other,
       the destination field is set to the multicast switch ID
       AllDSwitches.
    o  Immediate acknowledgments are sent directly to a specific
       neighbor in response to the receipt of duplicate link state
       advertisements.  These acknowledgments are sent immediately
       when the duplicate is received.
    The method used to send a Link State Acknowledgment packet --
    either delayed or immediate -- depends on the circumstances
    surrounding the receipt of the advertisement, as shown in Table 6.
    Note that switches with an interface state of Backup send

Kane Informational [Page 64] RFC 2642 Cabletron's VLS Protocol Specification August 1999

    acknowledgments differently than other switches because they play
    a slightly different role in the distribution process (see Section
    8.2.3).
                                Action taken in state
    Circumstances           Backup               Other states
    Advertisement was       No ack sent          No ack sent
    forwarded back out
    receiving interface
    Advertisement is        Delayed ack sent     Delayed ack
    more recent than        if advertisement     sent
    database copy, but      received from DS,
    was not forwarded       else do nothing
    back out receiving
    interface
    Advertisement was a     Delayed ack sent     No ack sent
    duplicate treated       if advertisement
    as an implied acknow-   received from DS,
    ledgment (step 4a of    else do nothing
    Section 8.2.2)
    Advertisement was a     Immediate ack        Immediate ack
    duplicate not treated   sent                 sent
    as an implied acknow-
    ledgment
    Advertisement age       Immediate ack        Immediate ack
    equal to MaxAge and     sent                 sent
    no current instance
    found in database
             Table 6: Sending Link State Acknowledgments
 Receiving an acknowledgment
    When the a Link State Acknowledgment packet is received, it is
    first subjected to a number of consistency checks.  In particular,
    the packet is associated with a specific neighbor. If the state of
    that neighbor is less than Exchange, the entire Link State
    Acknowledgment packet is discarded.
    Each acknowledgment contained in the packet is processed as
    follows:

Kane Informational [Page 65] RFC 2642 Cabletron's VLS Protocol Specification August 1999

    o  If the advertisement being acknowledged has an instance in the
       link state retransmission list for the sending neighbor, do the
       following:
       o  If the acknowledgment is for the same instance as that
          specified in the list (as determined by the procedure
          described in Section 7.1.1), remove the instance from the
          retransmission list.
       o  Otherwise, log the acknowledgment as questionable.

8.3 Aging the Link State Database

 Each link state advertisement has an age field, containing the
 advertisement's age, expressed in seconds.  When the advertisement is
 copied into a Link State Update packet for forwarding out a
 particular interface, the age is incremented by InfTransDelay seconds
 to account for the transmission time over the link.  An
 advertisement's age is never incremented past the value MaxAge.
 Advertisements with an age of MaxAge are not used to calculate best
 paths.
 If a link state advertisement's age reaches MaxAge, the switch
 flushes the advertisement from the switch fabric by doing the
 following:
 o  Originate a new instance of the advertisement with the age field
    set to MaxAge.  The distribution process will eventually result in
    the advertisement being removed from the retransmission lists of
    all switches in the fabric.
 o  Once the advertisement is no longer contained in the link state
    retransmission list of any neighbor and no neighbor is in a state
    of Exchange or Loading, remove  the advertisement from the local
    link state database.

8.3.1 Premature Aging of Advertisements

 A link state advertisement can be prematurely flushed from the switch
 fabric by forcing its age to MaxAge and redistributing the
 advertisement.
 A switch that was previously the designated switch for a multi-access
 network link but has lost that status due to a failover to the backup
 designated switch prematurely ages the network link advertisements it
 originated for the link.

Kane Informational [Page 66] RFC 2642 Cabletron's VLS Protocol Specification August 1999

 Premature aging also occurs when an advertisement's sequence number
 must wrap -- that is, when the current advertisement instance has a
 sequence number of 0x7fffffff.  In this circumstance, the
 advertisement is prematurely aged so that the next instance of the
 advertisement can be originated with a sequence number of 0x80000001
 and be recognized as the most recent instance.
 A switch may only prematurely age those link state advertisements for
 which it is the advertising switch.

9. Calculating the Best Paths

 Once an adjacency has been formed and the two switches have
 synchronized their databases, each switch in the adjacency calculates
 the best path(s) to all other switches in the fabric, using itself as
 the root of each path.  In this context, "best" path means that path
 with the lowest total cost metric across all hops.  If there are
 multiple paths with the same (lowest) total cost metric, they are all
 calculated.  Best paths are stored in the area data structure.
 Paths are calculated using the well-known Dijkstra algorithm. For a
 detailed description of this algorithm, the reader is referred to
 [Perlman], or any of a number of standard textbooks dealing with
 network routing.
 Note that whenever there is a change in an adjacency relationship, or
 any change that alters the topology of the switch fabric, the set of
 best paths must be recalculated.

10. Protocol Packets

 This section describes VLS protocol packets and link state
 advertisements.

