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rfc:rfc3847

Table of Contents

Network Working Group M. Shand Request for Comments: 3847 L. Ginsberg Category: Informational Cisco Systems

                                                             July 2004
                       Restart Signaling for
        Intermediate System to Intermediate System (IS-IS)

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 (2004).  All Rights Reserved.

Abstract

 This document describes a mechanism for a restarting router to signal
 to its neighbors that it is restarting, allowing them to reestablish
 their adjacencies without cycling through the down state, while still
 correctly initiating database synchronization.
 This document additionally describes a mechanism for a restarting
 router to determine when it has achieved LSP database synchronization
 with its neighbors and a mechanism to optimize LSP database
 synchronization, while minimizing transient routing disruption when a
 router starts.

Shand & Ginsberg Informational [Page 1] RFC 3847 Restart signaling for IS-IS July 2004

Table of Contents

 1.  Conventions used in this Document. . . . . . . . . . . . . . .  2
 2.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  3
 3.  Approach . . . . . . . . . . . . . . . . . . . . . . . . . . .  4
     3.1.  Timers . . . . . . . . . . . . . . . . . . . . . . . . .  4
     3.2.  Restart TLV. . . . . . . . . . . . . . . . . . . . . . .  5
           3.2.1.  Use of RR and RA Bits. . . . . . . . . . . . . .  6
           3.2.2.  Use of SA Bit. . . . . . . . . . . . . . . . . .  7
     3.3.  Adjacency (re)Acquisition. . . . . . . . . . . . . . . .  8
           3.3.1.  Adjacency Reacquisition During Restart . . . . .  8
           3.3.2.  Adjacency Acquisition During Start . . . . . . . 10
           3.3.3.  Multiple Levels. . . . . . . . . . . . . . . . . 12
     3.4.  Database Synchronization . . . . . . . . . . . . . . . . 12
           3.4.1.  LSP Generation and Flooding and SPF Computation. 13
                   3.4.1.1. Restarting. . . . . . . . . . . . . . . 13
                   3.4.1.2. Starting. . . . . . . . . . . . . . . . 15
 4.  State Tables . . . . . . . . . . . . . . . . . . . . . . . . . 15
     4.1.  Running Router . . . . . . . . . . . . . . . . . . . . . 16
     4.2.  Restarting Router. . . . . . . . . . . . . . . . . . . . 17
     4.3.  Starting Router. . . . . . . . . . . . . . . . . . . . . 18
 5.  Security Considerations. . . . . . . . . . . . . . . . . . . . 18
 6.  IANA Considerations. . . . . . . . . . . . . . . . . . . . . . 19
 7.  Normative References . . . . . . . . . . . . . . . . . . . . . 19
 8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 20
 9.  Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 20
 10. Full Copyright Statement . . . . . . . . . . . . . . . . . . . 21

1. Conventions used in this Document

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in BCP 14, RFC-2119 [3].
 If the control and forwarding functions in a router can be maintained
 independently, it is possible for the forwarding function state to be
 maintained across a resumption of control function operations.  This
 functionality is assumed when the terms "restart/restarting" are used
 in this document.
 The terms "start/starting" are used to refer to a router in which the
 control function has either commenced operations for the first time
 or has resumed operations but the forwarding functions have not been
 maintained in a prior state.
 The terms "(re)start/(re)starting" are used when the text is
 applicable to both a "starting" and a "restarting" router.

Shand & Ginsberg Informational [Page 2] RFC 3847 Restart signaling for IS-IS July 2004

2. Overview

 The Intermediate System to Intermediate System (IS-IS) routing
 protocol [RFC 1195, ISO/IEC 10589] is a link state intra-domain
 routing protocol.  Normally, when an IS-IS router is restarted,
 temporary disruption of routing occurs due to events in both the
 restarting router and the neighbors of the restarting router.
 The router which has been restarted computes its own routes before
 achieving database synchronization with its neighbors.  The results
 of this computation are likely to be non-convergent with the routes
 computed by other routers in the area/domain.
 Neighbors of the restarting router detect the restart event and cycle
 their adjacencies with the restarting router through the down state.
 The cycling of the adjacency state causes the neighbors to regenerate
 their LSPs describing the adjacency concerned.  This in turn causes a
 temporary disruption of routes passing through the restarting router.
 In certain scenarios, the temporary disruption of the routes is
 highly undesirable.  This document describes mechanisms to avoid or
 minimize the disruption due to both of these causes.
 When an adjacency is reinitialized as a result of a neighbor
 restarting, a router does three things:
 1. It causes its own LSP(s) to be regenerated, thus triggering SPF
    runs throughout the area (or in the case of Level 2, throughout
    the domain).
 2. It sets SRMflags on its own LSP database on the adjacency
    concerned.
 3. In the case of a Point-to-Point link, it transmits a (set of)
    CSNP(s) over the adjacency.
 In the case of a restarting router process, the first of these is
 highly undesirable, but the second is essential in order to ensure
 synchronization of the LSP database.
 The third action above minimizes the number of LSPs which must be
 exchanged and, if made reliable, provides a means of determining when
 the LSP databases of the neighboring routers have been synchronized.
 This is desirable whether the router is being restarted or not (so
 that the overload bit can be cleared in the router's own LSP, for
 example).

