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rfc:rfc6439

Internet Engineering Task Force (IETF) R. Perlman Request for Comments: 6439 Intel Labs Updates: 6325 D. Eastlake 3rd Category: Standards Track Y. Li ISSN: 2070-1721 Huawei Technologies

                                                           A. Banerjee
                                                         Cisco Systems
                                                                 F. Hu
                                                       ZTE Corporation
                                                         November 2011
          Routing Bridges (RBridges): Appointed Forwarders

Abstract

 The IETF TRILL (TRansparent Interconnection of Lots of Links)
 protocol provides least cost pair-wise data forwarding without
 configuration in multi-hop networks with arbitrary topology, safe
 forwarding even during periods of temporary loops, and support for
 multipathing of both unicast and multicast traffic.  TRILL
 accomplishes this by using IS-IS (Intermediate System to Intermediate
 System) link state routing and by encapsulating traffic using a
 header that includes a hop count.  Devices that implement TRILL are
 called "RBridges" (Routing Bridges).
 TRILL supports multi-access LAN (Local Area Network) links that can
 have multiple end stations and RBridges attached.  Where multiple
 RBridges are attached to a link, native traffic to and from end
 stations on that link is handled by a subset of those RBridges called
 "Appointed Forwarders", with the intent that native traffic in each
 VLAN (Virtual LAN) be handled by at most one RBridge.  The purpose of
 this document is to improve the documentation of the Appointed
 Forwarder mechanism; thus, it updates RFC 6325.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc6439.

Perlman, et al. Standards Track [Page 1] RFC 6439 RBridges: Appointed Forwarders November 2011

Copyright Notice

 Copyright (c) 2011 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Table of Contents

 1. Introduction ....................................................2
    1.1. Terminology and Acronyms ...................................3
 2. Appointed Forwarders and Their Appointment ......................4
    2.1. Appointment Effects of DRB Elections .......................5
    2.2. Appointment and Removal by the DRB .........................5
         2.2.1. Processing Forwarder Appointments ...................6
         2.2.2. Frequency of Appointments ...........................7
         2.2.3. Appointed Forwarders Limit ..........................8
    2.3. Local Configuration Action Appointment Effects .............8
    2.4. VLAN Mapping within a Link .................................9
 3. The Inhibition Mechanism ........................................9
 4. Inhibited Appointed Forwarder Behavior .........................11
 5. Multiple Ports on the Same Link ................................12
 6. Security Considerations ........................................12
 7. Acknowledgements ...............................................13
 8. References .....................................................13
    8.1. Normative References ......................................13
    8.2. Informative References ....................................13
 Appendix. VLAN Inhibition Example .................................14

1. Introduction

 The IETF TRILL (TRansparent Interconnection of Lots of Links)
 protocol [RFC6325] provides optimal pair-wise data frame forwarding
 without configuration in multi-hop networks with arbitrary topology,
 safe forwarding even during periods of temporary loops, and support
 for multipathing of both unicast and multicast traffic.  TRILL
 accomplishes this by using IS-IS (Intermediate System to Intermediate
 System) [IS-IS] [RFC1195] link state routing and encapsulating
 traffic using a header that includes a hop count.  The design

Perlman, et al. Standards Track [Page 2] RFC 6439 RBridges: Appointed Forwarders November 2011

