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Versions: (draft-shand-isis-restart) 00 01 02 03 04 05 RFC 3847

INTERNET DRAFT              IS-IS restart                  March 2003




Network Working Group                                          M. Shand
Internet Draft                                             Les Ginsberg
Expiration Date: September 2003                           Cisco Systems
                                                             March 2003






                      Restart signaling for IS-IS
                     draft-ietf-isis-restart-03.txt


Status of this Memo


   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026 [1].

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that
   other groups may also distribute working documents as Internet-
   Drafts. Internet-Drafts are draft documents valid for a maximum of
   six months and may be updated, replaced, or obsoleted by other
   documents at any time. It is inappropriate to use Internet-Drafts as
   reference material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

1.   Abstract

   The IS-IS routing protocol (RFC 1142 [2], ISO/IEC 10589 [3]) is a
   link state intra-domain routing protocol. Normally, when an IS-IS
   router is restarted, the neighboring routers detect the restart
   event and cycle their adjacencies with the restarting router through
   the down state. This is necessary in order to invoke the protocol
   mechanisms to ensure correct synchronization of the LSP database.
   However, the cycling of the adjacency state causes the neighbors to
   regenerate their LSPs describing the adjacency concerned. This in
   turn causes temporary disruption of routes passing through the
   restarting router.

   In certain scenarios such temporary disruption of the routes is
   highly undesirable.


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   This draft 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.

   When such a router is restarted, it is highly desirable that it does
   not recompute its own routes until it has achieved database
   synchronization with its neighbors. Recomputing its routes before
   synchronization is achieved will result in its own routes being
   temporarily incorrect.

   This draft additionally describes a mechanism for a restarting
   router to determine when it has achieved synchronization with its
   neighbors.

   This draft additionally describes a mechanism to optimize database
   synchronization and minimize transient routing disruption when a
   router starts.

2.   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 RFC-2119 [4].

   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 control function restart. 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 been started for the first time or
   has been restarted 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.

3.   Overview

   There are two related problems with the existing specification of
   IS-IS with regard to synchronization of LSP databases when a router
   is restarted.

   Firstly, when a routing process restarts and an adjacency to a
   neighboring router is reinitialized the neighboring routing process
   does three things:

     1. It reinitializes the adjacency and 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).

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     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.

   Secondly, whether or not the router is being restarted, it is
   desirable to be able to determine when the LSP databases of the
   neighboring routers have been synchronized (so that the overload bit
   can be cleared in the router's own LSP, for example). This document
   describes modifications to achieve this.

   It is assumed that the three-way handshake [5] is being used on
   Point-to-Point circuits.

4.   Approach

4.1    Timers

   Three additional timers, T1, T2 and T3 are required to support the
   functionality defined in this document.

   An instance of 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 T2 is maintained for each LSP database present in the
   system i.e. for a Level1/2 system, there will be an instance of 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 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 RA set.

4.2    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.



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     Type   211
     Length 3
     Value (3 octets)
        Flags (1 octet)
               Bit 1 - Restart Request (RR)
               Bit 2 - Restart Acknowledgment (RA)
               Bit 3 û Suppress adjacency advertisement(SA)
               Bits 4-8 û Reserved
       Remaining Time (2 octets)
               Remaining holding time (in seconds)
               (note: only required when RA bit is set)

4.2.1      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, and to request a set of CSNPs.

   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 Adjacency
   State" option (Point-to-Point circuits):

   a) The state of the adjacency is not changed. It is an
     implementation choice whether or not the holding time of the
     adjacency is refreshed. Not refreshing the holding time preserves
     the intention of the original holding time. Refreshing it may
     allow a longer grace period for the completion of the (re)start
     process. Whichever option is chosen, the "remaining time"
     transmitted according to (b) below MUST reflect the actual time
     after which the adjacency will now expire.

   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, having updated the "Point-to-
     Point Adjacency State" 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. This
     IIH SHOULD be transmitted before any LSPs or SNPs transmitted as a
     result of the receipt of the original IIH.