Shand & Ginsberg Informational [Page 3] RFC 3847 Restart signaling for IS-IS July 2004

 This document describes a mechanism for a restarting router to signal
 that it is restarting to its neighbors, and allow them to reestablish
 their adjacencies without cycling through the down state, while still
 correctly initiating database synchronization.
 This document additionally describes a mechanism for a restarting
 router to determine when it has achieved LSP database synchronization
 with its neighbors and a mechanism to optimize LSP database
 synchronization and minimize transient routing disruption when a
 router starts.
 It is assumed that the three-way handshake [4] is being used on
 Point-to-Point circuits.

3. Approach

3.1. Timers

 Three additional timers, T1, T2, and T3 are required to support the
 functionality defined in this document.
 An instance of the timer T1 is maintained per interface, and
 indicates the time after which an unacknowledged (re)start attempt
 will be repeated.  A typical value might be 3 seconds.
 An instance of the timer T2 is maintained for each LSP database
 present in the system, i.e., for a Level1/2 system, there will be an
 instance of the timer T2 for Level 1 and an instance for Level 2.
 This is the maximum time that the system will wait for LSPDB
 synchronization.  A typical value might be 60 seconds.
 A single instance of the timer T3 is maintained for the entire
 system.  It indicates the time after which the router will declare
 that it has failed to achieve database synchronization (by setting
 the overload bit in its own LSP).  This is initialized to 65535
 seconds, but is set to the minimum of the remaining times of received
 IIHs containing a restart TLV with the RA set and an indication that
 the neighbor has an adjacency in the "UP" state to the restarting
 router.
 NOTE: The timer T3 is only used by a restarting router.

Shand & Ginsberg Informational [Page 4] RFC 3847 Restart signaling for IS-IS July 2004

3.2. Restart TLV

 A new TLV is defined to be included in IIH PDUs.  The presence of
 this TLV indicates that the sender supports the functionality defined
 in this document and it carries flags that are used to convey
 information during a (re)start.  All IIHs transmitted by a router
 that supports this capability MUST include this TLV.
 Type   211
 Length # of octets in the value field (1 to (3 + ID Length))
 Value
                                  No. of octets
   +-----------------------+
   |   Flags               |     1
   +-----------------------+
   | Remaining Time        |     2
   +-----------------------+
   | Restarting Neighbor ID|     ID Length
   +-----------------------+
 Flags (1 octet)
    0  1  2  3  4  5  6  7
   +--+--+--+--+--+--+--+--+
   |  Reserved    |SA|RA|RR|
   +--+--+--+--+--+--+--+--+
   RR - Restart Request
   RA - Restart Acknowledgement
   SA - Suppress adjacency advertisement
 (Note: Remaining fields are required when the RA bit is set)
 Remaining Time (2 octets)
   Remaining holding time (in seconds)
 Restarting Neighbor System ID (ID Length octets)
 The system ID of the neighbor to which an RA refers.  Note:
 Implementations based on earlier versions of this document may not
 include this field in the TLV when the RA is set.  In this case, a
 router which is expecting an RA on a LAN circuit SHOULD assume that
 the acknowledgement is directed at the local system.

Shand & Ginsberg Informational [Page 5] RFC 3847 Restart signaling for IS-IS July 2004

3.2.1. Use of RR and RA Bits

 The RR bit is used by a (re)starting router to signal to its
 neighbors that a (re)start is in progress, that an existing adjacency
 SHOULD be maintained even under circumstances when the normal
 operation of the adjacency state machine would require the adjacency
 to be reinitialized, to request a set of CSNPs, and to request
 setting of the SRMflags.
 The RA bit is sent by the neighbor of a (re)starting router to
 acknowledge the receipt of a restart TLV with the RR bit set.
 When the neighbor of a (re)starting router receives an IIH with the
 restart TLV having the RR bit set, if there exists on this interface
 an adjacency in state "UP" with the same System ID, and in the case
 of a LAN circuit, with the same source LAN address, then,
 irrespective of the other contents of the "Intermediate System
 Neighbors" option (LAN circuits) or the "Point-to-Point Three-Way
 Adjacency" option (Point-to-Point circuits):
 a) the state of the adjacency is not changed.  If this is the first
    IIH with the RR bit set that this system has received associated
    with this adjacency, then the adjacency is marked as being in
    "Restart mode" and the adjacency holding time is refreshed -
    otherwise the holding time is not refreshed.  The "remaining time"
    transmitted according to (b) below MUST reflect the actual time
    after which the adjacency will now expire.  Receipt of a normal
    IIH with the RR bit reset will clear the "Restart mode" state.
    This procedure allows the restarting router to cause the neighbor
    to maintain the adjacency long enough for restart to successfully
    complete while also preventing repetitive restarts from
    maintaining an adjacency indefinitely.  Whether an adjacency is
    marked as being in "Restart mode" or not has no effect on
    adjacency state transitions.
 b) immediately (i.e., without waiting for any currently running timer
    interval to expire, but with a small random delay of a few 10s of
    milliseconds on LANs to avoid "storms") transmit over the
    corresponding interface an IIH including the restart TLV with the
    RR bit clear and the RA bit set, in the case of Point-to-Point
    adjacencies having updated the "Point-to-Point Three-Way
    Adjacency" option to reflect any new values received from the
    (re)starting router.  (This allows a restarting router to quickly
    acquire the correct information to place in its hellos.)  The
    "Remaining Time" MUST be set to the current time (in seconds)
    before the holding timer on this adjacency is due to expire.  If
    the corresponding interface is a LAN interface, then the
    Restarting Neighbor System ID SHOULD be set to the System ID of