 supports VLANs (Virtual Local Area Networks) and optimization of the
 distribution of multi-destination frames based on VLANs and IP-
 derived multicast groups.  Devices that implement TRILL are called
 "RBridges" (Routing Bridges).
 Section 2 of [RFC6327] explains the environment for which the TRILL
 protocol is designed and the differences between that environment and
 the typical Layer 3 routing environment.
 TRILL supports multi-access LAN (Local Area Network) links that can
 have multiple end stations and RBridges attached.  Where multiple
 RBridges are attached to a link, native traffic to and from end
 stations on that link is handled by a subset of those RBridges called
 "Appointed Forwarders", with the intent that native traffic in each
 VLAN be handled by at most one RBridge.  An RBridge can be Appointed
 Forwarder for many VLANs.
 The purpose of this document is to improve the documentation of the
 Appointed Forwarder mechanism; thus, it updates RFC 6325.  It
 includes reference implementation details.  Alternative
 implementations that interoperate on the wire are permitted.
 The Appointed Forwarder mechanism is irrelevant to any link on which
 end station service is not offered.  This includes links configured
 as point-to-point IS-IS links and any link with all RBridge ports on
 that link configured as trunk ports.  (In TRILL, configuration of a
 port as a "trunk port" just means that no end station service will be
 provided.  It does not imply that all VLANs are enabled on that
 port.)
 The Appointed Forwarder mechanism has no effect on the formation of
 adjacencies, the election of the Designated RBridge (DRB) for a link,
 MTU matching, or pseudonode formation.  Those topics are covered in
 [RFC6327].  Furthermore, Appointed Forwarder status has no effect on
 the forwarding of TRILL Data frames.  It only affects the handling of
 native frames.
 For other aspects of the TRILL base protocol, see [RFC6325] and
 [RFC6327].  Familiarity with [RFC6325] and [RFC6327] is assumed in
 this document.  In case of conflict between this document and
 [RFC6325], this document prevails.

1.1. Terminology and Acronyms

 This document uses the acronyms defined in [RFC6325].
 A "trunk port" is a port configured with the "end station service
 disable" bit on, as described in Section 4.9.1 of [RFC6325].

Perlman, et al. Standards Track [Page 3] RFC 6439 RBridges: Appointed Forwarders November 2011

 In this document, the term "link" means "bridged LAN", that is to say
 some combination of physical links with zero or more bridges, hubs,
 repeaters, or the like.
 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
 "OPTIONAL" in this document are to be interpreted as described in
 [RFC2119].

2. Appointed Forwarders and Their Appointment

 The Appointed Forwarder on a link for VLAN-x is the RBridge that
 ingresses native frames from the link and egresses native frames to
 the link in VLAN-x.  By default, the DRB (Designated RBridge) on a
 link is in charge of native traffic for all VLANs on the link.  The
 DRB may, if it wishes, act as Appointed Forwarder for any VLAN and it
 may appoint other RBridges that have ports on the link as Appointed
 Forwarder for one or more VLANs.
 It is important that there not be two Appointed Forwarders on a link
 that are ingressing and egressing native frames for the same VLAN at
 the same time.  Should this occur, it could form a loop where frames
 are not protected by a TRILL Hop Count for part of the loop.  (Such a
 condition can even occur through two Appointed Forwarders for two
 different VLANs, VLAN-x and VLAN-y, if ports or bridges inside the
 link are configured to map frames between VLAN-x and VLAN-y as
 discussed in Section 2.4.)  While TRILL tries to avoid such
 situations, for loop safety there is also an "inhibition" mechanism
 (see Section 3) that can cause an RBridge that is an Appointed
 Forwarder to not ingress or egress native frames.
 As discussed in Section 5, an RBridge may have multiple ports on a
 link.  As discussed in [RFC6327], if there are multiple ports with
 the same Media Access Control (MAC) address on a link, all but one
 will be suspended.  The case of multiple ports on a link for one
 RBridge and the case of multiple ports with the same MAC address on a
 link and combinations of these cases are fully accommodated; however,
 multiple ports on a link for one RBridge is expected to be a rare
 condition and duplicate MAC addresses are not recommended by either
 TRILL or IEEE 802.1 standards.
 Appointed Forwarder status has no effect on the forwarding of TRILL
 Data frames.  It only affects the handling of native frames.

Perlman, et al. Standards Track [Page 4] RFC 6439 RBridges: Appointed Forwarders November 2011

 There are three mechanisms by which an RBridge can be appointed or
 un-appointed as Appointed Forwarder: as a result of the DRB elections
 [RFC6327] as discussed in Section 2.1, as a result of action by the
 DRB as discussed in Section 2.2, as a result of a local configuration
 action as discussed in Section 2.3.

