draft-ietf-isis-restart-05.txt   rfc3847.txt 
Network Working Group M. Shand Network Working Group M. Shand
Internet Draft L. Ginsberg Request for Comments: 3847 L. Ginsberg
Expiration Date: July 2004 Cisco Systems Category: Informational Cisco Systems
January 2004 July 2004
Restart signaling for IS-IS Restart Signaling for
draft-ietf-isis-restart-05.txt Intermediate System to Intermediate System (IS-IS)
Status of this Memo Status of this Memo
This document is an Internet-Draft and is in full conformance with This memo provides information for the Internet community. It does
all provisions of Section 10 of RFC 2026. not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
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Copyright Notice Copyright (C) The Internet Society (2003). All Copyright (C) The Internet Society (2004). All Rights Reserved.
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Abstract Abstract
The IS-IS routing protocol (RFC 1195, ISO/IEC 10589) is a link state This document describes a mechanism for a restarting router to signal
intra-domain routing protocol. Normally, when an IS-IS router is to its neighbors that it is restarting, allowing them to reestablish
restarted, temporary disruption of routing occurs due to events in their adjacencies without cycling through the down state, while still
both the restarting router and the neighbors of the restarting correctly initiating database synchronization.
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 temporary disruption of routes passing through the
restarting router.
In certain scenarios the temporary disruption of the routes is This document additionally describes a mechanism for a restarting
highly undesirable. This draft describes mechanisms to avoid or router to determine when it has achieved LSP database synchronization
minimize the disruption due to both of these causes. with its neighbors and a mechanism to optimize LSP database
synchronization, while minimizing transient routing disruption when a
router starts.
Table of Contents Table of Contents
1. Conventions used in this document..............................3 1. Conventions used in this Document. . . . . . . . . . . . . . . 2
2. Overview.......................................................3 2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Approach.......................................................4 3. Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1 Timers.......................................................4 3.1. Timers . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.2 Restart TLV..................................................4 3.2. Restart TLV. . . . . . . . . . . . . . . . . . . . . . . 5
3.2.1 Use of RR and RA bits.....................................5 3.2.1. Use of RR and RA Bits. . . . . . . . . . . . . . 6
3.2.2 Use of SA bit.............................................7 3.2.2. Use of SA Bit. . . . . . . . . . . . . . . . . . 7
3.3 Adjacency (re)acquisition....................................8 3.3. Adjacency (re)Acquisition. . . . . . . . . . . . . . . . 8
3.3.1 Adjacency reacquisition during restart....................8 3.3.1. Adjacency Reacquisition During Restart . . . . . 8
3.3.2 Adjacency acquisition during start.......................10 3.3.2. Adjacency Acquisition During Start . . . . . . . 10
3.3.3 Multiple levels..........................................11 3.3.3. Multiple Levels. . . . . . . . . . . . . . . . . 12
3.4 Database synchronization....................................12 3.4. Database Synchronization . . . . . . . . . . . . . . . . 12
3.4.1 LSP generation and flooding and SPF computation..........12 3.4.1. LSP Generation and Flooding and SPF Computation. 13
3.4.1.1. Restarting..........................................13 3.4.1.1. Restarting. . . . . . . . . . . . . . . 13
3.4.1.2. Starting............................................14 3.4.1.2. Starting. . . . . . . . . . . . . . . . 15
4. State Tables..................................................16 4. State Tables . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.1 Running Router..............................................16 4.1. Running Router . . . . . . . . . . . . . . . . . . . . . 16
4.2 Restarting Router...........................................17 4.2. Restarting Router. . . . . . . . . . . . . . . . . . . . 17
4.3 Starting Router.............................................18 4.3. Starting Router. . . . . . . . . . . . . . . . . . . . . 18
5. Security Considerations.......................................19 5. Security Considerations. . . . . . . . . . . . . . . . . . . . 18
6. IANA Considerations...........................................19 6. IANA Considerations. . . . . . . . . . . . . . . . . . . . . . 19
7. Normative References..........................................20 7. Normative References . . . . . . . . . . . . . . . . . . . . . 19
8. Acknowledgments...............................................20 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 20
9. Authors' Addresses............................................20 9. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 20
10. Full Copyright Statement.....................................21 10. Full Copyright Statement . . . . . . . . . . . . . . . . . . . 21
1. Conventions used in this document 1. Conventions used in this Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
this document are to be interpreted as described in RFC-2119 [4]. document are to be interpreted as described in BCP 14, RFC-2119 [3].
If the control and forwarding functions in a router can be If the control and forwarding functions in a router can be maintained
maintained independently, it is possible for the forwarding function independently, it is possible for the forwarding function state to be
state to be maintained across a control function restart. This maintained across a resumption of control function operations. This
functionality is assumed when the terms "restart/restarting" are functionality is assumed when the terms "restart/restarting" are used
used in this document. in this document.
The terms "start/starting" are used to refer to a router in which The terms "start/starting" are used to refer to a router in which the
the control function has either been started for the first time or control function has either commenced operations for the first time
has been restarted but the forwarding functions have not been or has resumed operations but the forwarding functions have not been
maintained in a prior state. maintained in a prior state.
The terms "(re)start/(re)starting" are used when the text is The terms "(re)start/(re)starting" are used when the text is
applicable to both a "starting" and a "restarting" router. applicable to both a "starting" and a "restarting" router.
2. Overview 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 When an adjacency is reinitialized as a result of a neighbor
restarting, a router does three things: restarting, a router does three things:
1. It causes its own LSP(s) to be regenerated, thus triggering 1. It causes its own LSP(s) to be regenerated, thus triggering SPF
SPF runs throughout the area (or in the case of Level 2, runs throughout the area (or in the case of Level 2, throughout
throughout the domain). the domain).
2. It sets SRMflags on its own LSP database on the adjacency 2. It sets SRMflags on its own LSP database on the adjacency
concerned. concerned.
3. In the case of a Point-to-Point link it transmits a (set of) 3. In the case of a Point-to-Point link, it transmits a (set of)
CSNP(s) over the adjacency. CSNP(s) over the adjacency.
In the case of a restarting router process, the first of these is In the case of a restarting router process, the first of these is
highly undesirable, but the second is essential in order to ensure highly undesirable, but the second is essential in order to ensure
synchronization of the LSP database. synchronization of the LSP database.
The third action above minimizes the number of LSPs which must be The third action above minimizes the number of LSPs which must be
exchanged and, if made reliable, provides a means of determining exchanged and, if made reliable, provides a means of determining when
when the LSP databases of the neighboring routers have been the LSP databases of the neighboring routers have been synchronized.
synchronized. This is desirable whether the router is being This is desirable whether the router is being restarted or not (so
restarted or not (so that the overload bit can be cleared in the that the overload bit can be cleared in the router's own LSP, for
router's own LSP, for example). example).
This draft describes a mechanism for a restarting router to signal This document describes a mechanism for a restarting router to signal
that it is restarting to its neighbors, and allow them to that it is restarting to its neighbors, and allow them to reestablish
reestablish their adjacencies without cycling through the down their adjacencies without cycling through the down state, while still
state, while still correctly initiating database synchronization. correctly initiating database synchronization.