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 There are five distinct VLSP packet types, as listed in Table 7.
   Type  Packet Name       Function              Description
   1     Hello             Select DS/Backup DS   Section 10.6.1
   2     Database          Summarize database
           Description     contents              Section 10.6.2
   3     LS Request        Database download     Section 10.6.3
   4     LS Update         Database update       Section 10.6.4
   5     LS Ack            Flooding acknow-
                           ledgment              Section 10.6.5
                    Table 7: VLSP Packet Types
 All VLSP packets are encapsulated within a standard ISMP packet, with
 the VLS packet carried in the ISMP message body.  The ISMP packet is
 described in Section 10.1.
 Since it is important that the link state databases remain
 synchronized throughout the switch fabric, processing of both
 incoming and outgoing routing protocol packets should take priority
 over ordinary data packets.  Section 10.2 describes packet
 processing.
 All VLSP packets begin with network layer addressing information,
 described in Section 10.3, followed by a standard header, described
 in Section 10.4.
 With the exception of Hello packets, all VLSP packets deal with lists
 of link state advertisements.  The format of a link state
 advertisement is described in Section 11.

10.1 ISMP Packet Format

 All VLSP packets are encapsulated within a standard ISMP packet. ISMP
 packets are of variable length and have the following general
 structure:
 o  Frame header
 o  ISMP packet header
 o  ISMP message body

Kane Informational [Page 68] RFC 2642 Cabletron's VLS Protocol Specification August 1999

10.1.1 Frame Header

 ISMP packets are encapsulated within an IEEE 802-compliant frame
 using a standard header as shown below:
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 00 |                                                               |
    +      Destination address      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 04 |                               |                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+        Source address         +
 08 |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 12 |             Type              |                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
 16 |                                                               |
    +                                                               +
    :                                                               :
 Destination address
    This 6-octet field contains the Media Access Control (MAC) address
    of the multicast channel over which all switches in the fabric
    receive ISMP packets.  The destination address of all ISMP packets
    contain a value of 01-00-1D-00-00-00.
 Source address
    This 6-octet field contains the physical (MAC) address of the
    switch originating the ISMP packet.
 Type
    This 2-octet field identifies the type of data carried within the
    frame.  The type field of ISMP packets contains the value 0x81FD.

Kane Informational [Page 69] RFC 2642 Cabletron's VLS Protocol Specification August 1999

10.1.2 ISMP Packet Header

 The ISMP packet header consists of 6 octets, as shown below:
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 00 |///////////////////////////////////////////////////////////////|
    ://////// Frame header /////////////////////////////////////////:
    +//////// (14 octets)  /////////+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 12 |///////////////////////////////|            Version            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 16 |       ISMP message type       |        Sequence number        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 20 |                                                               |
    +                                                               +
    :                                                               :
 Frame header
    This 14-octet field contains the frame header.
    Version
    This 2-octet field contains the version number of the InterSwitch
    Message Protocol to which this ISMP packet adheres.  This document
    describes ISMP Version 2.0.           ISMP message type
    This 2-octet field contains a value indicating which type of ISMP
    message is contained within the message body.  Valid values are as
    follows:
       1    (reserved)
       2    Interswitch Keepalive messages
       3    Interswitch Link State messages
       4    Interswitch Spanning Tree BPDU messages and
            Interswitch Remote Blocking messages
       5    Interswitch Resolve and New User messages
       6    (reserved)
       7    Tag-Based Flood messages
       8    Interswitch Tap messages
    All VLS protocol messages have an ISMP message type of 3.

Kane Informational [Page 70] RFC 2642 Cabletron's VLS Protocol Specification August 1999

 Sequence number
    This 2-octet field contains an internally generated sequence
    number used by the various protocol handlers for internal
    synchronization of messages.

10.1.3 ISMP Message Body

 The ISMP message body is a variable-length field containing the
 actual data of the ISMP message.  The length and content of this
 field are determined by the value found in the message type field.
 VLSP packets are contained in the ISMP message body.

10.2 VLSP Packet Processing

 Note that with the exception of Hello packets, VLSP packets are sent
 only between adjacent neighbors.  Therefore, all packets travel a
 single hop.
 VLSP does not support fragmentation and reassembly of packets.
 Therefore, packets containing lists of link state advertisements or
 advertisement headers must be formatted such that they contain only
 as many advertisements or headers as will fit within the size
 constraints of a standard ethernet frame.
 When a protocol packet is received by a switch, it must first pass
 the following criteria before being accepted for further processing:
 o  The checksum number must be correct.
 o  The destination switch ID (as found in the network layer address
    information) must be the switch ID of the receiving switch, or one
    of the multicast switch IDs AllSPFSwitches or AllDSwitches.
    If the destination switch ID is the multicast switch ID
    AllDSwitches, the state of the receiving interface must be Point-
    to-Point, DS, or Backup.
 o  The source switch ID (as found in the network layer address
    information) must not be that of the receiving switch.  (That is,
    locally originated packets should be discarded.)
 At this point, if the packet is a Hello packet, it is accepted for
 further processing.

Kane Informational [Page 71] RFC 2642 Cabletron's VLS Protocol Specification August 1999

 Since all other packet types are only sent between adjacent
 neighbors, the packet must have been sent by one of the switch's
 active neighbors.  If the source switch ID matches the switch ID of
 one of the receiving switch's active neighbors (as stored in the
 interface data structure associated with the inport interface), the
 packet is accepted for further processing.  Otherwise, the packet is
 discarded.