2.1. Appointment Effects of DRB Elections

 When an RBridge believes that it has become the DRB on a link, by
 default, it can act as Appointed Forwarder for any VLANs on that link
 that it chooses as long as its port is not configured as a trunk port
 and has that VLAN enabled (or at least one of its ports meets these
 criteria, if it has more than one port on the link).
 An RBridge loses all Appointed Forwarder status when:
 1.  it decides that it has lost the status of being the DRB for a
     link; or
 2.  it observes a change in the RBridge that is the DRB for the link
     without itself becoming the DRB.
 In the rare corner case where an RBridge has more than one port on a
 link, one of which was previously the DRB election winner but has
 just lost the DRB election to a different port of the same RBridge
 (possibly due to management configuration of port priorities), there
 is no change in which RBridge is the DRB.  Therefore, neither of the
 above points applies and there is no change in Appointed Forwarder
 status.

2.2. Appointment and Removal by the DRB

 The DRB may appoint other RBridges on the link through inclusion of
 one or more Appointed Forwarders sub-TLVs [RFC6326] in a TRILL Hello
 it sends on the Designated VLAN out the port that won the DRB
 election.  When the DRB sends any appointments in a TRILL Hello, it
 must send all appointments for that link in that Hello.  Any previous
 appointment not included is implicitly revoked.
 Although the DRB does not need to announce the VLANs for which it has
 chosen to act as Appointed Forwarder by sending appoints for itself,
 if the DRB wishes to revoke all appointments for RBridges other than
 itself on the link, it is recommended that it send a TRILL Hello with
 an appointment for itself for some VLAN.
 The DRB MUST NOT send any appointments on a link unless its DRB
 inhibition timer (see Section 3) for that link is expired.

Perlman, et al. Standards Track [Page 5] RFC 6439 RBridges: Appointed Forwarders November 2011

 How the DRB decides what other RBridges on the link, if any, to
 appoint forwarder for which VLANs is beyond the scope of this
 document.

2.2.1. Processing Forwarder Appointments

 When a non-DRB RBridge that can offer end station service on a link
 receives a TRILL Hello that is not discarded for one of the reasons
 given in [RFC6327], it checks the source MAC address and the Port ID
 and System ID in the Hello to determine if it is from the winning DRB
 port.  If it is not from that port, any Appointed Forwarder sub-TLVs
 in the Hello are ignored, and there is no change in the receiving
 RBridge's Appointed Forwarder status.  Also, if no Appointed
 Forwarder sub-TLVs are present in the TRILL Hello, there is no change
 in the receiver's Appointed Forwarder status.
 However, if the TRILL Hello is from the winning DRB port and the
 Hello includes one or more Appointed Forwarder sub-TLVs, then the
 receiving RBridge becomes appointed for the VLANs that are both
 listed for it in the Hello and are enabled on the receiving port.
 (If the appointment includes VLAN IDs 0x000 or 0xFFF, they are
 ignored, but any other VLAN IDs are still effective.)  If the
 receiver was Appointed Forwarder for any other VLANs, its Appointed
 Forwarder status for such other VLANs is revoked.  For example, if
 none of these sub-TLVs in a Hello appoints the receiving RBridge,
 then it loses all Appointed Forwarder status and is no longer
 Appointed Forwarder for any VLAN on the port where the Hello was
 received.
 The handling of one or more Appointed Forwarder sub-TLVs in a Hello
 from the winning port that appoints the receiving RBridge is as
 follows.  An appointment in an Appointed Forwarder sub-TLV is for a
 specific RBridge and a contiguous interval of VLAN IDs; however, as
 stated above, it actually appoints that RBridge forwarder only for
 the VLAN(s) in that range that are enabled on one or more ports that
 RBridge has on the link (ignoring any ports configured as trunk ports
 or as IS-IS point-to-point ports).  If the RBridge was Appointed
 Forwarder for any additional VLANs beyond the VLANs for which it was
 being appointed, it loses Appointed Forwarder status for such
 additional VLANs.
 There is no reason for an RBridge to remember that it received a
 valid appointment message for a VLAN that was ineffective because the
 VLAN was not enabled on the port where the message was received or
 because the port was a trunk or point-to-point port.  It does not
 become Appointed Forwarder for such a VLAN just because that VLAN is
 later enabled or the port later reconfigured.