This draft additionally describes a mechanism for a restarting This document additionally describes a mechanism for a restarting
router to determine when it has achieved LSP database router to determine when it has achieved LSP database synchronization
synchronization with its neighbors and a mechanism to optimize LSP with its neighbors and a mechanism to optimize LSP database
database synchronization and minimize transient routing disruption synchronization and minimize transient routing disruption when a
when a router starts. router starts.
It is assumed that the three-way handshake [5] is being used on It is assumed that the three-way handshake [4] is being used on
Point-to-Point circuits. Point-to-Point circuits.
3. Approach 3. Approach
3.1 Timers 3.1. Timers
Three additional timers, T1, T2 and T3 are required to support the Three additional timers, T1, T2, and T3 are required to support the
functionality defined in this document. functionality defined in this document.
An instance of the timer T1 is maintained per interface, and An instance of the timer T1 is maintained per interface, and
indicates the time after which an unacknowledged (re)start attempt indicates the time after which an unacknowledged (re)start attempt
will be repeated. A typical value might be 3 seconds. will be repeated. A typical value might be 3 seconds.
An instance of the timer T2 is maintained for each LSP database 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 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. 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 This is the maximum time that the system will wait for LSPDB
synchronization. A typical value might be 60 seconds. synchronization. A typical value might be 60 seconds.
A single instance of the timer T3 is maintained for the entire A single instance of the timer T3 is maintained for the entire
system. It indicates the time after which the router will declare system. It indicates the time after which the router will declare
that it has failed to achieve database synchronization (by setting that it has failed to achieve database synchronization (by setting
the overload bit in its own LSP). This is initialized to 65535 the overload bit in its own LSP). This is initialized to 65535
seconds, but is set to the minimum of the remaining times of seconds, but is set to the minimum of the remaining times of received
received IIHs containing a restart TLV with RA set and an indication IIHs containing a restart TLV with the RA set and an indication that
that the neighbor has an adjacency in the UP state to the restarting the neighbor has an adjacency in the "UP" state to the restarting
router. router.
NOTE: The timer T3 is only used by a restarting router. NOTE: The timer T3 is only used by a restarting router.
3.2 Restart TLV 3.2. Restart TLV
A new TLV is defined to be included in IIH PDUs. The presence of A new TLV is defined to be included in IIH PDUs. The presence of
this TLV indicates that the sender supports the functionality this TLV indicates that the sender supports the functionality defined
defined in this document and it carries flags that are used to in this document and it carries flags that are used to convey
convey information during a (re)start. All IIHs transmitted by a information during a (re)start. All IIHs transmitted by a router
router that supports this capability MUST include this TLV. that supports this capability MUST include this TLV.
Type 211 Type 211
Length # of octets in the value field (1 to (3 + ID Length)) Length # of octets in the value field (1 to (3 + ID Length))
Value Value
No. of octets No. of octets
+-----------------------+ +-----------------------+
| Flags | 1 | Flags | 1
+-----------------------+ +-----------------------+
| Remaining Time | 2 | Remaining Time | 2
skipping to change at page 5, line 26 skipping to change at page 5, line 34
+-----------------------+ +-----------------------+
Flags (1 octet) Flags (1 octet)
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
+--+--+--+--+--+--+--+--+ +--+--+--+--+--+--+--+--+
| Reserved |SA|RA|RR| | Reserved |SA|RA|RR|
+--+--+--+--+--+--+--+--+ +--+--+--+--+--+--+--+--+
RR - Restart Request RR - Restart Request
RA - Restart Acknowledgment RA - Restart Acknowledgement
SA - Suppress adjacency advertisement SA - Suppress adjacency advertisement
(Note: Remaining fields are required when RA bit is set) (Note: Remaining fields are required when the RA bit is set)
Remaining Time (2 octets) Remaining Time (2 octets)
Remaining holding time (in seconds) Remaining holding time (in seconds)
Restarting Neighbor System ID (ID Length octets) Restarting Neighbor System ID (ID Length octets)
The system ID of the neighbor to which an RA refers. Note: The system ID of the neighbor to which an RA refers. Note:
Implementations based on earlier versions of this document Implementations based on earlier versions of this document may not
may not include this field in the TLV when RA is set. In include this field in the TLV when the RA is set. In this case, a
this case a router which is expecting an RA on a LAN router which is expecting an RA on a LAN circuit SHOULD assume that
circuit SHOULD assume that the acknowledgement is directed the acknowledgement is directed at the local system.
at the local system.
3.2.1 Use of RR and RA bits 3.2.1. Use of RR and RA Bits
The RR bit is used by a (re)starting router to signal to its 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 neighbors that a (re)start is in progress, that an existing adjacency
adjacency SHOULD be maintained even under circumstances when the SHOULD be maintained even under circumstances when the normal
normal operation of the adjacency state machine would require the operation of the adjacency state machine would require the adjacency
adjacency to be reinitialized, to request a set of CSNPs, and to to be reinitialized, to request a set of CSNPs, and to request
request setting of SRMflags. setting of the SRMflags.
The RA bit is sent by the neighbor of a (re)starting router to 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. 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 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 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 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, of a LAN circuit, with the same source LAN address, then,
irrespective of the other contents of the "Intermediate System irrespective of the other contents of the "Intermediate System
Neighbors" option (LAN circuits), or the "Point-to-Point Three-Way Neighbors" option (LAN circuits) or the "Point-to-Point Three-Way
Adjacency" option (Point-to-Point circuits): Adjacency" option (Point-to-Point circuits):
a) The state of the adjacency is not changed. If this is the first 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 IIH with the RR bit set that this system has received associated
with this adjacency then the adjacency is marked as being in with this adjacency, then the adjacency is marked as being in
"Restart mode" and the adjacency holding time is refreshed - "Restart mode" and the adjacency holding time is refreshed -
otherwise the holding time is not refreshed. The "remaining time" otherwise the holding time is not refreshed. The "remaining time"
transmitted according to (b) below MUST reflect the actual time transmitted according to (b) below MUST reflect the actual time
after which the adjacency will now expire. Receipt of a normal after which the adjacency will now expire. Receipt of a normal
IIH with RR bit reset will clear the "Restart mode" state. This IIH with the RR bit reset will clear the "Restart mode" state.
procedure allows the restarting router to cause the neighbor to This procedure allows the restarting router to cause the neighbor
maintain the adjacency long enough for restart to successfully to maintain the adjacency long enough for restart to successfully
complete while also preventing repetitive restarts from complete while also preventing repetitive restarts from
maintaining an adjacency indefinitely. Whether an adjacency is maintaining an adjacency indefinitely. Whether an adjacency is
marked as being in "Restart mode" or not has no effect on marked as being in "Restart mode" or not has no effect on
adjacency state transitions. adjacency state transitions.