Shand & Ginsberg Informational [Page 6] RFC 3847 Restart signaling for IS-IS July 2004

    the router from whom the IIH with the RR bit set was received.
    This is required to correctly associate the acknowledgement and
    holding time in the case where multiple systems on a LAN restart
    at approximately the same time.  This IIH SHOULD be transmitted
    before any LSPs or SNPs are transmitted as a result of the receipt
    of the original IIH.
 c) if the corresponding interface is a Point-to-Point interface, or
    if the receiving router has the highest LnRouterPriority (with
    highest source MAC address breaking ties) among those routers to
    which the receiving router has an adjacency in state "UP" on this
    interface whose IIHs contain the restart TLV, excluding
    adjacencies to all routers which are considered in "Restart mode"
    (note the actual DIS is NOT changed by this process), initiate the
    transmission over the corresponding interface of a complete set of
    CSNPs, and set SRMflags on the corresponding interface for all
    LSPs in the local LSP database.
 Otherwise (i.e., if there was no adjacency in the "UP" state to the
 system ID in question), process the IIH as normal by reinitializing
 the adjacency and setting the RA bit in the returned IIH.

3.2.2. Use of the SA Bit

 The SA bit is used by a starting router to request that its neighbor
 suppress advertisement of the adjacency to the starting router in the
 neighbor's LSPs.
 A router which is starting has no maintained forwarding function
 state.  This may or may not be the first time the router has started.
 If this is not the first time the router has started, copies of LSPs
 generated by this router in its previous incarnation may exist in the
 LSP databases of other routers in the network.  These copies are
 likely to appear "newer" than LSPs initially generated by the
 starting router due to the reinitialization of LSP fragment sequence
 numbers by the starting router.  This may cause temporary blackholes
 to occur until the normal operation of the update process causes the
 starting router to regenerate and flood copies of its own LSPs with
 higher sequence numbers.  The temporary blackholes can be avoided if
 the starting router's neighbors suppress advertising an adjacency to
 the starting router until the starting router has been able to
 propagate newer versions of LSPs generated by previous incarnations.
 When a router receives an IIH with the restart TLV having the SA bit
 set, if there exists on this interface an adjacency in state "UP"
 with the same System ID, and in the case of a LAN circuit, with the
 same source LAN address, then the router MUST suppress advertisement
 of the adjacency to the neighbor in its own LSPs.  Until an IIH with

Shand & Ginsberg Informational [Page 7] RFC 3847 Restart signaling for IS-IS July 2004

 the SA bit clear has been received, the neighbor advertisement MUST
 continue to be suppressed.  If the adjacency transitions to the "UP"
 state, the new adjacency MUST NOT be advertised until an IIH with the
 SA bit clear has been received.
 Note that a router which suppresses advertisement of an adjacency
 MUST NOT use this adjacency when performing its SPF calculation.  In
 particular, if an implementation follows the example guidelines
 presented in [2] Annex C.2.5 Step 0:b) "pre-load TENT with the local
 adjacency database", the suppressed adjacency MUST NOT be loaded into
 TENT.

3.3. Adjacency (Re)Acquisition

 Adjacency (re)acquisition is the first step in (re)initialization.
 Restarting and starting routers will make use of the RR bit in the
 restart TLV, though each will use it at different stages of the
 (re)start procedure.

3.3.1. Adjacency Reacquisition During Restart

 The restarting router explicitly notifies its neighbor that the
 adjacency is being reacquired, and hence that it SHOULD NOT
 reinitialize the adjacency.  This is achieved by setting the RR bit
 in the restart TLV.  When the neighbor of a restarting router
 receives an IIH with the restart TLV having the RR bit set, if there
 exists on this interface an adjacency in state "UP" with the same
 System ID, and in the case of a LAN circuit, with the same source LAN
 address, then the procedures described in 3.2.1 are followed.
 A router that does not support the restart capability will ignore the
 restart TLV and reinitialize the adjacency as normal, returning an
 IIH without the restart TLV.
 On restarting, a router initializes the timer T3, starts the timer T2
 for each LSPDB, and for each interface (and in the case of a LAN
 circuit, for each level) starts the timer T1 and transmits an IIH
 containing the restart TLV with the RR bit set.
 On a Point-to-Point circuit the restarting router SHOULD set the
 "Adjacency Three-Way State" to "Init", because the receipt of the
 acknowledging IIH (with RA set) MUST cause the adjacency to enter the
 "UP" state immediately.
 On a LAN circuit the LAN-ID assigned to the circuit SHOULD be the
 same as that used prior to the restart.  In particular, for any
 circuits for which the restarting router was previously DIS, the use
 of a different LAN-ID would necessitate the generation of a new set