Perlman, et al. Standards Track [Page 6] RFC 6439 RBridges: Appointed Forwarders November 2011

 It should be straightforward for the DRB to send, within one Hello,
 the appointments for several dozen VLAN IDs or several dozen blocks
 of contiguous VLAN IDs.  Should the VLANs the DRB wishes to appoint
 be inconveniently distributed, for example, the proverbial case where
 the DRB RB1 wishes to appoint RB2 forwarder for all even-numbered
 VLANs and appoint RB3 forwarder for all odd-numbered VLANs, the
 following method may be used.  The network manager normally controls
 what VLANs are enabled on RBridge port.  Thus, the network manager
 can appoint an RBridge forwarder for an arbitrary set of scattered
 VLANs by enabling only those VLANs on the relevant port (or ports)
 and then having the DRB send an appointment that appears to appoint
 the target RBridge forwarder for all VLANs.  However, for proper
 operation and inter-RBridge communication, the Designated VLAN for a
 link SHOULD be enabled on all RBridge ports on that link, and it may
 not be desired to appoint the RBridge forwarder for the Designated
 VLAN.  Thus, in the general case, it would require two appointments,
 although it would still only require one appointment if the
 Designated VLAN were an extreme low or high value such as VLAN 0xFFE
 or the default VLAN 1.
 For example, assume the DRB wants RB2 to be Appointed Forwarder for
 all even-numbered VLANs and the Designated VLAN for the link is VLAN
 101.  The network manager could cause all even-numbered VLANs plus
 VLAN 101 to be enabled on the relevant port of RB2 and then, with the
 desired effect, cause the DRB to send appointments to RB2 appointing
 it forwarder for all VLANs from 1 through 100 and from 102 through
 4,094.
 Should the network manager have misconfigured the enabled VLANs and
 Appointed Forwarders, resulting in two RBridges believing they are
 Appointed Forwarders for the same VLAN, then item 4 in Section 3 will
 cause one or more of the RBridges to be inhibited for that VLAN.

2.2.2. Frequency of Appointments

 It is not necessary for the DRB to include the forwarder appointments
 in every TRILL Hello that it sends on the Designated VLAN for a link.
 For loop safety, every RBridge is required to indicate, in every
 TRILL Hello it sends in VLAN-x on a link, whether it is an Appointed
 Forwarder for VLAN-x for that link (see item 4 in Section 3).  It is
 also RECOMMENDED that the DRB have all VLANs for which end station
 service will be offered on the link as well as the Designated VLAN,
 enabled.  Thus, the DRB will generally be informed by other RBridges
 on the link of the VLANs for which they believe they are Appointed
 Forwarder.  If this matches the appointments the DRB wishes to make,
 it is not required to re-send its forwarder appointments; however,
 for robustness, especially in cases such as VLAN misconfigurations in

Perlman, et al. Standards Track [Page 7] RFC 6439 RBridges: Appointed Forwarders November 2011

 a bridged LAN link, it is RECOMMENDED that the DRB send its forwarder
 appointments on the Designated VLAN at least once per its Holding
 Time on the port that won the DRB election.