b) immediately (i.e. without waiting for any currently running timer 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 interval to expire, but with a small random delay of a few 10s of
milliseconds on LANs to avoid "storms"), transmit over the milliseconds on LANs to avoid "storms") transmit over the
corresponding interface an IIH including the restart TLV with 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 RR bit clear and the RA bit set, in the case of Point-to-Point
adjacencies having updated the "Point-to-Point Three-Way adjacencies having updated the "Point-to-Point Three-Way
Adjacency" option to reflect any new values received from the Adjacency" option to reflect any new values received from the
(re)starting router. (This allows a restarting router to quickly (re)starting router. (This allows a restarting router to quickly
acquire the correct information to place in its hellos.) The acquire the correct information to place in its hellos.) The
"Remaining Time" MUST be set to the current time (in seconds) "Remaining Time" MUST be set to the current time (in seconds)
before the holding timer on this adjacency is due to expire. If before the holding timer on this adjacency is due to expire. If
the corresponding interface is a LAN interface, then the the corresponding interface is a LAN interface, then the
Restarting Neighbor System ID SHOULD be set to the System ID of Restarting Neighbor System ID SHOULD be set to the System ID of
the router from whom the IIH with RR bit set was received. This the router from whom the IIH with the RR bit set was received.
is required to correctly associate the acknowledgement and This is required to correctly associate the acknowledgement and
holding time in the case where multiple systems on a LAN restart holding time in the case where multiple systems on a LAN restart
at approximately the same time. This IIH SHOULD be transmitted at approximately the same time. This IIH SHOULD be transmitted
before any LSPs or SNPs transmitted as a result of the receipt of before any LSPs or SNPs are transmitted as a result of the receipt
the original IIH. of the original IIH.
c) if the corresponding interface is a Point-to-Point interface, or c) if the corresponding interface is a Point-to-Point interface, or
if the receiving router has the highest LnRouterPriority (with if the receiving router has the highest LnRouterPriority (with
highest source MAC address breaking ties) among those routers to highest source MAC address breaking ties) among those routers to
which the receiving router has an adjacency in state "Up" on this which the receiving router has an adjacency in state "UP" on this
interface whose IIHs contain the restart TLV, excluding interface whose IIHs contain the restart TLV, excluding
adjacencies to all routers which are considered in "Restart mode" adjacencies to all routers which are considered in "Restart mode"
(note the actual DIS is NOT changed by this process), initiate (note the actual DIS is NOT changed by this process), initiate the
the transmission over the corresponding interface of a complete transmission over the corresponding interface of a complete set of
set of CSNPs, and set SRMflags on the corresponding interface for CSNPs, and set SRMflags on the corresponding interface for all
all LSPs in the local LSP database. LSPs in the local LSP database.
Otherwise (i.e. if there was no adjacency in the "UP" state to the 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 system ID in question), process the IIH as normal by reinitializing
the adjacency, and setting the RA bit in the returned IIH. the adjacency and setting the RA bit in the returned IIH.
3.2.2 Use of SA bit 3.2.2. Use of the SA Bit
The SA bit is used by a starting router to request that its neighbor The SA bit is used by a starting router to request that its neighbor
suppress advertisement of the adjacency to the starting router in suppress advertisement of the adjacency to the starting router in the
the neighbor's LSPs. neighbor's LSPs.
A router which is starting has no maintained forwarding function A router which is starting has no maintained forwarding function
state. This may or may not be the first time the router has started. 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 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 generated by this router in its previous incarnation may exist in the
the LSP databases of other routers in the network. These copies are LSP databases of other routers in the network. These copies are
likely to appear "newer" than LSPs initially generated by the likely to appear "newer" than LSPs initially generated by the
starting router due to the reinitialization of LSP fragment sequence starting router due to the reinitialization of LSP fragment sequence
numbers by the starting router. This may cause temporary blackholes numbers by the starting router. This may cause temporary blackholes
to occur until the normal operation of the update process causes the to occur until the normal operation of the update process causes the
starting router to regenerate and flood copies of its own LSPs with starting router to regenerate and flood copies of its own LSPs with
higher sequence numbers. The temporary blackholes can be avoided if higher sequence numbers. The temporary blackholes can be avoided if
the starting router's neighbors suppress advertising an adjacency to the starting router's neighbors suppress advertising an adjacency to
the starting router until the starting router has been able to the starting router until the starting router has been able to
propagate newer versions of LSPs generated by previous incarnations. propagate newer versions of LSPs generated by previous incarnations.
When a router receives an IIH with the restart TLV having the SA bit 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" 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 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 same source LAN address, then the router MUST suppress advertisement
of the adjacency to the neighbor in its own LSPs. Until an IIH with 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 the SA bit clear has been received, the neighbor advertisement MUST
continue to be suppressed. If the adjacency transitions to the UP continue to be suppressed. If the adjacency transitions to the "UP"
state, the new adjacency MUST NOT be advertised until an IIH with state, the new adjacency MUST NOT be advertised until an IIH with the
the SA bit clear has been received. SA bit clear has been received.
Note that a router which suppresses advertisement of an adjacency Note that a router which suppresses advertisement of an adjacency
MUST NOT use this adjacency when performing its SPF calculation. In MUST NOT use this adjacency when performing its SPF calculation. In
particular, if an implementation follows the example guidelines particular, if an implementation follows the example guidelines
presented in [3] Annex C.2.5 Step 0:b) "pre-load TENT with the local 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 adjacency database", the suppressed adjacency MUST NOT be loaded into
into TENT. TENT.
3.3 Adjacency (re)acquisition 3.3. Adjacency (Re)Acquisition
Adjacency (re)acquisition is the first step in (re)initialization. Adjacency (re)acquisition is the first step in (re)initialization.
Restarting and starting routers will make use of the RR bit in the 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 restart TLV, though each will use it at different stages of the
(re)start procedure. (re)start procedure.
3.3.1 Adjacency reacquisition during restart 3.3.1. Adjacency Reacquisition During Restart
The restarting router explicitly notifies its neighbor that the The restarting router explicitly notifies its neighbor that the
adjacency is being reacquired, and hence that it SHOULD NOT adjacency is being reacquired, and hence that it SHOULD NOT
reinitialize the adjacency. This is achieved by setting the RR bit reinitialize the adjacency. This is achieved by setting the RR bit
in the restart TLV. When the neighbor of a restarting router 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 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 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 System ID, and in the case of a LAN circuit, with the same source LAN
LAN address, then the procedures described in 4.2.1 are followed. address, then the procedures described in 3.2.1 are followed.
A router that does not support the restart capability will ignore A router that does not support the restart capability will ignore the
the restart TLV and reinitialize the adjacency as normal, returning restart TLV and reinitialize the adjacency as normal, returning an
an IIH without the restart TLV. IIH without the restart TLV.
On restarting, a router initializes the timer T3, starts the timer On restarting, a router initializes the timer T3, starts the timer T2
T2 for each LSPDB and for each interface (and in the case of a LAN 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 circuit, for each level) starts the timer T1 and transmits an IIH
containing the restart TLV with the RR bit set. containing the restart TLV with the RR bit set.