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   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
     whose IIHs contain the restart TLV, excluding the transmitting
     router (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.

4.2.2      Use of 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 neighbors 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 the neighbor of a starting 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
   advertisement of the adjacency to the starting router in LSPs should
   be suppressed. Until an IIH with the SA bit clear has been received,
   the adjacency advertisement should continue to be suppressed. If the
   adjacency transitions to the UP state, the new adjacency should not
   be advertised until an IIH with the SA bit clear has been received.

4.3    Adjacency (re)acquisition

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



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4.3.1      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 4.2.1 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 timer T2
   for each LSPDB and for each interface (and in the case of a LAN
   circuit, for each level) starts a timer T1 and transmits an IIH
   containing the restart TLV with the RR bit set.

   On a Point-to-Point circuit the "Point-to-Point Adjacency State"
   SHOULD be set to "Init", because the receipt of the acknowledging
   IIH (with RA set) MUST cause the adjacency to enter "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
   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 w 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 reinitialization). On expiry of the timer T1, it is
   restarted and the IIH is retransmitted as above.

   On receipt of an IIH by the restarting router, a local adjacency is
   established as usual, and if the IIH contains a restart TLV with the
   RA bit set, the receipt of the acknowledgement over that interface
   is noted.

   T3 is set to the minimum of its current value and the value of the
   "Remaining Time" field in the received IIH.

   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. In this case the neighbor will have reinitialized
   the adjacency as normal, which in the case of a Point-to-Point link

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   will guarantee that SRMflags have been set on its database, thus
   ensuring eventual LSPDB synchronization. In the case of a LAN
   interface, the usual operation of the update process will also
   ensure that synchronization is eventually achieved. However, since
   no CSNP is guaranteed to be received over this interface, T1 is
   cancelled immediately without waiting for a CSNP. Synchronization
   may therefore be deemed complete even though there are some LSPs
   which are held(only) by this neighbor (see section 4.4).

   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
   handshake information. The presence of "Neighbor System ID" or
   "Neighbor Extended Local Circuit ID" information which does not
   match the values currently in use by the local system is ignored
   (since the IIH may have been transmitted before the neighbor had
   received the new values 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 information in the Intermediate
   Systems Neighbors option is recorded and used for the generation of
   subsequent IIHs as normal.

   When BOTH a complete set of CSNP(s) (for each active level, in the
   case of a pt-pt circuit) and an acknowledgement have been received
   over the interface, the timer T1 is cancelled.

   Once T3 has expired or 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 4.4.1.1 the SPF would never be run.
   Therefore timer T1 is cancelled after some pre-determined number of
   expirations (which MAY be 1). (By this time any existing adjacency
   on a remote system would probably have expired anyway.)

   A router which supports restart SHOULD ensure that the holding time
   of any IIHs it transmits is greater than the expected time to
   complete a restart. However, where this is impracticable or
   undesirable a router MAY transmit one or more normal IIHs
   (containing a restart TLV with RR and RA clear) after the initial
   RR/RA exchange, but before synchronization has been achieved, in
   order to extend the holding time of the neighbors adjacencies beyond
   that indicated in the remaining time field of the neighbors IIH with
   the RA bit set.

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4.3.2      Adjacency acquisition during start

   The starting router wants to ensure that in the event a neighboring
   router has an adjacency to the starting router in the UP state (from
   a previous incarnation of the starting router) that 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 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.

   On receipt of an IIH with SA bit set the behavior described in 4.2.2
   is followed.

   On starting, a router initializes the timer T3, and 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. On 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).

   On receipt of an IIH with RR bit set(regardless of whether SA is set
   or not) the behavior described in 4.2.1 is followed.

   When an IIH is received by the starting router and the IIH contains
   a restart TLV with the RA bit set, the receipt of the
   acknowledgement over that interface is noted.