Shand & Ginsberg Informational [Page 8] RFC 3847 Restart signaling for IS-IS July 2004

 of pseudonode LSPs, and corresponding changes in all the LSPs
 referencing them from other routers on the LAN.  By preserving the
 LAN-ID across the restart, this churn can be prevented.  To enable a
 restarting router to learn the LAN-ID used prior to restart, the
 LAN-ID specified in an IIH with RR set MUST be ignored.
 Transmission of "normal" IIHs is inhibited until the conditions
 described below are met (in order to avoid causing an unnecessary
 adjacency initialization).  Upon expiry of the timer T1, it is
 restarted and the IIH is retransmitted as above.
 When a restarting router receives an IIH a local adjacency is
 established as usual, and if the IIH contains a restart TLV with the
 RA bit set (and on LAN circuits with a Restart Neighbor System ID
 which matches that of the local system), the receipt of the
 acknowledgement over that interface is noted.  When the RA bit is set
 and the state of the remote adjacency is "UP", then the timer T3 is
 set to the minimum of its current value and the value of the
 "Remaining Time" field in the received IIH.
 On a Point-to-Point link, receipt of an IIH not containing the
 restart TLV is also treated as an acknowledgement, since it indicates
 that the neighbor is not restart capable.  However, since no CSNP is
 guaranteed to be received over this interface, the timer T1 is
 cancelled immediately without waiting for a complete set of CSNP(s).
 Synchronization may therefore be deemed complete even though there
 are some LSPs which are held (only) by this neighbor (see section
 3.4).  In this case we also want to be certain that the neighbor will
 reinitialize the adjacency in order to guarantee that the SRMflags
 have been set on its database, thus ensuring eventual LSPDB
 synchronization.  This is guaranteed to happen except in the case
 where the Adjacency Three-Way State in the received IIH is "UP" and
 the Neighbor Extended Local Circuit ID matches the extended local
 circuit ID assigned by the restarting router.  In this case the
 restarting router MUST force the adjacency to reinitialize by setting
 the local Adjacency Three-Way State to "DOWN" and sending a normal
 IIH.
 In the case of a LAN interface, receipt of an IIH not containing the
 restart TLV is unremarkable since synchronization can still occur so
 long as at least one of the non-restarting neighboring routers on the
 LAN supports restart.  Therefore T1 continues to run in this case.
 If none of the neighbors on the LAN are restart capable, T1 will
 eventually expire after the locally defined number of retries.
 In the case of a Point-to-Point circuit, the "LocalCircuitID" and
 "Extended Local Circuit ID" information contained in the IIH can be
 used immediately to generate an IIH containing the correct 3-way

Shand & Ginsberg Informational [Page 9] RFC 3847 Restart signaling for IS-IS July 2004

 handshake information.  The presence of "Neighbor Extended Local
 Circuit ID" information which does not match the value currently in
 use by the local system is ignored (since the IIH may have been
 transmitted before the neighbor had received the new value from the
 restarting router), but the adjacency remains in the initializing
 state until the correct information is received.
 In the case of a LAN circuit, the source neighbor information (e.g.,
 SNPAAddress) is recorded and used for adjacency establishment and
 maintenance as normal.
 When BOTH a complete set of CSNP(s) (for each active level, in the
 case of a point-to-point circuit) and an acknowledgement have been
 received over the interface, the timer T1 is cancelled.
 Once the timer T1 has been cancelled, subsequent IIHs are transmitted
 according to the normal algorithms, but including the restart TLV
 with both RR and RA clear.
 If a LAN contains a mixture of systems, only some of which support
 the new algorithm, database synchronization is still guaranteed, but
 the "old" systems will have reinitialized their adjacencies.
 If an interface is active, but does not have any neighboring router
 reachable over that interface, the timer T1 would never be cancelled,
 and according to clause 3.4.1.1, the SPF would never be run.
 Therefore timer T1 is cancelled after some pre-determined number of
 expirations (which MAY be 1).

3.3.2. Adjacency Acquisition During Start

 The starting router wants to ensure that in the event that a
 neighboring router has an adjacency to the starting router in the
 "UP" state (from a previous incarnation of the starting router), this
 adjacency is reinitialized.  The starting router also wants
 neighboring routers to suppress advertisement of an adjacency to the
 starting router until LSP database synchronization is achieved.  This
 is achieved by sending IIHs with the RR bit clear and the SA bit set
 in the restart TLV.  The RR bit remains clear and the SA bit remains
 set in subsequent transmissions of IIHs until the adjacency has
 reached the "UP" state and the initial T1 timer interval (see below)
 has expired.
 Receipt of an IIH with the RR bit clear will result in the
 neighboring router utilizing normal operation of the adjacency state
 machine. This will ensure that any old adjacency on the neighboring
 router will be reinitialized.

