draft-ietf-isis-restart-02.txt   draft-ietf-isis-restart-03.txt 
Network Working Group M. Shand Network Working Group M. Shand
Internet Draft Cisco Systems Internet Draft Les Ginsberg
Expiration Date: May 2003 Expiration Date: September 2003 Cisco Systems
Nov 2002 March 2003
Restart signaling for IS-IS Restart signaling for IS-IS
draft-ietf-isis-restart-02.txt draft-ietf-isis-restart-03.txt
Status of this Memo Status of this Memo
This document is an Internet-Draft and is in full conformance with This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026 [1]. all provisions of Section 10 of RFC2026 [1].
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that Task Force (IETF), its areas, and its working groups. Note that
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skipping to change at page 1, line 34 skipping to change at page 1, line 34
The list of current Internet-Drafts can be accessed at The list of current Internet-Drafts can be accessed at
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The list of Internet-Draft Shadow Directories can be accessed at The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html. http://www.ietf.org/shadow.html.
1. Abstract 1. Abstract
The IS-IS routing protocol (RFC 1142 [2], ISO/IEC 10589 [3]) is a The IS-IS routing protocol (RFC 1142 [2], ISO/IEC 10589 [3]) is a
link state intra-domain routing protocol. Normally, when an IS-IS link state intra-domain routing protocol. Normally, when an IS-IS
router is re-started, the neighboring routers detect the restart router is restarted, the neighboring routers detect the restart
event and cycle their adjacencies with the restarting router through event and cycle their adjacencies with the restarting router through
the down state. This is necessary in order to invoke the protocol the down state. This is necessary in order to invoke the protocol
mechanisms to ensure correct re-synchronization of the LSP database. mechanisms to ensure correct synchronization of the LSP database.
However, the cycling of the adjacency state causes the neighbors to However, the cycling of the adjacency state causes the neighbors to
regenerate their LSPs describing the adjacency concerned. This in regenerate their LSPs describing the adjacency concerned. This in
turn causes temporary disruption of routes passing through the turn causes temporary disruption of routes passing through the
restarting router. restarting router.
In certain scenarios such temporary disruption of the routes is In certain scenarios such temporary disruption of the routes is
highly undesirable. highly undesirable.
This draft describes a mechanism for a restarting router to signal This draft describes a mechanism for a restarting router to signal
that it is restarting to its neighbors, and allow them to re- that it is restarting to its neighbors, and allow them to
establish their adjacencies without cycling through the down state, reestablish their adjacencies without cycling through the down
while still correctly initiating database synchronization. state, while still correctly initiating database synchronization.
When such a router is restarted, it is highly desirable that it does When such a router is restarted, it is highly desirable that it does
not re-compute its own routes until it has achieved database not recompute its own routes until it has achieved database
synchronization with its neighbors. Re-computing its routes before synchronization with its neighbors. Recomputing its routes before
synchronization is achieved will result in its own routes being synchronization is achieved will result in its own routes being
temporarily incorrect. temporarily incorrect.
This draft additionally describes a mechanism for a restarting This draft additionally describes a mechanism for a restarting
router to determine when it has achieved synchronization with its router to determine when it has achieved synchronization with its
neighbors. neighbors.
This draft additionally describes a mechanism to optimize database
synchronization and minimize transient routing disruption when a
router starts.
2. Conventions used in this document 2. 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 document are to be interpreted as described in RFC-2119 [4]. this document are to be interpreted as described in RFC-2119 [4].
If the control and forwarding functions in a router can be
maintained independently, it is possible for the forwarding function
state to be maintained across a control function restart. This
functionality is assumed when the terms "restart/restarting" are
used in this document.
The terms "start/starting" are used to refer to a router in which
the control function has either been started for the first time or
has been restarted but the forwarding functions have not been
maintained in a prior state.
The terms "(re)start/(re)starting" are used when the text is
applicable to both a "starting" and a "restarting" router.
3. Overview 3. Overview
There are two related problems with the existing specification of There are two related problems with the existing specification of
IS-IS with regard to re-synchronization of LSP databases when a IS-IS with regard to synchronization of LSP databases when a router
router is re-started. is restarted.