   T3 is set to the minimum of its current value and the value of the
   "Remaining Time" field in the received IIH.

   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. In this case the neighbor will have reinitialized
   the adjacency as normal, which in the case of a Point-to-Point link
   will guarantee that SRMflags have been set on its database, thus
   ensuring eventual LSPDB synchronization. In the case of a LAN
   interface, the usual operation of the update process will also
   ensure that synchronization is eventually achieved. However, since
   no CSNP is guaranteed to be received over this interface, T1 is
   cancelled immediately without waiting for a CSNP. Synchronization


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   may therefore be deemed complete even though there are some LSPs
   which are held(only) by this neighbor (see section 4.4).

   When BOTH a complete set of CSNP(s) (for each active level, in the
   case of a pt-pt 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.

   Once T3 has expired or been cancelled, subsequent IIHs are
   transmitted according to the normal algorithms, but including the
   restart TLV with RR, RA, and SA bits clear.

   Timer T1 is cancelled after some pre-determined number of
   expirations (which MAY be 1).

   During the period when T1 is active, according to the rules defined
   in 4.3 the neighbor of the starting router may choose not to update
   the holding time for an adjacency because the RR bit is set in the
   received IIH. To prevent holding time expiration a starting router
   MAY transmit one or more IIHs containing a restart TLV with RR and
   RA bits clear and SA bit set after the initial RR/RA exchange.

   When T2 is cancelled or expires transmission of "normal" IIHs (with
   RR,RA, and SA bits clear) will begin.

4.3.3      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 pt-pt interface, only a single IIH (indicating support for both
   levels) is required, but it MUST perform the CSNP synchronizations
   independently for each level.

4.4    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 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.



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   When (re)starting, a router starts the timer T3 and an instance of
   timer T2 for each LSPDB as described in 4.3.1 or 4.3.2. 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. 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 the LSP had arrived 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 T1 has been cancelled for all the
   interfaces that have an adjacency at this level, the timer T2 is
   cancelled.

   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) which 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.

4.4.1      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.



4.4.1.1.         Restarting

   In order to avoid causing unnecessary routing churn in other
   routers, it is highly desirable that the 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 should be 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

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   previous versions). If this can be achieved, the new versions will
   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 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) should 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 that should be propagated can be included in
   the Level-n LSP(s).

   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 an own LSP -- and hence churn in remote
   routers. Even if the local system has one or more 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 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 '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) should then be
   purged. Note that it is possible that a Designated Router change may
   have taken place, and consequently the router should purge those


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   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 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 other
   routers should therefore 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 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 own LSP(s) will continue to have the
   overload bit set until timer T2 has expired or been cancelled.

4.4.1.2.         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 routers LSP is still present in the
   network) while any timer T2 is running.

   When all the T2 timers have been cancelled, the own LSP(s) MAY be
   regenerated with the overload bit clear (assuming the router isn't
   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 'own' LSPs (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 4.3.2) in
   all IIHs that it sends.

   When all T2 timers have been cancelled, the starting router MUST
   transmit IIHs with the SA bit clear.





Shand, Ginsberg            Expires Sep 2003                  [Page 12]

INTERNET DRAFT              IS-IS restart                    Mar 2003


5.   Security Considerations

   This memo does not create any new security issues for the IS-IS
   protocol. Security considerations for the base IS-IS protocol are
   covered in [2] and [3].

6.   References


   1  Bradner, S., "The Internet Standards Process -- Revision 3", BCP
      9, RFC 2026, October 1996.

   2  Callon, R., "OSI IS-IS for IP and Dual Environment," RFC 1195,
      December 1990.

   3  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.

   4  Bradner, S., "Key words for use in RFCs to Indicate Requirement
      Levels", BCP 14, RFC 2119, March 1997

   5  Katz, D., "Three-Way Handshake for IS-IS Point-to-Point
      Adjacencies", RFC 3373, September 2002

7.   Acknowledgments

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

8.   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



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