10.3 Network Layer Address Information

 As mentioned in Section 2.2.1, portions of the VLS protocol (as
 derived from OSPF) are dependent on certain network layer addresses
 -- in particular, the AllSPFSwitches and AllDSwitches multicast
 addresses that drive the distribution of link state advertisements
 throughout the switch fabric.  In order to facilitate the
 implementation of the protocol at the physical MAC layer, network
 layer address information is encapsulated in the VSLP packets.  This
 information immediately follows the ISMP frame and packet header and
 immediately precedes the VLSP packet header, as shown below:
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    :                  frame header / ISMP header                   :
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 00 |                                                               |
    :                      Unused (20 octets)                       :
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 20 |                                                               |
    +                       Source switch ID                        +
 24 |                                                               |
    +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 28 |                               |                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
 32 |                                                               |
    +                     Destination switch ID                     +
 36 |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 40 |                                                               |
    :                          VLSP header                          :
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Kane Informational [Page 72] RFC 2642 Cabletron's VLS Protocol Specification August 1999

 Source switch ID
    This 10-octet field contains the switch ID of the sending switch.
 Destination switch ID
    This 10-octet field contains the switch ID of the packet
    destination.  The value here is set as follows:
    o  Hello packets are addressed to the multicast switch ID
       AllSPFSwitches.
    o  The designated switch and the backup designated switch address
       initial Link State Update packets and Link State Acknowledgment
       packets to the multicast switch ID AllSPFSwitches.
    o  All other switches address initial Link State Update packets
       and Link State Acknowledgment packets to the multicast switch
       ID AllDSwitches.
    o  Retransmissions of Link State Update packets are always
       addressed directly to the nonresponding switch.
    o  Database Description packets and Link State Request are always
       addressed directly to the other switch participating in the
       database exchange process.
 VLSP header
    This 30-octet field contains the VLSP standard header.  See
    Section 10.4.

10.4 VLSP Packet Header

 Every VLSP packet starts with a common 30-octet header.  This header,
 along with the data found in the network layer address information,
 contains all the data necessary to determine whether the packet
 should be accepted for further processing. (See Section 10.1.)
 The format of the VLSP header is shown below.  Note that the header
 starts at offset 36 of the ISMP message body, following the network
 layer address information.

Kane Informational [Page 73] RFC 2642 Cabletron's VLS Protocol Specification August 1999

      0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    :                  frame header / ISMP header                   :
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 00 |                                                               |
    :               Network layer address information               :
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 40 |    (unused)   |     Type      |         Packet length         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 44 |                                                               |
    +                       Source switch ID                        +
 48 |                                                               |
    +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 52 |                               |         Area ID . . .         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 56 |         Area ID . . .         |           Checksum            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 60 |            Autype             |                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+        Authentication         +
 64 |                                                               |
    +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 68 |                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Type
    This 1-octet field contains the packet type.  Possible values are
    as follows:
       1   Hello
       2   Database Description
       3   Link State Request
       4   Link State Update
       5   Link State Acknowledgment
 Packet length
    This 2-octet field contains the length of the protocol packet, in
    bytes, calculated from the start of the VLSP header, at offset 20
    of the ISMP message body.  If the packet length is not an integral
    number of 16-bit words, the packet is padded with an octet of zero
    (see the description of the checksum field, below).

Kane Informational [Page 74] RFC 2642 Cabletron's VLS Protocol Specification August 1999

 Switch ID
    This 10-octet field contains the switch ID of the sending switch.
 Area ID
    This 4-octet field contains the area identifier.  Since VLSP does
    not support multiple areas, the value here is always zero.
 Checksum
    This 2-octet field contains the packet checksum value.  The
    checksum is calculated as the 16-bit one's complement of the one's
    complement sum of all the 16-bit words in the packet, beginning
    with the VLSP header, excluding the authentication field.  If the
    packet length is not an integral number of 16-bit words, the
    packet is padded with an octet of zero before calculating the
    checksum.
 AuType
    This 2-octet field identifies the authentication scheme to be used
    for the packet.  Since authentication is not supported by this
    version of VLSP, this field contains zero.
 Authentication
    This 8-octet field is reserved for use by the authentication
    scheme.  Since authentication is not supported by this version of
    VLSP, this field contains zeroes.

10.5 Options Field

 Hello packets and Database Description packets, as well as link state
 advertisements, contain a 1-octet options field.  Using this field, a
 switch can communicate its optional capabilities to other VLSP
 switches.  The receiving switch can then choose whether or not to
 support those optional capabilities.  Thus, switches of differing
 capabilities potentially can be mixed within a single VLSP routing
 domain.
 Two optional capabilities are currently defined in the options field:
 routing based on Type of Service (TOS) and support for external
 routing beyond the local switch fabric.  These two capabilities are
 specified in the options field as shown below.