On a Point-to-Point circuit the restarting router SHOULD set the On a Point-to-Point circuit the restarting router SHOULD set the
"Adjacency Three-Way State" to "Init", because the receipt of the "Adjacency Three-Way State" to "Init", because the receipt of the
acknowledging IIH (with RA set) MUST cause the adjacency to enter acknowledging IIH (with RA set) MUST cause the adjacency to enter the
"Up" state immediately. "UP" state immediately.
On a LAN circuit the LAN-ID assigned to the circuit SHOULD be the 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 same as that used prior to the restart. In particular, for any
circuits for which the restarting router was previously DIS, the use 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 a different LAN-ID would necessitate the generation of a new set
of pseudonode LSPs, and corresponding changes in all the LSPs of pseudonode LSPs, and corresponding changes in all the LSPs
referencing them from other routers on the LAN. By preserving the referencing them from other routers on the LAN. By preserving the
LAN-ID across the restart, this churn can be prevented. To enable a 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 restarting router to learn the LAN-ID used prior to restart, the
LAN-ID specified in an IIH with RR set MUST be ignored. LAN-ID specified in an IIH with RR set MUST be ignored.
Transmission of "normal" IIHs is inhibited until the conditions Transmission of "normal" IIHs is inhibited until the conditions
described below are met (in order to avoid causing an unnecessary described below are met (in order to avoid causing an unnecessary
adjacency initialization). On expiry of the timer T1, it is adjacency initialization). Upon expiry of the timer T1, it is
restarted and the IIH is retransmitted as above. restarted and the IIH is retransmitted as above.
When a restarting router receives an IIH a local adjacency is When a restarting router receives an IIH a local adjacency is
established as usual, and if the IIH contains a restart TLV with the 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 RA bit set (and on LAN circuits with a Restart Neighbor System ID
which matches that of the local system), the receipt of the which matches that of the local system), the receipt of the
acknowledgement over that interface is noted. When the RA bit is set 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 and the state of the remote adjacency is "UP", then the timer T3 is
to the minimum of its current value and the value of the "Remaining set to the minimum of its current value and the value of the
Time" field in the received IIH. "Remaining Time" field in the received IIH.
On a Point-to-Point link, receipt of an IIH not containing the On a Point-to-Point link, receipt of an IIH not containing the
restart TLV is also treated as an acknowledgement, since it restart TLV is also treated as an acknowledgement, since it indicates
indicates that the neighbor is not restart capable. However, since that the neighbor is not restart capable. However, since no CSNP is
no CSNP is guaranteed to be received over this interface, the timer guaranteed to be received over this interface, the timer T1 is
T1 is cancelled immediately without waiting for a complete set of cancelled immediately without waiting for a complete set of CSNP(s).
CSNP(s). Synchronization may therefore be deemed complete even Synchronization may therefore be deemed complete even though there
though there are some LSPs which are held (only) by this neighbor are some LSPs which are held (only) by this neighbor (see section
(see section 4.4). In this case we also want to be certain that the 3.4). In this case we also want to be certain that the neighbor will
neighbor will reinitialize the adjacency in order to guarantee that reinitialize the adjacency in order to guarantee that the SRMflags
SRMflags have been set on its database, thus ensuring eventual LSPDB have been set on its database, thus ensuring eventual LSPDB
synchronization. This is guaranteed to happen except in the case synchronization. This is guaranteed to happen except in the case
where the Adjacency Three-Way State in the received IIH is UP and where the Adjacency Three-Way State in the received IIH is "UP" and
the Neighbor Extended Local Circuit ID matches the extended local the Neighbor Extended Local Circuit ID matches the extended local
circuit ID assigned by the restarting router. In this case the circuit ID assigned by the restarting router. In this case the
restarting router MUST force the adjacency to reinitialize by restarting router MUST force the adjacency to reinitialize by setting
setting the local Adjacency Three-Way State to DOWN and sending a the local Adjacency Three-Way State to "DOWN" and sending a normal
normal IIH. IIH.
In the case of a LAN interface, receipt of an IIH not containing the In the case of a LAN interface, receipt of an IIH not containing the
restart TLV is unremarkable since synchronization can still occur so restart TLV is unremarkable since synchronization can still occur so
long as at least one of the non-restarting neighboring routers on long as at least one of the non-restarting neighboring routers on the
the LAN supports restart. Therefore T1 continues to run in this LAN supports restart. Therefore T1 continues to run in this case.
case. If none of the neighbors on the LAN are restart capable, T1 If none of the neighbors on the LAN are restart capable, T1 will
will eventually expire after the locally defined number of retries. eventually expire after the locally defined number of retries.
In the case of a Point-to-Point circuit, the "LocalCircuitID" and In the case of a Point-to-Point circuit, the "LocalCircuitID" and
"Extended Local Circuit ID" information contained in the IIH can be "Extended Local Circuit ID" information contained in the IIH can be
used immediately to generate an IIH containing the correct 3-way used immediately to generate an IIH containing the correct 3-way
handshake information. The presence of "Neighbor Extended Local handshake information. The presence of "Neighbor Extended Local
Circuit ID" information which does not match the value currently in Circuit ID" information which does not match the value currently in
use by the local system is ignored (since the IIH may have been use by the local system is ignored (since the IIH may have been
transmitted before the neighbor had received the new value from the transmitted before the neighbor had received the new value from the
restarting router), but the adjacency remains in the initializing restarting router), but the adjacency remains in the initializing
state until the correct information is received. state until the correct information is received.
In the case of a LAN circuit the source neighbor information (e.g. In the case of a LAN circuit, the source neighbor information (e.g.,
SNPAAddress) is recorded and used for adjacency establishment and SNPAAddress) is recorded and used for adjacency establishment and
maintenance as normal. maintenance as normal.
When BOTH a complete set of CSNP(s) (for each active level, in the 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 case of a point-to-point circuit) and an acknowledgement have been
over the interface, the timer T1 is cancelled. received over the interface, the timer T1 is cancelled.
Once the timer T3 has expired or been cancelled, subsequent IIHs are Once the timer T1 has been cancelled, subsequent IIHs are transmitted
transmitted according to the normal algorithms, but including the according to the normal algorithms, but including the restart TLV
restart TLV with both RR and RA clear. with both RR and RA clear.
If a LAN contains a mixture of systems, only some of which support If a LAN contains a mixture of systems, only some of which support
the new algorithm, database synchronization is still guaranteed, but the new algorithm, database synchronization is still guaranteed, but
the "old" systems will have reinitialized their adjacencies. the "old" systems will have reinitialized their adjacencies.
If an interface is active, but does not have any neighboring router If an interface is active, but does not have any neighboring router
reachable over that interface the timer T1 would never be cancelled, 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. 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 Therefore timer T1 is cancelled after some pre-determined number of
expirations (which MAY be 1). expirations (which MAY be 1).