Shand & Ginsberg Informational [Page 10] RFC 3847 Restart signaling for IS-IS July 2004

 Upon receipt of an IIH with the SA bit set, the behavior described in
 3.2.2 is followed.
 Upon starting, a router starts timer T2 for each LSPDB.
 For each interface (and in the case of a LAN circuit, for each
 level), when an adjacency reaches the "UP" state, the starting router
 starts a timer T1 and transmits an IIH containing the restart TLV
 with the RR bit clear and SA bit set.  Upon expiry of the timer T1,
 it is restarted and the IIH is retransmitted with both RR and SA bits
 set (only the RR bit has changed state from earlier IIHs).
 Upon receipt of an IIH with the RR bit set (regardless of whether the
 SA is set or not), the behavior described in 3.2.1 is followed.
 When an IIH is received by the starting router and the IIH contains a
 restart TLV with the RA bit set (and on LAN circuits with a Restart
 Neighbor System ID which matches that of the local system), the
 receipt of the acknowledgement over that interface is noted.
 On a Point-to-Point link, receipt of an IIH not containing the
 restart TLV is also treated as an acknowledgement, since it indicates
 that the neighbor is not restart capable.  Since the neighbor will
 have reinitialized the adjacency, this guarantees that SRMflags have
 been set on its database, thus ensuring eventual LSPDB
 synchronization.  However, since no CSNP is guaranteed to be received
 over this interface, the timer T1 is cancelled immediately without
 waiting for a complete set of CSNP(s).  Synchronization may therefore
 be deemed complete even though there are some LSPs which are held
 (only) by this neighbor (see section 3.4).
 In the case of a LAN interface, receipt of an IIH not containing the
 restart TLV is unremarkable since synchronization can still occur so
 long as at least one of the non-restarting neighboring routers on the
 LAN supports restart.  Therefore T1 continues to run in this case.
 If none of the neighbors on the LAN are restart capable, T1 will
 eventually expire after the locally defined number of retries.  The
 usual operation of the update process will ensure that
 synchronization is eventually achieved.
 When BOTH a complete set of CSNP(s) (for each active level, in the
 case of a point-to-point circuit) and an acknowledgement have been
 received over the interface, the timer T1 is cancelled.  Subsequent
 IIHs sent by the starting router have the RR and RA bits clear and
 the SA bit set in the restart TLV.
 Timer T1 is cancelled after some pre-determined number of expirations
 (which MAY be 1).

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 When the T2 timer(s) are cancelled or expire, transmission of
 "normal" IIHs (with RR, RA, and SA bits clear) will begin.

3.3.3. Multiple Levels

 A router which is operating as both a Level 1 and a Level 2 router on
 a particular interface MUST perform the above operations for each
 level.
 On a LAN interface, it MUST send and receive both Level 1 and Level 2
 IIHs and perform the CSNP synchronizations independently for each
 level.
 On a point-to-point interface, only a single IIH (indicating support
 for both levels) is required, but it MUST perform the CSNP
 synchronizations independently for each level.

3.4. Database Synchronization

 When a router is started or restarted it can expect to receive a (set
 of) CSNP(s) over each interface.  The arrival of the CSNP(s) is now
 guaranteed, since an IIH with the RR bit set will be retransmitted
 until the CSNP(s) are correctly received.
 The CSNPs describe the set of LSPs that are currently held by each
 neighbor.  Synchronization will be complete when all these LSPs have
 been received.
 When (re)starting, a router starts an instance of timer T2 for each
 LSPDB as described in 3.3.1 or 3.3.2.  In addition to normal
 processing of the CSNPs, the set of LSPIDs contained in the first
 complete set of CSNP(s) received over each interface is recorded,
 together with their remaining lifetime.  In the case of a LAN
 interface, a complete set of CSNPs MUST consist of CSNPs received
 from neighbor(s) which are not restarting.  If there are multiple
 interfaces on the (re)starting router, the recorded set of LSPIDs is
 the union of those received over each interface.  LSPs with a
 remaining lifetime of zero are NOT so recorded.
 As LSPs are received (by the normal operation of the update process)
 over any interface, the corresponding LSPID entry is removed (it is
 also removed if an LSP arrives before the CSNP containing the
 reference).  When an LSPID has been held in the list for its
 indicated remaining lifetime, it is removed from the list.  When the
 list of LSPIDs is empty and the timer T1 has been cancelled for all
 the interfaces that have an adjacency at this level, the timer T2 is
 cancelled.