2.2.3. Appointed Forwarders Limit

 The mechanism of DRB forwarder appointment and the limited length of
 TRILL Hellos impose a limit on the number of RBridges on a link that
 can be Appointed Forwarders.  To obtain a conservative estimate,
 assume that no more than 1000 bytes are available in a TRILL Hello
 for such appointments.  Assume it is desired to appoint various
 RBridges on a link forwarder for arbitrary non-intersecting sets of
 VLANs.  Using the technique discussed above would generally require
 two appointments, or 12 bytes, per RBridge.  With allowance for
 sub-TLV and TLV overhead, appointments for 83 RBridges would fit in
 under 1000 bytes.  Including the DRB, this implies a link with 84 or
 more RBridges attached.  Links with more than a handful of RBridges
 attached are expected to be rare.
 Note: If the Designated VLAN were an extreme low or high value, such
 as VLAN 1, which is the default and may be a common value in
 practice, only 6 bytes per RBridge would be required.  This would
 permit twice as many different Appointed Forwarder RBridges than
 indicated by the general analysis above or, alternatively, would take
 only half as much space to appoint the same number of Appointed
 Forwarders.
 Unnecessary changes in Appointed Forwarders SHOULD NOT be made as
 they may result in transient lack of end station service.  Large
 numbers of Appointed Forwarders on a link (in excess of 65) are NOT
 RECOMMENDED due to the complexity of their establishment and
 maintenance.

2.3. Local Configuration Action Appointment Effects

 Disabling VLAN-x at an RBridge port cancels any Appointed Forwarder
 status that RBridge has for VLAN-x unless VLAN-x is enabled on some
 other port that the RBridge has connected to the same link.
 Configuring a port as a trunk port or point-to-point port revokes any
 Appointed Forwarder status that depends on enabled VLANs at that
 port.
 Causing a port to no longer be configured as a trunk or point-to-
 point port or enabling VLAN-x on a port does not, in itself, cause
 the RBridge to become an Appointed Forwarder for the link that port
 is on.  However, such actions can allow the port's RBridge to become
 Appointed Forwarder by choice if it is the DRB or by appointment, if
 it is not the DRB on the link.

Perlman, et al. Standards Track [Page 8] RFC 6439 RBridges: Appointed Forwarders November 2011

2.4. VLAN Mapping within a Link

 TRILL Hellos include a field that is set to the VLAN in which they
 are sent.  If they arrive on a different VLAN, then VLAN mapping is
 occurring within the link.  (Such VLAN mapping within a link between
 RBridges should not be confused with VLAN mapping inside an RBridge
 [VLANMAP]).  VLAN mapping between VLAN-x and VLAN-y can lead to a
 loop if the Appointed Forwarders for the VLANs are different.  If
 such mapping within a link was allowed and occurred on two or more
 links so that there was a cycle of VLAN mappings, a broadcast frame,
 for example, would loop forever.
 To prevent this potential problem, if the DRB on a link detects VLAN
 mapping by receiving a Hello in VLAN-x that was sent on VLAN-y, it
 MUST make or revoke appointments so as to assure that the same
 RBridge (possibly the DRB) is the Appointed Forwarder on the link for
 both VLAN-x and VLAN-y.

3. The Inhibition Mechanism

 An RBridge has, for every link on which it can offer end station
 service (that is every link for which it can act as an Appointed
 Forwarder), the following timers denominated in seconds:
  1. a DRB inhibition timer,
  1. a root change inhibition timer, and
  1. up to 4,094 VLAN inhibition timers, one for each legal VLAN ID.
 The DRB and root change inhibition timers MUST be implemented.
 The loss of native traffic due to inhibition will be minimized by
 logically implementing a VLAN inhibition timer per each VLAN for
 which end station service will ever be offered by the RBridge on the
 link; this SHOULD be done.  (See the Appendix for an example
 motivating VLAN inhibition timers.)  However, if implementation
 limitations make a full set of such timers impractical, the VLAN
 inhibition timers for more than one VLAN can, with care, be merged
 into one timer.  In particular, an RBridge MUST NOT merge the VLAN
 inhibition timers together for two VLANs if it is the Appointer
 Forwarder for one and not for the other, as this can lead to
 unnecessary indefinitely prolonged inhibition.  In the limit, there
 will be safe operations, albeit with more native frame loss than
 would otherwise be required, even if only two VLAN inhibition timers
 are provided: one for VLANs for which the RBridge is the Appointed
 Forwarder and one for all other VLANs.  At least two VLAN inhibition