3.3.2 Adjacency acquisition during start 3.3.2. Adjacency Acquisition During Start
The starting router wants to ensure that in the event a neighboring The starting router wants to ensure that in the event that a
router has an adjacency to the starting router in the UP state (from neighboring router has an adjacency to the starting router in the
a previous incarnation of the starting router) that this adjacency "UP" state (from a previous incarnation of the starting router), this
is reinitialized. The starting router also wants neighboring routers adjacency is reinitialized. The starting router also wants
to suppress advertisement of an adjacency to the starting router neighboring routers to suppress advertisement of an adjacency to the
until LSP database synchronization is achieved. This is achieved by starting router until LSP database synchronization is achieved. This
sending IIHs with the RR bit clear and the SA bit set in the restart is achieved by sending IIHs with the RR bit clear and the SA bit set
TLV. The RR bit remains clear and the SA bit remains set in in the restart TLV. The RR bit remains clear and the SA bit remains
subsequent transmissions of IIHs until the adjacency has reached the set in subsequent transmissions of IIHs until the adjacency has
UP state and the initial T1 timer interval (see below) has expired. 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 Receipt of an IIH with the RR bit clear will result in the
router utilizing normal operation of the adjacency state machine. neighboring router utilizing normal operation of the adjacency state
This will ensure that any old adjacency on the neighboring router machine. This will ensure that any old adjacency on the neighboring
will be reinitialized. router will be reinitialized.
On receipt of an IIH with SA bit set the behavior described in 3.2.2 Upon receipt of an IIH with the SA bit set, the behavior described in
is followed. 3.2.2 is followed.
On starting, a router starts timer T2 for each LSPDB. Upon starting, a router starts timer T2 for each LSPDB.
For each interface (and in the case of a LAN circuit, for each For each interface (and in the case of a LAN circuit, for each
level), when an adjacency reaches the UP state, the starting router level), when an adjacency reaches the "UP" state, the starting router
starts a timer T1 and transmits an IIH containing the restart TLV 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 with the RR bit clear and SA bit set. Upon expiry of the timer T1,
is restarted and the IIH is retransmitted with both RR and SA bits 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). 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 Upon receipt of an IIH with the RR bit set (regardless of whether the
set or not) the behavior described in 3.2.1 is followed. 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 When an IIH is received by the starting router and the IIH contains a
a restart TLV with the RA bit set (and on LAN circuits with a restart TLV with the RA bit set (and on LAN circuits with a Restart
Restart Neighbor System ID which matches that of the local system), Neighbor System ID which matches that of the local system), the
the receipt of the acknowledgement over that interface is noted. receipt of the acknowledgement over that interface is noted.
On a Point-to-Point link, receipt of an IIH not containing the On a Point-to-Point link, receipt of an IIH not containing the
restart TLV is also treated as an acknowledgement, since it restart TLV is also treated as an acknowledgement, since it indicates
indicates that the neighbor is not restart capable. Since the that the neighbor is not restart capable. Since the neighbor will
neighbor will have reinitialized the adjacency this guarantees that have reinitialized the adjacency, this guarantees that SRMflags have
SRMflags have been set on its database, thus ensuring eventual LSPDB been set on its database, thus ensuring eventual LSPDB
synchronization. However, since no CSNP is guaranteed to be received synchronization. However, since no CSNP is guaranteed to be received
over this interface, the timer T1 is cancelled immediately without over this interface, the timer T1 is cancelled immediately without
waiting for a complete set of CSNP(s). Synchronization may therefore waiting for a complete set of CSNP(s). Synchronization may therefore
be deemed complete even though there are some LSPs which are held be deemed complete even though there are some LSPs which are held
(only) by this neighbor (see section 4.4). (only) by this neighbor (see section 3.4).
In the case of a LAN interface, receipt of an IIH not containing the In the case of a LAN interface, receipt of an IIH not containing the
restart TLV is unremarkable since synchronization can still occur so restart TLV is unremarkable since synchronization can still occur so
long as at least one of the non-restarting neighboring routers on long as at least one of the non-restarting neighboring routers on the
the LAN supports restart. Therefore T1 continues to run in this LAN supports restart. Therefore T1 continues to run in this case.
case. If none of the neighbors on the LAN are restart capable, T1 If none of the neighbors on the LAN are restart capable, T1 will
will eventually expire after the locally defined number of retries. eventually expire after the locally defined number of retries. The
The usual operation of the update process will ensure that usual operation of the update process will ensure that
synchronization is eventually achieved. synchronization is eventually achieved.
When BOTH a complete set of CSNP(s) (for each active level, in the 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 case of a point-to-point circuit) and an acknowledgement have been
over the interface, the timer T1 is cancelled. Subsequent IIHs sent received over the interface, the timer T1 is cancelled. Subsequent
by the starting router have the RR and RA bits clear and the SA bit IIHs sent by the starting router have the RR and RA bits clear and
set in the restart TLV. the SA bit set in the restart TLV.
Timer T1 is cancelled after some pre-determined number of Timer T1 is cancelled after some pre-determined number of expirations
expirations (which MAY be 1). (which MAY be 1).
When the T2 timer(s) are cancelled or expire transmission of When the T2 timer(s) are cancelled or expire, transmission of
"normal" IIHs (with RR, RA, and SA bits clear) will begin. "normal" IIHs (with RR, RA, and SA bits clear) will begin.
3.3.3 Multiple levels 3.3.3. Multiple Levels
A router which is operating as both a Level 1 and a Level 2 router A router which is operating as both a Level 1 and a Level 2 router on
on a particular interface MUST perform the above operations for each a particular interface MUST perform the above operations for each
level. level.
On a LAN interface, it MUST send and receive both Level 1 and On a LAN interface, it MUST send and receive both Level 1 and Level 2
Level 2 IIHs and perform the CSNP synchronizations independently for IIHs and perform the CSNP synchronizations independently for each
each level. level.
On a pt-pt interface, only a single IIH (indicating support for both On a point-to-point interface, only a single IIH (indicating support
levels) is required, but it MUST perform the CSNP synchronizations for both levels) is required, but it MUST perform the CSNP
independently for each level. synchronizations independently for each level.
3.4 Database synchronization 3.4. Database Synchronization
When a router is started or restarted it can expect to receive a When a router is started or restarted it can expect to receive a (set
(set of) CSNP(s) over each interface. The arrival of the CSNP(s) is of) CSNP(s) over each interface. The arrival of the CSNP(s) is now
now guaranteed, since an IIH with RR bit set will be retransmitted guaranteed, since an IIH with the RR bit set will be retransmitted
until the CSNP(s) are correctly received. until the CSNP(s) are correctly received.
The CSNPs describe the set of LSPs that are currently held by each The CSNPs describe the set of LSPs that are currently held by each
neighbor. Synchronization will be complete when all these LSPs have neighbor. Synchronization will be complete when all these LSPs have
been received. been received.