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 At this point, the local database is guaranteed to contain all the
 LSP(s) (either the same sequence number, or a more recent sequence
 number) that were present in the neighbors' databases at the time of
 (re)starting.  LSPs that arrived in a neighbor's database after the
 time of (re)starting may or may not be present, but the normal
 operation of the update process will guarantee that they will
 eventually be received.  At this point, the local database is deemed
 to be "synchronized".
 Since LSPs mentioned in the CSNP(s) with a zero remaining lifetime
 are not recorded, and those with a short remaining lifetime are
 deleted from the list when the lifetime expires, cancellation of the
 timer T2 will not be prevented by waiting for an LSP that will never
 arrive.

3.4.1. LSP Generation and Flooding and SPF Computation

 The operation of a router starting, as opposed to restarting, is
 somewhat different.  These two cases are dealt with separately below.

3.4.1.1. Restarting

 In order to avoid causing unnecessary routing churn in other routers,
 it is highly desirable that the router's own LSPs generated by the
 restarting system are the same as those previously present in the
 network (assuming no other changes have taken place).  It is
 important therefore not to regenerate and flood the LSPs until all
 the adjacencies have been re-established and any information required
 for propagation into the local LSPs is fully available.  Ideally, the
 information is loaded into the LSPs in a deterministic way, such that
 the same information occurs in the same place in the same LSP (and
 hence the LSPs are identical to their previous versions).  If this
 can be achieved, the new versions may not even cause SPF to be run in
 other systems.  However, provided the same information is included in
 the set of LSPs (albeit in a different order, and possibly different
 LSPs), the result of running the SPF will be the same and will not
 cause churn to the forwarding tables.
 In the case of a restarting router, none of the router's own LSPs are
 transmitted, nor are the router's own forwarding tables updated while
 the timer T3 is running.
 Redistribution of inter-level information MUST be regenerated before
 this router's LSP is flooded to other nodes.  Therefore, the Level-n
 non-pseudonode LSP(s) MUST NOT be flooded until the other level's T2
 timer has expired and its SPF has been run.  This ensures that any
 inter-level information which is to be propagated can be included in
 the Level-n LSP(s).

Shand & Ginsberg Informational [Page 13] RFC 3847 Restart signaling for IS-IS July 2004

 During this period, if one of the router's own (including
 pseudonodes) LSPs is received, which the local router does not
 currently have in its own database, it is NOT purged.  Under normal
 operation, such an LSP would be purged, since the LSP clearly should
 not be present in the global LSP database.  However, in the present
 circumstances, this would be highly undesirable, because it could
 cause premature removal of a router's own LSP - and hence churn in
 remote routers.  Even if the local system has one or more of the
 router's own LSPs (which it has generated, but not yet transmitted),
 it is still not valid to compare the received LSP against this set,
 since it may be that as a result of propagation between Level 1 and
 Level 2 (or vice versa), a further router's own LSP will need to be
 generated when the LSP databases have synchronized.
 During this period a restarting router SHOULD send CSNPs as it
 normally would.  Information about the router's own LSPs MAY be
 included, but if it is included it MUST be based on LSPs which have
 been received, not on versions which have been generated (but not yet
 transmitted).  This restriction is necessary to prevent premature
 removal of an LSP from the global LSP database.
 When the timer T2 expires or is cancelled indicating that
 synchronization for that level is complete, the SPF for that level is
 run in order to derive any information which is required to be
 propagated to another level, but the forwarding tables are not yet
 updated.
 Once the other level's SPF has run and any inter-level propagation
 has been resolved, the router's own LSPs can be generated and
 flooded.  Any own LSPs which were previously ignored, but which are
 not part of the current set of own LSPs (including pseudonodes) MUST
 then be purged.  Note that it is possible that a Designated Router
 change may have taken place, and consequently the router SHOULD purge
 those pseudonode LSPs which it previously owned, but which are now no
 longer part of its set of pseudonode LSPs.
 When all the T2 timers have expired or been cancelled, the timer T3
 is cancelled and the local forwarding tables are updated.
 If the timer T3 expires before all the T2 timers have expired or been
 cancelled, this indicates that the synchronization process is taking
 longer than the minimum holding time of the neighbors.  The router's
 own LSP(s) for levels which have not yet completed their first SPF
 computation are then flooded with the overload bit set to indicate
 that the router's LSPDB is not yet synchronized (and therefore other
 routers MUST NOT compute routes through this router).  Normal
 operation of the update process resumes and the local forwarding
 tables are updated.  In order to prevent the neighbor's adjacencies

Shand & Ginsberg Informational [Page 14] RFC 3847 Restart signaling for IS-IS July 2004

 from expiring, IIHs with the normal interface value for the holding
 time are transmitted over all interfaces with neither RR nor RA set
 in the restart TLV.  This will cause the neighbors to refresh their
 adjacencies.  The router's own LSP(s) will continue to have the
 overload bit set until timer T2 has expired or been cancelled.