Kane Informational [Page 75] RFC 2642 Cabletron's VLS Protocol Specification August 1999

                           +-+-+-+-+-+-+-+-+
                           |0|0|0|0|0|0|E|T|
                           +-+-+-+-+-+-+-+-+
                           The options field
 T-bit
    The T-bit specifies the switch's Type of Service (TOS) capability.
    If the T-bit is set, the switch supports routing based on nonzero
    types of service.
 E-bit
    The E-bit specifies the switch's external routing capability. If
    the E-bit is set, the switch supports external routing.
 Note:  The current version of VLSP supports neither of these
 capabilities.  Therefore, both the T-bit and the E-bit are clear and
 the options field contains a value of zero.

10.6 Packet Formats

 This section contains detailed descriptions of the five VLS protocol
 packets.

10.6.1 Hello Packets

 Hello packets are sent periodically over multi-access switch
 interfaces in order to discover and maintain neighbor relationships.
 Note:  Hello packets are not sent over point-to-point network links.
 For point-to-point links, the VLS protocol relies on the VlanHello
 protocol [IDhello] to notify it of neighboring switches.
 Since all switches connected to a common network link must agree on
 certain interface parameters, these parameters are included in each
 Hello packet.  A switch receiving a Hello packet that contains
 parameters inconsistent with its own view of the interface will not
 establish a neighbor relationship with the sending switch.
 The format of a Hello packet is shown below.

Kane Informational [Page 76] RFC 2642 Cabletron's VLS Protocol Specification August 1999

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  00 |                                                               |
     :              Network layer addressing / VLSP header           :
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  70 |                      (unused -- must be 0)                    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  74 |         HelloInt              |    Options    |   Priority    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  78 |                            DeadInt                            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  82 |                                                               |
     +                      Designated switch ID                     +
  86 |                                                               |
     +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
  90 |                               |                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
  94 |                                                               |
     +                   Backup designated switch ID                 +
  98 |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 102 |                                                               |
     +                                                               +
     :                          Neighbor list                        :
     +                                                               +
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Network layer addressing / VLSP header
    This 70-octet field contains the network layer addressing
    information and the standard VLS protocol packet header.  The
    packet header type field contains a value of 1.
 HelloInt
    This 2-octet field contains the interval, in seconds, at which
    this switch sends Hello packets.
 Options
    This 1-octet field contains the optional capabilities supported by
    the switch, as described in Section 10.5.

Kane Informational [Page 77] RFC 2642 Cabletron's VLS Protocol Specification August 1999

 Priority
    This 1-octet field contains the switch priority used in selecting
    the designated switch and backup designated switch (see Section
    6.3.1).  If the value here is zero, the switch is ineligible to
    become the designated switch or the backup designated switch.
 DeadInt
    This 4-octet field contains the length of time, in seconds, that
    neighboring switches will wait before declaring the interface down
    once they stop receiving Hello packets over the interface.  The
    value here is equal to the value of SwitchDeadInterval, as found
    in the interface data structure.
 Designated switch
    This 10-octet field contains the switch ID of the designated
    switch for this network link, as currently understood by the
    sending switch.  This value is set to zero if the designated
    switch selection process has not yet begun.
 Backup designated switch
    This 10-octet field contains the switch ID of the backup
    designated switch for the network link, as currently understood by
    the sending switch.  This value is set to zero if the backup
    designated switch selection process has not yet begun.
 Neighbor list
    This variable-length field contains a list of switch IDs of each
    switch from which the sending switch has received a valid Hello
    packet within the last SwitchDeadInterval seconds.

10.6.2 Database Description Packets

 Database Description packets are exchanged while an adjacency is
 being formed between two neighboring switches and are used to
 describe the contents of the topological database.  For a complete
 description of the database exchange process, see Section 7.2.
 The format of a Database Description packet is shown below.

Kane Informational [Page 78] RFC 2642 Cabletron's VLS Protocol Specification August 1999

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 00 |                                                               |
    :              Network layer addressing / VLSP header           :
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 70 |     (unused -- must be 0)     |    Options    |     Flags     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 74 |                        Sequence number                        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 78 |                                                               |
    +                                                               +
    :                 Link state advertisement headers              :
    +                                                               +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Network layer addressing / VLSP header
    This 70-octet field contains the network layer addressing
    information and the standard VLS protocol packet header.  The
    packet header type field contains a value of 2.
 Options
    This 1-octet field contains the optional capabilities supported by
    the switch, as described in Section 10.5.
 Flags
    This 1-octet field contains a set of bit flags that are used to
    coordinate the database exchange process.  The format of this
    octet is as follows:
                        +-+-+-+-+-+-+-+-+
                        |0|0|0|0|0|I|M|MS
                        +-+-+-+-+-+-+-+-+
 I-bit (Init)
    The I-bit is used to signal the start of the exchange.  It is set
    while the two switches negotiate the master/slave relationship and
    the starting sequence number.