When (re)starting, a router starts an instance of timer T2 for each When (re)starting, a router starts an instance of timer T2 for each
LSPDB as described in 4.3.1 or 4.3.2. In addition to normal 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 processing of the CSNPs, the set of LSPIDs contained in the first
complete set of CSNP(s) received over each interface is recorded, complete set of CSNP(s) received over each interface is recorded,
together with their remaining lifetime. In the case of a LAN together with their remaining lifetime. In the case of a LAN
interface, a complete set of CSNPs MUST consist of CSNPs received interface, a complete set of CSNPs MUST consist of CSNPs received
from neighbor(s) which are not restarting. If there are multiple from neighbor(s) which are not restarting. If there are multiple
interfaces on the (re)starting router, the recorded set of LSPIDs is interfaces on the (re)starting router, the recorded set of LSPIDs is
the union of those received over each interface. LSPs with a the union of those received over each interface. LSPs with a
remaining lifetime of zero are NOT so recorded. remaining lifetime of zero are NOT so recorded.
As LSPs are received (by the normal operation of the update process) As LSPs are received (by the normal operation of the update process)
over any interface, the corresponding LSPID entry is removed (it is over any interface, the corresponding LSPID entry is removed (it is
also removed if the LSP had arrived before the CSNP containing the also removed if an LSP arrives before the CSNP containing the
reference). When an LSPID has been held in the list for its reference). When an LSPID has been held in the list for its
indicated remaining lifetime, it is removed from the list. When the 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 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 the interfaces that have an adjacency at this level, the timer T2 is
cancelled. cancelled.
At this point the local database is guaranteed to contain all the At this point, the local database is guaranteed to contain all the
LSP(s) (either the same sequence number, or a more recent sequence LSP(s) (either the same sequence number, or a more recent sequence
number) which were present in the neighbors' databases at the time number) that were present in the neighbors' databases at the time of
of (re)starting. LSPs that arrived in a neighbor's database after (re)starting. LSPs that arrived in a neighbor's database after the
the time of (re)starting may or may not be present, but the normal time of (re)starting may or may not be present, but the normal
operation of the update process will guarantee that they will operation of the update process will guarantee that they will
eventually be received. At this point the local database is deemed eventually be received. At this point, the local database is deemed
to be "synchronized". to be "synchronized".
Since LSPs mentioned in the CSNP(s) with a zero remaining lifetime Since LSPs mentioned in the CSNP(s) with a zero remaining lifetime
are not recorded, and those with a short remaining lifetime are are not recorded, and those with a short remaining lifetime are
deleted from the list when the lifetime expires, cancellation of the 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 timer T2 will not be prevented by waiting for an LSP that will never
arrive. arrive.
3.4.1 LSP generation and flooding and SPF computation 3.4.1. LSP Generation and Flooding and SPF Computation
The operation of a router starting, as opposed to restarting is The operation of a router starting, as opposed to restarting, is
somewhat different. These two cases are dealt with separately below. somewhat different. These two cases are dealt with separately below.
3.4.1.1. Restarting 3.4.1.1. Restarting
In order to avoid causing unnecessary routing churn in other In order to avoid causing unnecessary routing churn in other routers,
routers, it is highly desirable that the own LSPs generated by the it is highly desirable that the router's own LSPs generated by the
restarting system are the same as those previously present in the restarting system are the same as those previously present in the
network (assuming no other changes have taken place). It is network (assuming no other changes have taken place). It is
important therefore not to regenerate and flood the LSPs until all important therefore not to regenerate and flood the LSPs until all
the adjacencies have been re-established and any information the adjacencies have been re-established and any information required
required for propagation into the local LSPs is fully available. for propagation into the local LSPs is fully available. Ideally, the
Ideally, the information is loaded into the LSPs in a deterministic information is loaded into the LSPs in a deterministic way, such that
way, such that the same information occurs in the same place in the the same information occurs in the same place in the same LSP (and
same LSP (and hence the LSPs are identical to their previous hence the LSPs are identical to their previous versions). If this
versions). If this can be achieved, the new versions may not even can be achieved, the new versions may not even cause SPF to be run in
cause SPF to be run in other systems. However, provided the same other systems. However, provided the same information is included in
information is included in the set of LSPs (albeit in a different the set of LSPs (albeit in a different order, and possibly different
order, and possibly different LSPs), the result of running the SPF LSPs), the result of running the SPF will be the same and will not
will be the same and will not cause churn to the forwarding tables. cause churn to the forwarding tables.
In the case of a restarting router, none of the router's own LSPs In the case of a restarting router, none of the router's own LSPs are
are transmitted, nor are the router's own forwarding tables updated transmitted, nor are the router's own forwarding tables updated while
while the timer T3 is running. the timer T3 is running.
Redistribution of inter-level information MUST be regenerated before Redistribution of inter-level information MUST be regenerated before
this router's LSP is flooded to other nodes. Therefore the Level-n 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 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 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 inter-level information which is to be propagated can be included in
the Level-n LSP(s). the Level-n LSP(s).
During this period, if one of the router's own (including During this period, if one of the router's own (including
pseudonodes) LSPs is received, which the local router does not pseudonodes) LSPs is received, which the local router does not
currently have in its own database, it is NOT purged. Under normal currently have in its own database, it is NOT purged. Under normal
operation, such an LSP would be purged, since the LSP clearly should operation, such an LSP would be purged, since the LSP clearly should
not be present in the global LSP database. However, in the present not be present in the global LSP database. However, in the present
circumstances, this would be highly undesirable, because it could circumstances, this would be highly undesirable, because it could
cause premature removal of an own LSP - and hence churn in remote cause premature removal of a router's own LSP - and hence churn in
routers. Even if the local system has one or more own LSPs (which it remote routers. Even if the local system has one or more of the
has generated, but not yet transmitted) it is still not valid to router's own LSPs (which it has generated, but not yet transmitted),
compare the received LSP against this set, since it may be that as a it is still not valid to compare the received LSP against this set,
result of propagation between Level 1 and Level 2 (or vice versa) a since it may be that as a result of propagation between Level 1 and
further own LSP will need to be generated when the LSP databases Level 2 (or vice versa), a further router's own LSP will need to be
have synchronized. generated when the LSP databases have synchronized.
During this period a restarting router SHOULD send CSNPs as it During this period a restarting router SHOULD send CSNPs as it
normally would. Information about the router's own LSPs MAY be 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 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 been received, not on versions which have been generated (but not yet
yet transmitted). This restriction is necessary to prevent premature transmitted). This restriction is necessary to prevent premature
removal of an LSP from the global LSP database. removal of an LSP from the global LSP database.
When the timer T2 expires or is cancelled indicating that When the timer T2 expires or is cancelled indicating that
synchronization for that level is complete, the SPF for that level synchronization for that level is complete, the SPF for that level is
is run in order to derive any information which is required to be run in order to derive any information which is required to be
propagated to another level, but the forwarding tables are not yet propagated to another level, but the forwarding tables are not yet
updated. updated.
Once the other level's SPF has run and any inter-level propagation 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 has been resolved, the router's own LSPs can be generated and
own LSPs which were previously ignored, but which are not part of flooded. Any own LSPs which were previously ignored, but which are
the current set of own LSPs (including pseudonodes) MUST then be not part of the current set of own LSPs (including pseudonodes) MUST
purged. Note that it is possible that a Designated Router change may then be purged. Note that it is possible that a Designated Router
have taken place, and consequently the router SHOULD purge those change may have taken place, and consequently the router SHOULD purge
pseudonode LSPs which it previously owned, but which are now no those pseudonode LSPs which it previously owned, but which are now no
longer part of its set of pseudonode LSPs. longer part of its set of pseudonode LSPs.