3.4.1.2. Starting

 In the case of a starting router, as soon as each adjacency is
 established, and before any CSNP exchanges, the router's own zeroth
 LSP is transmitted with the overload bit set.  This prevents other
 routers from computing routes through the router until it has
 reliably acquired the complete set of LSPs.  The overload bit remains
 set in subsequent transmissions of the zeroth LSP (such as will occur
 if a previous copy of the router's own zeroth LSP is still present in
 the network) while any timer T2 is running.
 When all the T2 timers have been cancelled, the router's own LSP(s)
 MAY be regenerated with the overload bit clear (assuming the router
 is not in fact overloaded, and there is no other reason, such as
 incomplete BGP convergence, to keep the overload bit set) and flooded
 as normal.
 Other LSPs owned by this router (including pseudonodes) are generated
 and flooded as normal, irrespective of the timer T2.  The SPF is also
 run as normal and the RIB and FIB updated as routes become available.
 To avoid the possible formation of temporary blackholes, the starting
 router sets the SA bit in the restart TLV (as described in 3.3.2) in
 all IIHs that it sends.
 When all T2 timers have been cancelled, the starting router MUST
 transmit IIHs with the SA bit clear.

4. State Tables

 This section presents state tables which summarize the behaviors
 described in this document.  Other behaviors, in particular adjacency
 state transitions and LSP database update operation, are NOT included
 in the state tables except where this document modifies the behaviors
 described in [2] and [4].
 The states named in the columns of the tables below are a mixture of
 states that are specific to a single adjacency (ADJ suppressed, ADJ
 Seen RA, ADJ Seen CSNP) and states which are indicative of the state
 of the protocol instance (Running, Restarting, Starting, SPF Wait).

Shand & Ginsberg Informational [Page 15] RFC 3847 Restart signaling for IS-IS July 2004

 Three state tables are presented from the point of view of a running
 router, a restarting router, and a starting router.

4.1. Running Router

Event | Running | ADJ suppressed

RX RR | Maintain ADJ State |

           | Send RA              |
           | Set SRM,send CSNP    |
           |  (Note 1)            |
           | Update Hold Time,    |
           |  set Restart Mode    |
           |  (Note 2)            |

————-+———————-+————————- RX RR clr | Clr Restart mode | ————-+———————-+————————- RX SA | Suppress IS neighbor |

           |   TLV in LSP(s)      |
           | Goto ADJ Suppressed  |

————-+———————-+————————- RX SA clr | |Unsuppress IS neighbor

           |                      |   TLV in LSP(s)
           |                      |Goto Running
 Note 1: CSNPs are sent by routers in accordance with Section 3.2.1c
 Note 2: If Restart Mode clear

Shand & Ginsberg Informational [Page 16] RFC 3847 Restart signaling for IS-IS July 2004

4.2. Restarting Router

Event | Restarting | ADJ Seen | ADJ Seen | SPF Wait

          |                    |    RA     |   CSNP    |

Router | Send IIH/RR | | |

restarts  | ADJ Init           |           |           |
          | Start T1,T2,T3     |           |           |

————+——————–+———–+———–+———— RX RR | Send RA | | | ————+——————–+———–+———–+———— RX RA | Adjust T3 | | Cancel T1 |

          | Goto ADJ Seen RA   |           | Adjust T3 |

———– +——————–+———–+———–+———— RX CSNP set| Goto ADJ Seen CSNP | Cancel T1 | | ————+——————–+———–+———–+———— RX IIH w/o | Cancel T1 (Point- | | | Restart TLV| to-point only) | | | ————+——————–+———–+———–+———— T1 Expires | Send IIH/RR |Send IIH/RR|Send IIH/RR|

          | Restart T1         | Restart T1| Restart T1|

————+——————–+———–+———–+———— T1 Expires | Send IIH/ | Send IIH/ | Send IIH/ |

nth time  |   normal           |   normal  |   normal  |

————+——————–+———–+———–+———— T2 expires | Trigger SPF | | |

          | Goto SPF Wait      |           |           |

————+——————–+———–+———–+———— T3 expires | Set OL | | |

          | Flood local LSPs   |           |           |
          | Update fwd plane   |           |           |

————+——————–+———–+———–+———— LSP DB Sync| Cancel T2, and T3 | | |

          | Trigger SPF        |           |           |
          | Goto SPF wait      |           |           |

————+——————–+———–+———–+———— All SPF | | | | Clear OL

done      |                    |           |           | Update fwd
          |                    |           |           |  plane
          |                    |           |           | Flood local
          |                    |           |           |   LSPs
          |                    |           |           | Goto Running

Shand & Ginsberg Informational [Page 17] RFC 3847 Restart signaling for IS-IS July 2004

4.3. Starting Router

Event | Starting | ADJ Seen RA| ADJ Seen CSNP

Router | Send IIH/SA | |

starts     | Start T1,T2       |            |

————-+——————-+————+————— RX RR | Send RA | | ————-+——————-+————+————— RX RA | Goto ADJ Seen RA | | Cancel T1 ————-+——————-+————+————— RX CSNP Set | Goto ADJ Seen CSNP| Cancel T1 | ————-+——————-+————+————— RX IIH w | Cancel T1 | |

no Restart | (Point-to-Point   |            |
TLV        |   only)           |            |