Kane Informational [Page 79] RFC 2642 Cabletron's VLS Protocol Specification August 1999

 M-bit (More)
    The M-bit is set to indicate that more Database Description
    packets to follow.
 MS-bit (Master/Slave)
    The MS-bit is used to indicate which switch is the master of the
    exchange.  If the bit is set, the sending switch is the master
    during the database exchange process.  If the bit is clear, the
    switch is the slave.
 Sequence number
    This 4-octet field is used to sequence the Database Description
    packets during the database exchange process.  The two switches
    involved in the exchange process agree on the initial value of the
    sequence number during the master/slave negotiation.  The number
    is then incremented for each Database Description packet in the
    exchange.
    To acknowledge each Database Description packet sent by the
    master, the slave sends a Database Description packet that echoes
    the sequence number of the packet being acknowledged.
 Link state advertisement headers
    This variable-length field contains a list of link state headers
    that describe a portion of the master's topological database.
    Each header uniquely identifies a link state advertisement and its
    current instance.  (See Section 11.1 for a detailed description of
    a link state advertisement header.)  The number of headers
    included in the list is calculated implicitly from the length of
    the packet, as stored in the VLSP packet header (see Section
    10.4).

10.6.3 Link State Request Packets

 Link State Request packets are used to request those pieces of the
 neighbor's database that the sending switch has discovered (during
 the database exchange process) are more up-to-date than instances in
 its own database.  Link State Request packets are sent as the last
 step in bringing up an adjacency.  (See Section 7.3.)
 The format of a Link State Request packet is shown below.

Kane Informational [Page 80] RFC 2642 Cabletron's VLS Protocol Specification August 1999

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 00 |                                                               |
    :              Network layer addressing / VLSP header           :
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 70 |                        Link state type                        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 74 |                                                               |
    +                         Link state ID                         +
 88 |                                                               |
    +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 82 |                               |                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
 86 |                                                               |
    +                      Advertising switch ID                    +
 90 |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 94 |                                                               |
    :                            . . .                              :
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Network layer addressing / VLSP header
    This 70-octet field contains the network layer addressing
    information and the standard VLS protocol packet header.  The
    packet header type field contains a value of 3.
 Link state type
    This 4-octet field contains the link state type of the requested
    link state advertisement, as stored in the advertisement header.
 Link state ID
    This 10-octet field contains the link state ID of the requested
    link state advertisement, as stored in the advertisement header.
 Advertising switch
    This 10-octet field contains the switch ID of advertising switch
    for the requested link state advertisement, as stored in the
    advertisement header.

Kane Informational [Page 81] RFC 2642 Cabletron's VLS Protocol Specification August 1999

    Note that the last three fields uniquely identify the
    advertisement, but not its instance.  The receiving switch will
    respond with its most recent instance of the specified
    advertisement.
    Multiple link state advertisements can be requested in a single
    Link State Request packet by repeating the link state type, ID,
    and advertising switch for each requested advertisement.  The
    number of advertisements requested is calculated implicitly from
    the length of the packet, as stored in the VLSP packet header.

10.6.4 Link State Update Packets

 Link State Update packets are used to respond to a Link State Request
 packet or to advertise a new instance of one or more link state
 advertisements.  Link State Update packets are acknowledged with Link
 State Acknowledgment packets.  For more information on the use of
 Link State Update packets, see Section 7 and Section 8.
 The format of a Link State Update packet is shown below.
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 00 |                                                               |
    :              Network layer addressing / VLSP header           :
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 70 |                        # advertisements                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 74 |                                                               |
    +                                                               +
    :                    Link state advertisements                  :
    +                                                               +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Network layer addressing / VLSP header
    This 70-octet field contains the network layer addressing
    information and the standard VLS protocol packet header.  The
    packet header type field contains a value of 4.
 # advertisements
    This 4-octet field contains the number of link state
    advertisements included in the packet.

Kane Informational [Page 82] RFC 2642 Cabletron's VLS Protocol Specification August 1999

 Link state advertisements
    This variable-length field contains a list of link state
    advertisements.  For a detailed description of the different types
    of link state advertisements, see Section 11.

10.6.5 Link State Acknowledgment Packets

 Link State Acknowledgment Packets are used to explicitly acknowledge
 one or more Link State Update packets, thereby making the
 distribution of link state advertisements reliable.  (See Section
 8.2.6.)
 The format of a Link State Acknowledgment packet is shown below.
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 00 |                                                               |
    :              Network layer addressing / VLSP header           :
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 70 |                                                               |
    +                                                               +
    :                 Link state advertisement headers              :
    +                                                               +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Network layer addressing / VLSP header
    This 70-octet field contains the network layer addressing
    information and the standard VLS protocol packet header.  The
    packet header type field contains a value of 5.
 Link state advertisement headers
    This variable-length field contains a list of link state headers
    that are being acknowledged by this packet.  Each header uniquely
    identifies a link state advertisement and its current instance.
    (See Section 11.1 for a detailed description of a link state
    advertisement header.)  The number of headers included in the list
    is calculated implicitly from the length of the packet, as stored
    in the VLSP packet header (see Section 10.4).