When all the T2 timers have expired or been cancelled, the timer T3 When all the T2 timers have expired or been cancelled, the timer T3
is cancelled and the local forwarding tables are updated. is cancelled and the local forwarding tables are updated.
If the timer T3 expires before all the T2 timers have expired or If the timer T3 expires before all the T2 timers have expired or been
been cancelled, this indicates that the synchronization process is cancelled, this indicates that the synchronization process is taking
taking longer than minimum holding time of the neighbors. The longer than the minimum holding time of the neighbors. The router's
router's own LSP(s) for levels which have not yet completed their own LSP(s) for levels which have not yet completed their first SPF
first SPF computation are then flooded with the overload bit set to computation are then flooded with the overload bit set to indicate
indicate that the router's LSPDB is not yet synchronized (and that the router's LSPDB is not yet synchronized (and therefore other
therefore other routers MUST NOT compute routes through this routers MUST NOT compute routes through this router). Normal
router). Normal operation of the update process resumes and the operation of the update process resumes and the local forwarding
local forwarding tables are updated. In order to prevent the tables are updated. In order to prevent the neighbor's adjacencies
neighbor's adjacencies from expiring, IIHs with the normal interface from expiring, IIHs with the normal interface value for the holding
value for the holding time are transmitted over all interfaces with time are transmitted over all interfaces with neither RR nor RA set
neither RR nor RA set in the restart TLV. This will cause the in the restart TLV. This will cause the neighbors to refresh their
neighbors to refresh their adjacencies. The own LSP(s) will continue adjacencies. The router's own LSP(s) will continue to have the
to have the overload bit set until timer T2 has expired or been overload bit set until timer T2 has expired or been cancelled.
cancelled.
3.4.1.2. Starting 3.4.1.2. Starting
In the case of a starting router, as soon as each adjacency is In the case of a starting router, as soon as each adjacency is
established, and before any CSNP exchanges, the router's own zeroth established, and before any CSNP exchanges, the router's own zeroth
LSP is transmitted with the overload bit set. This prevents other LSP is transmitted with the overload bit set. This prevents other
routers from computing routes through the router until it has routers from computing routes through the router until it has
reliably acquired the complete set of LSPs. The overload bit remains reliably acquired the complete set of LSPs. The overload bit remains
set in subsequent transmissions of the zeroth LSP (such as will set in subsequent transmissions of the zeroth LSP (such as will occur
occur if a previous copy of the routers LSP is still present in the if a previous copy of the router's own zeroth LSP is still present in
network) while any timer T2 is running. the network) while any timer T2 is running.
When all the T2 timers have been cancelled, the own LSP(s) MAY be When all the T2 timers have been cancelled, the router's own LSP(s)
regenerated with the overload bit clear (assuming the router isn't MAY be regenerated with the overload bit clear (assuming the router
in fact overloaded, and there is no other reason, such as incomplete is not in fact overloaded, and there is no other reason, such as
BGP convergence, to keep the overload bit set), and flooded as incomplete BGP convergence, to keep the overload bit set) and flooded
normal. as normal.
Other own LSPs (including pseudonodes) are generated and flooded as Other LSPs owned by this router (including pseudonodes) are generated
normal, irrespective of the timer T2. The SPF is also run as normal and flooded as normal, irrespective of the timer T2. The SPF is also
and the RIB and FIB updated as routes become available. run as normal and the RIB and FIB updated as routes become available.
To avoid the possible formation of temporary blackholes the starting 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 router sets the SA bit in the restart TLV (as described in 3.3.2) in
all IIHs that it sends. all IIHs that it sends.
When all T2 timers have been cancelled the starting router MUST When all T2 timers have been cancelled, the starting router MUST
transmit IIHs with the SA bit clear. transmit IIHs with the SA bit clear.
4. State Tables 4. State Tables
This section presents state tables which summarize the behaviors This section presents state tables which summarize the behaviors
described in this document. Other behaviors, in particular adjacency described in this document. Other behaviors, in particular adjacency
state transitions and LSP database update operation, are NOT state transitions and LSP database update operation, are NOT included
included in the state tables except where this document modifies the in the state tables except where this document modifies the behaviors
behaviors described in [3] and [5]. described in [2] and [4].
The states named in the columns of the tables below are a mixture of 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 states that are specific to a single adjacency (ADJ suppressed, ADJ
Seen RA, ADJ Seen CSNP) and states which are indicative of the state Seen RA, ADJ Seen CSNP) and states which are indicative of the state
of the protocol instance (Running, Restarting, Starting, SPF Wait). of the protocol instance (Running, Restarting, Starting, SPF Wait).
Three state tables are presented from the point of view of a running Three state tables are presented from the point of view of a running
router, a restarting router, and a starting router. router, a restarting router, and a starting router.
4.1 Running Router 4.1. Running Router
Event | Running | ADJ suppressed Event | Running | ADJ suppressed
============================================================== ==============================================================
RX RR | Maintain ADJ State | RX RR | Maintain ADJ State |
| Send RA | | Send RA |
| Set SRM,send CSNP | | Set SRM,send CSNP |
| (Note 1) | | (Note 1) |
| Update Hold Time, | | Update Hold Time, |
| set Restart Mode | | set Restart Mode |
| (Note 2) | | (Note 2) |
skipping to change at page 17, line 5 skipping to change at page 17, line 5
| Goto ADJ Suppressed | | Goto ADJ Suppressed |
-------------+----------------------+------------------------- -------------+----------------------+-------------------------
RX SA clr | |Unsuppress IS neighbor RX SA clr | |Unsuppress IS neighbor
| | TLV in LSP(s) | | TLV in LSP(s)
| |Goto Running | |Goto Running
============================================================== ==============================================================
Note 1: CSNPs are sent by routers in accordance with Section 3.2.1c Note 1: CSNPs are sent by routers in accordance with Section 3.2.1c
Note 2: If Restart Mode clear Note 2: If Restart Mode clear
4.2 Restarting Router 4.2. Restarting Router
Event | Restarting | ADJ Seen | ADJ Seen | SPF Wait Event | Restarting | ADJ Seen | ADJ Seen | SPF Wait
| | RA | CSNP | | | RA | CSNP |
=================================================================== ===================================================================
Router | Send IIH/RR | | | Router | Send IIH/RR | | |
restarts | ADJ Init | | | restarts | ADJ Init | | |
| Start T1,T2,T3 | | | | Start T1,T2,T3 | | |
------------+--------------------+-----------+-----------+------------ ------------+--------------------+-----------+-----------+------------
RX RR | Send RA | | | RX RR | Send RA | | |
------------+--------------------+-----------+-----------+------------ ------------+--------------------+-----------+-----------+------------
skipping to change at page 17, line 46 skipping to change at page 17, line 46
------------+--------------------+-----------+-----------+------------ ------------+--------------------+-----------+-----------+------------
LSP DB Sync| Cancel T2, and T3 | | | LSP DB Sync| Cancel T2, and T3 | | |
| Trigger SPF | | | | Trigger SPF | | |
| Goto SPF wait | | | | Goto SPF wait | | |
------------+--------------------+-----------+-----------+------------ ------------+--------------------+-----------+-----------+------------
All SPF | | | | Clear OL All SPF | | | | Clear OL
done | | | | Update fwd done | | | | Update fwd
| | | | plane | | | | plane
| | | | Flood local | | | | Flood local
| | | | LSPs | | | | LSPs
| | | | Goto Runing | | | | Goto Running
====================================================================== ======================================================================
4.3 Starting Router 4.3. Starting Router
Event | Starting | ADJ Seen RA| ADJ Seen CSNP Event | Starting | ADJ Seen RA| ADJ Seen CSNP
============================================================= =============================================================
Router | Send IIH/SA | | Router | Send IIH/SA | |
starts | Start T1,T2 | | starts | Start T1,T2 | |
-------------+-------------------+------------+--------------- -------------+-------------------+------------+---------------
RX RR | Send RA | | RX RR | Send RA | |
-------------+-------------------+------------+--------------- -------------+-------------------+------------+---------------
RX RA | Goto ADJ Seen RA | | Cancel T1 RX RA | Goto ADJ Seen RA | | Cancel T1
-------------+-------------------+------------+--------------- -------------+-------------------+------------+---------------
skipping to change at page 19, line 13 skipping to change at page 19, line 6
============================================================== ==============================================================
5. Security Considerations 5. Security Considerations
Any new security issues raised by the procedures in this document Any new security issues raised by the procedures in this document
depend upon the ability of an attacker to inject a false but depend upon the ability of an attacker to inject a false but
apparently valid IIH, the ease/difficulty of which has not been apparently valid IIH, the ease/difficulty of which has not been
altered. altered.