————-+——————-+————+————— ADJ UP | Start T1 | |

           | Send local LSPs   |            |
           |  w OL             |            |

————-+——————-+————+————— T1 Expires | Send IIH/RR |Send IIH/RR | Send IIH/RR

           |   and SA          |   and SA   |   and SA
           | Restart T1        |Restart T1  | Restart T1

————-+——————-+————+————— T1 Expires | Send IIH/SA |Send IIH/SA | Send IIH/SA nth time | | | ————-+——————-+————+————— T2 expires | Clear OL | |

           | Send IIH normal   |            |
           | Goto Running      |            |

————-+——————-+————+————— LSP DB Sync | Cancel T2 | |

           | Clear OL          |            |
           | Send IIH normal   |            |

5. Security Considerations

 Any new security issues raised by the procedures in this document
 depend upon the ability of an attacker to inject a false but
 apparently valid IIH, the ease/difficulty of which has not been
 altered.

Shand & Ginsberg Informational [Page 18] RFC 3847 Restart signaling for IS-IS July 2004

 If the RR bit is set in a false IIH, neighbors who receive such an
 IIH will continue to maintain an existing adjacency in the "UP" state
 and may (re)send a complete set of CSNPs.  While the latter action is
 wasteful, neither action causes any disruption in correct protocol
 operation.
 If the RA bit is set in a false IIH, a (re)starting router which
 receives such an IIH may falsely believe that there is a neighbor on
 the corresponding interface which supports the procedures described
 in this document.  In the absence of receipt of a complete set of
 CSNPs on that interface, this could delay the completion of (re)start
 procedures by requiring the timer T1 to time out the locally defined
 maximum number of retries.  This behavior is the same as would occur
 on a LAN where none of the (re)starting router's neighbors support
 the procedures in this document and is covered in Sections 3.3.1 and
 3.3.2.
 If an SA bit is set in a false IIH, this could cause suppression of
 the advertisement of an IS neighbor which could either continue for
 an indefinite period, or occur intermittently with the result being a
 possible loss of reachability to some destinations in the network
 and/or increased frequency of LSP flooding and SPF calculation.
 The possibility of IS-IS PDU spoofing can be reduced by the use of
 authentication as described in [1] and [2], and especially the use of
 cryptographic authentication as described in [5].

6. IANA Considerations

 This document defines the following IS-IS TLV that is listed in the
 IS-IS TLV code-point registry:
 Type        Description                            IIH   LSP   SNP
 ----        -----------------------------------    ---   ---   ---
 211         Restart TLV                              y     n     n

7. Normative References

 [1]  Callon, R., "OSI IS-IS for IP and Dual Environment", RFC 1195,
      December 1990.
 [2]  ISO, "Intermediate system to Intermediate system routeing
      information exchange protocol for use in conjunction with the
      Protocol for providing the Connectionless-mode Network Service
      (ISO 8473)," ISO/IEC 10589:2002, Second Edition.
 [3]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
      Levels", BCP 14, RFC 2119, March 1997.

Shand & Ginsberg Informational [Page 19] RFC 3847 Restart signaling for IS-IS July 2004

 [4]  Katz, D. and R. Saluja, "Three-Way Handshake for IS-IS Point-
      to-Point Adjacencies", RFC 3373, September 2002.
 [5]  Li, T. and R. Atkinson, "Intermediate System to Intermediate
      System (IS-IS) Cryptographic Authentication", RFC 3567, July
      2003.

8. Acknowledgements

 The authors would like to acknowledge contributions made by Jeff
 Parker, Radia Perlman, Mark Schaefer, Naiming Shen, Nischal Sheth,
 Russ White, and Rena Yang.

9. Authors' Addresses

 Mike Shand
 Cisco Systems
 250 Longwater Avenue,
 Reading,
 Berkshire,
 RG2 6GB
 UK
 Phone: +44 208 824 8690
 EMail: mshand@cisco.com
 Les Ginsberg
 Cisco Systems
 510 McCarthy Blvd.
 Milpitas, Ca. 95035 USA
 EMail: ginsberg@cisco.com

Shand & Ginsberg Informational [Page 20] RFC 3847 Restart signaling for IS-IS July 2004

10. Full Copyright Statement

 Copyright (C) The Internet Society (2004).  This document is subject
 to the rights, licenses and restrictions contained in BCP 78, and
 except as set forth therein, the authors retain all their rights.
 This document and the information contained herein are provided on an
 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
 ENGINEERING TASK FORCE DISCLAIM 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.

Intellectual Property

 The IETF takes no position regarding the validity or scope of any
 Intellectual Property Rights or other rights that might be claimed to
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 might or might not be available; nor does it represent that it has
 made any independent effort to identify any such rights.  Information
 on the procedures with respect to rights in RFC documents can be
 found in BCP 78 and BCP 79.
 Copies of IPR disclosures made to the IETF Secretariat and any
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 The IETF invites any interested party to bring to its attention any
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 this standard.  Please address the information to the IETF at ietf-
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Acknowledgement

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

Shand & Ginsberg Informational [Page 21]

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