Perlman, et al. Standards Track [Page 9] RFC 6439 RBridges: Appointed Forwarders November 2011

 timers MUST be implemented.  Where a VLAN inhibition timer represents
 more than one VLAN, an update or test that would have been done to
 the timer for any of the VLANs is performed on the merged timer.
 These timers are set as follows:
 1.  On booting or management reset, each port will have its own set
     of timers, even if two or more such ports are on the same link,
     because the RBridge will not have had a chance to learn that yet.
     All inhibition timers are set to expired except the DRB
     inhibition timer that is set in accordance with item 2 below.
     The DRB inhibition timer is handled differently because each port
     will initially believe it is the DRB.
 2.  When an RBridge decides that it has become the DRB on a link,
     including when it is first booted or reset by management, it sets
     the DRB inhibition timer to the Holding Time of its port on that
     link that won the DRB election.
 3.  When an RBridge decides that it has lost DRB status on a link, it
     sets the DRB inhibition timer to expired.
     Note: In the rare corner case where one port of an RBridge was
     the DRB election winner, but later lost the DRB election to a
     different port of the same RBridge on that link (perhaps due to
     management configuration of port priority), neither 2 nor 3 above
     applies, and the DRB timer is not changed.
 4.  When an RBridge RB1 receives a TRILL Hello asserting that the
     sender is the Appointed Forwarder that either (1) arrives on
     VLAN-x or (2) was sent on VLAN-x as indicated inside the Hello,
     then RB1 sets its VLAN-x inhibition timer for the link to the
     maximum of that timer's existing value and the Holding Time in
     the received Hello.  An RBridge MUST maintain VLAN inhibition
     timers for a link to which it connects if it can offer end
     station service on that link even if it is not currently
     Appointed Forwarder for any VLAN on that link.
 5.  When an RBridge RB1 enables VLAN-x on a port connecting to a link
     and VLAN-x was previously not enabled on any of RB1's ports on
     that link, it sets its VLAN inhibition timer for VLAN-x for that
     link to its Holding Time for that port.  This is done even if the
     port is configured as a trunk or point-to-point port as long as
     there is some chance it might later be configured not to be a
     trunk or point-to-point port.

Perlman, et al. Standards Track [Page 10] RFC 6439 RBridges: Appointed Forwarders November 2011

 6.  When an RBridge detects a change in the common spanning tree root
     bridge on a port, it sets its root change inhibition timer for
     the link to an amount of time that defaults to 30 seconds and is
     configurable to any value from 30 down to zero seconds.  This
     condition will not occur unless the RBridge is receiving Bridge
     PDU (BPDUs) on the port from an attached bridged LAN.  It is safe
     to configure this inhibition time to the settling time of an
     attached bridged LAN.  For example, if it is known that Rapid
     Spanning Tree Protocol (RSTP [802.1Q]) is running throughout the
     attached bridged LAN, it should be safe to configure this
     inhibition time to 7 seconds or, if the attached bridges have
     been configured to have a minimum Bridge Hello Timer, safe to
     configure it to 4 seconds.  Note that, while an RBridge could
     determine what version of spanning tree is running on the
     physical link between it and any directly connected bridge by
     examination of the BPDUs it receives, it could not tell if
     inter-bridge links beyond those directly connected bridges were
     running classic Spanning Tree Protocol (STP), which might require
     the root change inhibition timer to be set to 30 seconds for
     safety.
 7.  When an RBridge decides that one of its ports (or a set of its
     ports) P1 is on the same link as another of its ports (or set of
     its ports) P2, then the inhibition timers are merged to a single
     set of inhibition timers by using the maximum value of the
     corresponding timers.
 8.  When an RBridge decides that a set of its ports that it had been
     treating as being on the same link are no longer on the same
     link, those ports will necessarily be on two or more links (one
     link per port in the limit).  This is handled by cloning a copy
     of the timers for each of the two or more links to which the
     RBridge has decided these ports connect.