If the RR bit is set in a false IIH, neighbors who receive such an 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 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 and may (re)send a complete set of CSNPs. While the latter action is
wasteful, neither action causes any disruption in correct protocol wasteful, neither action causes any disruption in correct protocol
operation. operation.
If the RA bit is set in a false IIH, a (re)starting router which 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 receives such an IIH may falsely believe that there is a neighbor on
the corresponding interface which supports the procedures described the corresponding interface which supports the procedures described
in this document. In the absence of receipt of a complete set of in this document. In the absence of receipt of a complete set of
CSNPs on that interface, this could delay the completion of CSNPs on that interface, this could delay the completion of (re)start
(re)start procedures by requiring the timer T1 to time out the procedures by requiring the timer T1 to time out the locally defined
locally defined maximum number of retries. This behavior is the same maximum number of retries. This behavior is the same as would occur
as would occur on a LAN where none of the (re)starting router's on a LAN where none of the (re)starting router's neighbors support
neighbors support the procedures in this document and is covered in the procedures in this document and is covered in Sections 3.3.1 and
Sections 3.3.1 and 3.3.2. 3.3.2.
If an SA bit is set in a false IIH, this could cause suppression of 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 the advertisement of an IS neighbor which could either continue for
an indefinite period or occur intermittently with the result being an indefinite period, or occur intermittently with the result being a
possible loss of reachability to some destinations in the network possible loss of reachability to some destinations in the network
and/or increased frequency of LSP flooding and SPF calculation. and/or increased frequency of LSP flooding and SPF calculation.
The possibility of IS-IS PDU spoofing can be reduced by the use of The possibility of IS-IS PDU spoofing can be reduced by the use of
authentication as described in [2] and [3], and especially the use authentication as described in [1] and [2], and especially the use of
of cryptographic authentication as described in [6]. cryptographic authentication as described in [5].
6. IANA Considerations 6. IANA Considerations
This document defines the following new ISIS TLV that needs to be This document defines the following IS-IS TLV that is listed in the
reflected in the ISIS TLV code-point registry: IS-IS TLV code-point registry:
Type Description IIH LSP SNP Type Description IIH LSP SNP
---- ----------------------------------- --- --- --- ---- ----------------------------------- --- --- ---
211 Restart TLV y n n 211 Restart TLV y n n
7. Normative References 7. Normative References
1 Bradner, S., "The Internet Standards Process -- Revision 3", BCP [1] Callon, R., "OSI IS-IS for IP and Dual Environment", RFC 1195,
9, RFC 2026, October 1996.
2 Callon, R., "OSI IS-IS for IP and Dual Environment," RFC 1195,
December 1990. December 1990.
3 ISO, "Intermediate system to Intermediate system routeing [2] ISO, "Intermediate system to Intermediate system routeing
information exchange protocol for use in conjunction with the information exchange protocol for use in conjunction with the
Protocol for providing the Connectionless-mode Network Service Protocol for providing the Connectionless-mode Network Service
(ISO 8473)," ISO/IEC 10589:2002, Second Edition. (ISO 8473)," ISO/IEC 10589:2002, Second Edition.
4 Bradner, S., "Key words for use in RFCs to Indicate Requirement [3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997 Levels", BCP 14, RFC 2119, March 1997.
5 Katz, D., and Saluja, R., "Three-Way Handshake for IS-IS Point- [4] Katz, D. and R. Saluja, "Three-Way Handshake for IS-IS Point-
to-Point Adjacencies", RFC 3373, September 2002 to-Point Adjacencies", RFC 3373, September 2002.
6 Li, T., and Atkinson, R., "Intermediate System to Intermediate [5] Li, T. and R. Atkinson, "Intermediate System to Intermediate
System (IS-IS) Cryptographic Authentication", RFC 3567, July System (IS-IS) Cryptographic Authentication", RFC 3567, July
2003 2003.
7 Narten, T. and Alvestrand, H., "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26 , RFC 2434, October 1998
8. Acknowledgments 8. Acknowledgements
The authors would like to acknowledge contributions made by Jeff The authors would like to acknowledge contributions made by Jeff
Parker, Radia Perlman, Mark Schaefer, Naiming Shen, Nischal Sheth, Parker, Radia Perlman, Mark Schaefer, Naiming Shen, Nischal Sheth,
Russ White, and Rena Yang. Russ White, and Rena Yang.
9. Authors' Addresses 9. Authors' Addresses
Mike Shand Mike Shand
Cisco Systems Cisco Systems
250 Longwater Avenue, 250 Longwater Avenue,
Reading, Reading,
Berkshire, Berkshire,
RG2 6GB RG2 6GB
UK UK
Phone: +44 208 824 8690 Phone: +44 208 824 8690
Email: mshand@cisco.com
EMail: mshand@cisco.com
Les Ginsberg Les Ginsberg
Cisco Systems Cisco Systems
510 McCarthy Blvd. 510 McCarthy Blvd.
Milpitas, Ca. 95035 USA Milpitas, Ca. 95035 USA
Email: ginsberg@cisco.com
EMail: ginsberg@cisco.com
10. Full Copyright Statement 10. Full Copyright Statement
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to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
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