Kane Informational [Page 83] RFC 2642 Cabletron's VLS Protocol Specification August 1999

11. Link State Advertisement Formats

 Link state advertisements are used to describe various pieces of the
 routing topology within the switch fabric.  Each switch in the fabric
 maintains a complete set of all link state advertisements generated
 throughout the fabric.  (Section 8.1 describes the circumstances
 under which a link state advertisement is originated.  Section 8.2
 describes how advertisements are distributed throughout the switch
 fabric.) This collection of advertisements, known as the link state
 (or topological) database, is used to calculate a set of best paths
 to all other switches in the fabric.
 There are two types of link state advertisement, as listed in Table
 8.
      Type   Name            Function             Description
      1      Switch link     Lists all network    Section 11.2
             advertisement   linksattached to
                             a switch
      2      Network link    Lists all adjacen-   Section 11.3
             advertisement   cies on a network
                             link
              Table 8: Link State Advertisement Types
 Each link state advertisement begins with a standard header,
 described in Section 11.1.

11.1 Link State Advertisement Headers

 All link state advertisements begin with a common 32-octet header.
 This header contains information that uniquely identifies the
 advertisement -- its type, link state ID, and the switch ID of its
 advertising switch.  Also, since multiple instances of a link state
 advertisement can exist concurrently in the switch fabric, the header
 contains information that permits a switch to determine which
 instance is the most recent -- the age, sequence number and checksum.
 The format of the link state advertisement header is shown below.

Kane Informational [Page 84] RFC 2642 Cabletron's VLS Protocol Specification August 1999

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 00 |              Age              |    Options    |    LS Type    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 04 |                                                               |
    +                         Link state ID                         +
 08 |                                                               |
    +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 12 |                               |                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
 16 |                                                               |
    +                      Advertising switch ID                    +
 20 |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 24 |                         Sequence number                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 28 |           Checksum            |             Length            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Age
    This 2-octet field contains the time, in seconds, since this
    instance of the link state advertisement was originated.
 Options
    This 1-octet field contains the optional capabilities supported by
    the advertising switch, as described in Section 10.5.
 LS type
    This 1-octet field contains the type of the link state
    advertisement.  Possible values are:
       1   Switch link advertisement
       2   Network link advertisement
 Link state ID
    This 10-octet field identifies the switch that originates
    advertisements for the link.  The content of this field depends on
    the advertisement's type.
    o  For a switch link advertisement, this field contains the switch
       ID of the originating switch

Kane Informational [Page 85] RFC 2642 Cabletron's VLS Protocol Specification August 1999

       o  For a network link advertisement, this field contains the
       switch ID of the designated switch for the link
 Note:  In VLSP, the link state ID of an advertisement is always the
 same as the advertising switch.  This level of redundancy results
 from the fact that OSPF uses additional types of link state
 advertisements for which the originating switch is not the
 advertising switch.
 Advertising switch
    This 10-octet field contains the switch ID of the switch that
    originated the link state advertisement.
 Sequence number
    This 4-octet field is used to sequence the instances of a
    particular link state advertisement.  The number is incremented
    for each new instance.
 Checksum
    This 2-octet field contains the checksum of the complete contents
    of the link state advertisement, excluding the age field.  The
    checksum used is commonly referred to as the Fletcher checksum and
    is documented in [RFC905].  Note that since this checksum is
    calculated for each separate advertisement, a protocol packet
    containing lists of advertisements or advertisement headers will
    contain multiple checksum values.
 Length
    This 2-octet field contains the total length, in octets, of the
    link state advertisement, including the header.

11.2 Switch Link Advertisements

 A switch link advertisement is used to describe all functioning
 network links of a switch, including the cost of using each link.
 Each functioning switch in the fabric originates one, and only one,
 switch link advertisement -- all of the switch's links must be
 described in a single advertisement.  A switch originates its first
 switch link advertisement (containing no links) when it first becomes
 functional.  It then originates a new instance of the advertisement
 each time any of its neighbor states changes such that the contents
 of the advertisement changes.   See Section 8.1 for details on
 originating a switch link advertisement.

Kane Informational [Page 86] RFC 2642 Cabletron's VLS Protocol Specification August 1999

 The format of a switch link advertisement is shown below.
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 00 |                                                               |
    :                       Link state header                       :
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 32 |      (unused -- must be 0)    |            # links            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 36 |                                                               |
    +                            Link ID                            +
 40 |                                                               |
    +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 44 |                               |                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
 48 |                                                               |
    +                           Link data                           +
 52 |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 56 |   Link type   |     # TOS     |         TOS 0 metric          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 60 |                                                               |
    :                            . . .                              :
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Link state header
    This 32-octet field contains the standard link state advertisement
    header.  The type field contains a 1, and the link state ID field
    contains the switch ID of the advertising switch.
 # links
    This 2-octet field contains the number of links described by this
    advertisement.  This value must be equal to the total number of
    functioning network links attached to the switch.