4. Inhibited Appointed Forwarder Behavior

 An Appointed Forwarder for a link is inhibited for VLAN-x if:
 1.  its DRB inhibition timer for that link is not expired, or
 2.  its root change inhibition timer for that link is not expired, or
 3.  its VLAN inhibition timer for that link for VLAN-x is not
     expired.
 If a VLAN-x Appointed Forwarder for a link is inhibited and receives
 a TRILL Data frame whose encapsulated frame is in VLAN-x and would
 normally be egressed to that link, it decapsulates the native frame

Perlman, et al. Standards Track [Page 11] RFC 6439 RBridges: Appointed Forwarders November 2011

 as usual.  However, it does not output it to or queue it for that
 link, although, if appropriate (for example, the frame is multi-
 destination), it may output it to or queue it for other links.
 If a VLAN-x Appointed Forwarder for a link is inhibited and receives
 a native frame in VLAN-x that would normally be ingressed from that
 link, the native frame is ignored except for address learning.
 An RBridge with one or more unexpired inhibition timers, possibly
 including an unexpired inhibition timer for VLAN-x, is still required
 to indicate in TRILL Hellos it sends on VLAN-x whether or not it is
 Appointed Forwarder for VLAN-x for the port on which it sends the
 Hello.
 Inhibition has no effect on the receipt or forwarding of TRILL Data
 frames.

5. Multiple Ports on the Same Link

 An RBridge may have multiple ports on the same link.  Some of these
 ports may be suspended due to MAC address duplication as described in
 [RFC6327].  Suspended ports never ingress or egress native frames.
 If an RBridge has one or more non-suspended ports on a link and those
 ports offer end station service, that is, those ports are not
 configured as point-to-point or trunk ports, then that RBridge is
 eligible to be an Appointed Forwarder for that link.  It can become
 Appointed Forwarder either by its choice, because it is the DRB, or
 by appointment by the DRB as described in Sections 2.1 and 2.2.
 If an RBridge that is the Appointed Forwarder for VLAN-x on a link
 has multiple non-suspended ports on that link, it may load share the
 task of ingressing and egressing VLAN-x native frames across those
 ports however it chooses, as long as there is no case in which a
 frame it egresses onto the link from one port can be ingressed on
 another of its ports, creating a loop.  If the RBridge is the
 Appointed Forwarder for multiple VLANs, a straightforward thing to do
 would be to partition those VLANs among the ports it has on the link.

6. Security Considerations

 This memo provides improved documentation of the TRILL Appointed
 Forwarder mechanism.  It does not change the security considerations
 of the TRILL base protocol.  See Section 6 of [RFC6325].

Perlman, et al. Standards Track [Page 12] RFC 6439 RBridges: Appointed Forwarders November 2011

7. Acknowledgements

 The authors of [RFC6325] and [RFC6327], those listed in the
 Acknowledgements section of [RFC6325] and [RFC6327], and Ron Bonica,
 Stewart Bryant, Linda Dunbar, Les Ginsberg, Erik Nordmark, Dan
 Romascanu, and Mike Shand are hereby thanked for their contributions.

8. References

 Normative and Informative references for this document are listed
 below.

8.1. Normative References

 [802.1Q]    IEEE 802.1, "IEEE Standard for Local and metropolitan
             area networks - Virtual Bridged Local Area Networks",
             IEEE Std 802.1Q-2011, May 2011.
 [IS-IS]     ISO/IEC 10589:2002, Second Edition, "Intermediate System
             to Intermediate System Intra-Domain Routeing Exchange
             Protocol for use in Conjunction with the Protocol for
             Providing the Connectionless-mode Network Service (ISO
             8473)", 2002.
 [RFC1195]   Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
             dual environments", RFC 1195, December 1990.
 [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC6325]   Perlman, R., Eastlake 3rd, D., Dutt, D., Gai, S., and A.
             Ghanwani, "Routing Bridges (RBridges): Base Protocol
             Specification", RFC 6325, July 2011.
 [RFC6326]   Eastlake, D., Banerjee, A., Dutt, D., Perlman, R., and A.
             Ghanwani, "Transparent Interconnection of Lots of Links
             (TRILL) Use of IS-IS", RFC 6326, July 2011.
 [RFC6327]   Eastlake 3rd, D., Perlman, R., Ghanwani, A., Dutt, D.,
             and V. Manral, "Routing Bridges (RBridges): Adjacency",
             RFC 6327, July 2011.

