RFC 3847
Restart Signaling for Intermediate System to Intermediate System (IS-IS)
Pages: 21
Obsoleted by: 5306
Obsoleted by: 5306
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.
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 . . . . . . . . . . . . . . . . . . . 211. 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.
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).
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.
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.
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
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
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
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
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.
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).
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.
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).
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
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).
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
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 ======================================================================
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.
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 n7. 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.
[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
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