Kane Informational [Page 87] RFC 2642 Cabletron's VLS Protocol Specification August 1999

 Link ID
    This 10-octet field identifies the other switch that originates
    link state advertisements for the link, providing a key for
    accessing other link state advertisements for the link.  The value
    here is based on the link type, as follows:
    o  For point-to-point links, this field contains the switch ID of
       the neighbor switch connected to the other end of the link.
    o  For multi-access links, this field contains the switch ID of
       the designated switch for the link.
 Link data
    This 10-octet field contains additional data necessary to
    calculate the set of best paths.  Typically, this field contains
    the interface ID of the link.
 Link type
    This 1-octet field contains the type of link being described.
    Possible values are as follows:
       1   Point-to-point link
       2   Multi-access link
 # TOS
    This 1-octet field contains the number of nonzero type of service
    metrics specified for the link.  Since the current version of VLSP
    does not support routing based on nonzero types of service, this
    field contains a value of zero.
 TOS 0 metric
    This 2-octet field contains the cost of using this link for the
    zero TOS.  This value is expressed in the link state metric and
    must be greater than zero.
 Note that the last five fields are repeated for all functioning
 network links attached to the advertising switch.  If the interface
 state of attached link changes, the switch must originate a new
 instance of the switch link advertisement.

Kane Informational [Page 88] RFC 2642 Cabletron's VLS Protocol Specification August 1999

11.3 Network Link Advertisements

 A network link advertisement is originated by the designated switch
 of each multi-access network link.  The advertisement describes all
 switches attached to the link that are currently fully adjacent to
 the designated switch, including the designated switch itself.  See
 Section 8.1 for details on originating a switch link advertisement.
 Network link advertisements are not generated for point-to-point
 network links.
 The format of a network link advertisement is show below.
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 00 |                                                               |
    :                       Link state header                       :
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 32 |                           (unused)                            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 36 |                                                               |
    +                                                               +
    :                          Switch list                          :
    +                                                               +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Link state header
    This 32-octet field contains the standard link state advertisement
    header.  The type field contains a 2, and the link state ID field
    contains the switch ID of the designated switch.
 Switch list
    The switch IDs of all switches attached to the network link that
    are currently fully adjacent to the designated switch. The
    designated switch includes itself in this list.

12. Protocol Parameters

 This section contains a compendium of the parameters used in the VLS
 protocol.

Kane Informational [Page 89] RFC 2642 Cabletron's VLS Protocol Specification August 1999

12.1 Architectural Constants

 Several VLS protocol parameters have fixed architectural values. The
 name of each architectural constant follows, together with its value
 and a short description of its function.
 AllSPFSwitches
    The multicast switch ID to which Hello packets and certain other
    protocol packets are addressed, as specified in the destination
    switch ID field of the network layer address information (see
    Section 10.3).  The value of AllSPFSwitches is E0-00-00-05-00-00-
    00-00.
 AllDSwitches
    The multicast switch ID to which Link State Update packets and
    Link State Acknowledgment packets are addressed, as specified in
    the destination switch ID field of the network layer address
    information (see Section 10.3), when they are destined for the
    designated switch or the backup designated switch of a network
    link.  The value of AllDSwitches is E0-00-00-06-00-00-00-00.
 LSRefreshTime
    The interval at which the set of best paths recalculated if no
    other state changes have forced a recalculation.  The value of
    LSRefreshTime is set to 1800 seconds (30 minutes).
 MinLSInterval
    The minimum time between distinct originations of any particular
    link state advertisement.  The value of MinLSInterval is set to 5
    seconds.
 MaxAge
    The maximum age that a link state advertisement can attain. When
    an advertisement's age reaches MaxAge, it is redistributed
    throughout the switch fabric.  When the originating switch
    receives an acknowledgment for the advertisement, indicating that
    the advertisement has been removed from all neighbor Link state
    retransmission lists, the advertisement is removed from the
    originating switch's database.  Advertisements having age MaxAge
    are not used to calculate the set of best paths.  The value of
    MaxAge must be greater than LSRefreshTime.  The value of MaxAge is
    set to 3600 seconds (1 hour).

Kane Informational [Page 90] RFC 2642 Cabletron's VLS Protocol Specification August 1999

 MaxAgeDiff
    The maximum time disparity in ages that can occur for a single
    link state instance as it is distributed throughout the switch
    fabric.  Most of this time is accounted for by the time the
    advertisement sits on switch output queues (and therefore not
    aging) during the distribution process. The value of MaxAgeDiff is
    set to 900 seconds (15 minutes).
 LSInfinity
    The link state metric value indicating that the destination is
    unreachable.  It is defined to be a binary value of all ones.

12.2 Configurable Parameters

 Many of the switch interface parameters used by VLSP may be made
 configurable if the implementer so desires.  These parameters are
 listed below.  Sample default values are given for some of the
 parameters.
 Note that some of these parameters specify properties of the
 individual interfaces and their attached network links.  These
 parameters must be consistent across all the switches attached to
 that link.
 Interface output cost(s)
    The cost of sending a packet over the interface, expressed in the
    link state metric.  This is advertised as the link cost for this
    interface in the switch's switch link advertisement. The interface
    output cost must always be greater than zero.
 RxmtInterval
    The number of seconds between link state advertisement
    retransmissions for adjacencies established on this interface.
    This value is also used when retransmitting Database Description
    packets and Link State Request packets. This value must be greater
    than the expected round-trip delay between any two switches on the
    attached link.  However, the value should be conservative or
    needless retransmissions will result.  A typical value for a local
    area network would be 5 seconds.