8.2. Informative References

 [VLANMAP]   Perlman, R., Dutt, D., Banerjee, A., Rijhsinghani, A.,
             and D.  Eastlake, "RBridges: Campus VLAN and Priority
             Regions", Work in Progress, October 2011.

Perlman, et al. Standards Track [Page 13] RFC 6439 RBridges: Appointed Forwarders November 2011

Appendix. VLAN Inhibition Example

 The per-VLAN inhibition timers (or the equivalent) are needed to be
 loop safe in the case of misconfigured bridges on a link.
 For a simple example, assume that RB1 and RB2 are the only RBridges
 on the link, that RB1 is higher priority to be the DRB, and that they
 both want VLAN 1 (the default) to be the Designated VLAN.  However,
 there is a bridge between them configured so that RB1 can see all the
 frames sent by RB2 but none of the frames from RB1 can get through to
 RB2.
 Both will think they are the DRB.  RB1 because it is higher priority
 even though it sees the Hellos from RB2, and RB2 because it doesn't
 see the Hellos from RB1 and therefore thinks it is highest priority.
 Say RB1 chooses to act as Appointed Forwarder for VLANs 2 and 3 while
 RB2 chooses to act as Appointed Forwarder for VLANs 3 and 4.  There
 is no problem with VLANs 2 and 4 but if you do not do something about
 it, you could have a loop involving VLAN 3.  RB1 will see the Hellos
 RB2 issues on VLAN 3 declaring itself Appointed Forwarder, so RB1
 will be inhibited on VLAN 3.  RB2 does not see the Hellos issued by
 RB1 on VLAN 3, so RB2 will become uninhibited and will handle VLAN 3
 native traffic.
 However, this situation may change.  RB2 might crash, the bridge
 might crash, or RB2 might be reconfigured so it no longer tried to
 act as Appointed Forwarder for VLAN 3, or other issues may occur.
 So, RB1 has to maintain a VLAN 3 inhibition timer, and if it sees no
 Hellos from any other RBridge on the link claiming to be Appointed
 Forwarder for VLAN 3 in a long enough time, then RB1 becomes
 uninhibited for that VLAN on the port in question and can handle end
 station traffic in VLAN 3.

Perlman, et al. Standards Track [Page 14] RFC 6439 RBridges: Appointed Forwarders November 2011

Authors' Addresses

 Radia Perlman
 Intel Labs
 2200 Mission College Blvd.
 Santa Clara, CA 95054 USA
 Phone: +1-408-765-8080
 EMail: Radia@alum.mit.edu
 Donald Eastlake 3rd
 Huawei Technologies
 155 Beaver Street
 Milford, MA 01757 USA
 Phone: +1-508-333-2270
 EMail: d3e3e3@gmail.com
 Yizhou Li
 Huawei Technologies
 101 Software Avenue,
 Nanjing 210012, China
 Phone: +86-25-56622310
 EMail: liyizhou@huawei.com
 Ayan Banerjee
 Cisco Systems
 170 West Tasman Drive
 San Jose, CA 95134 USA
 Phone: +1-408-333-7149
 EMail: ayabaner@cisco.com
 Fangwei Hu
 ZTE Corporation
 889 Bibo Road
 Shanghai 201203
 China
 Phone: +86-21-68896273
 EMail: hu.fangwei@zte.com.cn

Perlman, et al. Standards Track [Page 15]

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