Kane Informational [Page 91] RFC 2642 Cabletron's VLS Protocol Specification August 1999

 InfTransDelay
    The estimated number of seconds it takes to transmit a Link State
    Update packet over this interface.  Link state advertisements
    contained in the Link State Update packet must have their age
    incremented by this amount before transmission.  This value must
    take into account the transmission and propagation delays for the
    interface and must be greater than zero.  A typical value for a
    local area network would be 1 second.
 Switch priority
    An 8-bit unsigned integer.  When two switches attached to the same
    network link contend for selection as the designated switch, the
    switch with the highest priority takes precedence.  If both
    switches have the same priority, the switch with the highest base
    MAC address becomes the designated switch.  A switch whose switch
    priority is set to zero is ineligible to become the designated
    switch on the attached link.
 HelloInterval
    The length of time, in seconds, between the Hello packets that the
    switch sends over the interface.  This value is advertised in the
    switch's Hello packets.  It must be the same for all switches
    attached to a common network link.  The smaller this value is set,
    the faster topological changes will be detected.  However, a
    smaller interval will also generate more routing traffic.  A
    typical value for a local area network would be 10 seconds.
 SwitchDeadInterval
    The length of time, in seconds, that neighboring switches will
    wait before declaring the interface down once they stop receiving
    Hello packets over the interface.  This value is advertised in the
    switch's Hello packets.  It must be the same for all switches
    attached to a common network link and should be some multiple of
    the HelloInterval parameter.  A typical value would be 4 times
    HelloInterval.

Kane Informational [Page 92] RFC 2642 Cabletron's VLS Protocol Specification August 1999

13. End Notes

 [1] During calculation of the set of best paths, a network link
 advertisement must be located based solely on its link state ID.
 Note, however, that the lookup in this case is still well defined,
 since no two network advertisements can have the same link state ID.
 [2] It is instructive to see what happens when the designated switch
 for a network link fails.  Call the designated switch for the link S1
 and the backup designated switch S2.  If switch S1 fails (or its
 interface to the link goes down), the other switches on the link will
 detect S1's absence within SwitchDeadInterval seconds.  All switches
 may not detect this condition at precisely the same time.  The
 switches that detect S1's absence before S2 does will temporarily
 select S2 as both designated switch and backup designated switch.
 When S2 detects that S1 is down, it will move itself to designated
 switch.  At this time, the remaining switch with the highest switch
 priority will be selected as the backup designated switch.
 [3] Note that it is possible for a switch to resynchronize any of its
 fully established adjacencies by setting the neighbor state back to
 ExStart.  This causes the switch on the other end of the adjacency to
 process a SeqNumberMismatch event and also revert to the ExStart
 state.
 [4] When two advertisements have different checksum values, they are
 assumed to be separate instances.  This can occur when a switch
 restarts and loses track of its previous sequence number. In this
 case, since the two advertisements have the same sequence number, it
 is not possible to determine which advertisement is actually newer.
 If the wrong advertisement is accepted as newer, the originating
 switch will originate another instance.
 [5] An instance of an advertisement is originated with an age of
 MaxAge only when it is to be flushed from the database.  This is done
 either when the advertisement has naturally aged to MaxAge, or (more
 typically) when the sequence number must wrap. Therefore, a received
 instance with an age of MaxAge must be processed as the most recent
 in order to flush it properly from the database.
 [6] MaxAgeDiff is an architectural constant that defines the maximum
 disparity in ages, in seconds, that can occur for a single link state
 instance as it is distributed throughout the switch fabric.  If two
 advertisements differ by more than this amount, they are assumed to
 be different instances of the same advertisement. This can occur when
 a switch restarts and loses track of its previous sequence number.

Kane Informational [Page 93] RFC 2642 Cabletron's VLS Protocol Specification August 1999

 [7] This is how the link state request list is emptied, causing the
 neighbor state to change to Full.

14. Security Considerations

 Security concerns are not addressed in this document.

15. References

 [Perlman]    Perlman, R.,  Interconnections: Bridges and Routers.
              Addison-Wesley Publishing Company.  1992.
 [RFC905]     McKenzie, A., "ISO Transport Protocol specification ISO
              DP 8073", RFC 905, April 1984.
 [RFC2328]    Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.
 [RFC1700]    Reynolds, J. and J. Postel, "Assigned Numbers", STD 2,
              RFC 1700, October 1994.
 [IDsfvlan]   Ruffen, D., Len, T. and J. Yanacek, "Cabletron's
              SecureFast VLAN Operational Model", RFC 2643, August
              1999.
 [IDhello]    Hamilton, D. and D. Ruffen, "Cabletron's VlanHello
              Protocol Specification", RFC 2641, August 1999.

16. Author's Address

 Laura Kane
 Cabletron Systems, Inc.
 Post Office Box 5005
 Rochester, NH  03866-5005
 Phone:(603) 332-9400
 EMail:  lkane@ctron.com

Kane Informational [Page 94] RFC 2642 Cabletron's VLS Protocol Specification August 1999

17. Full Copyright Statement

 Copyright (C) The Internet Society (1999).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Kane Informational [Page 95]

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