Firstly, when a routing process restarts, and an adjacency to a Firstly, when a routing process restarts and an adjacency to a
neighboring router is re-initialized the neighboring routing process neighboring router is reinitialized the neighboring routing process
does three things does three things:
1. It re-initializes the adjacency and causes its own LSP(s) to be 1. It reinitializes the adjacency and causes its own LSP(s) to be
regenerated, thus triggering SPF runs throughout the area (or regenerated, thus triggering SPF runs throughout the area (or
in the case of Level 2, throughout the domain). in the case of Level 2, throughout 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
re-synchronization of the LSP database. synchronization of the LSP database.
Secondly, whether or not the router is being re-started, it is Secondly, whether or not the router is being restarted, it is
desirable to be able to determine when the LSP databases of the desirable to be able to determine when the LSP databases of the
neighboring routers have been synchronized (so that the overload bit neighboring routers have been synchronized (so that the overload bit
can be cleared in the router's own LSP, for example). This document can be cleared in the router's own LSP, for example). This document
describes modifications to achieve this. describes modifications to achieve this.
It is assumed that the three-way handshake [5] is being used on It is assumed that the three-way handshake [5] is being used on
Point-to-Point circuits. Point-to-Point circuits.
4. Approach 4. Approach
4.1 Timers 4.1 Timers
A router that is restart capable maintains three additional timers, Three additional timers, T1, T2 and T3 are required to support the
T1, T2 and T3. functionality defined in this document.
An instance of T1 is maintained per interface, and indicates the An instance of T1 is maintained per interface, and indicates the
time after which an unacknowledged restart attempt will be repeated. time after which an unacknowledged (re)start attempt will be
A typical value might be 3 seconds. repeated. A typical value might be 3 seconds.
An instance of T2 is maintained for each LSP database present in the An instance of T2 is maintained for each LSP database present in the
system. I.e. for a level1/2 system, there will be an instance of T2 system i.e. for a Level1/2 system, there will be an instance of T2
for Level 1 and one for level 2. This is the maximum time that the for Level 1 and an instance for Level 2. This is the maximum time
system will wait for LSPDB synchronization. A typical value might be that the system will wait for LSPDB synchronization. A typical value
60 seconds. might be 60 seconds.
A single instance of T3 is maintained for the entire system. It A single instance of T3 is maintained for the entire system. It
indicates the time after which the router will declare that it has indicates the time after which the router will declare that it has
failed to achieve database synchronization (by setting the overload failed to achieve database synchronization (by setting the overload
bit in its own LSP). This is initialized to 65535 seconds, but is bit in its own LSP). This is initialized to 65535 seconds, but is
set to the minimum of the remaining times of received IIHs set to the minimum of the remaining times of received IIHs
containing a restart TLV with RA set. containing a restart TLV with RA set.
4.2 Adjacency re-acquisition 4.2 Restart TLV
Adjacency re-acquisition is the first step in re-initialization. The A new TLV is defined to be included in IIH PDUs. The presence of
restarting router explicitly notifies its neighbor that the this TLV indicates that the sender supports the functionality
adjacency is being re-acquired, and hence that it should not re- defined in this document and it carries flags that are used to
initialize the adjacency. This is achieved by the inclusion of a new convey information during a (re)start. All IIHs transmitted by a
"re-start" option (TLV) in the IIH PDU. The presence of this TLV router that supports this capability MUST include this TLV.
indicates that the sender supports the new restart capability and it
carries flags that are used to convey information during a restart.
All IIHs transmitted by a router that supports this capability MUST
include this TLV.
Type 211 Type 211
Length 3 Length 3
Value (3 octets) Value (3 octets)
Flags (1 octet) Flags (1 octet)
Bit 1 - Restart Request (RR) Bit 1 - Restart Request (RR)
Bit 2 - Restart Acknowledgment (RA) Bit 2 - Restart Acknowledgment (RA)
Bits 3-8 Reserved Bit 3 Suppress adjacency advertisement(SA)
Bits 4-8 Reserved
Remaining Time (2 octets) Remaining Time (2 octets)
Remaining holding time (in seconds) Remaining holding time (in seconds)
(note: only required when RA bit is set) (note: only required when RA bit is set)
On receipt of an IIH with the "re-start" TLV having the RR bit set,
if there exists on this interface an adjacency in state "Up" with
the same System ID, and in the case of a LAN circuit, with the same
source LAN address, then, irrespective of the other contents of the
"Intermediate System Neighbors" option (LAN circuits), or the
"Point-to-Point Adjacency State" option (Point-to-Point circuits): -
a) Do not change the state of the adjacency. It is an implementation 4.2.1 Use of RR and RA bits
choice whether or not the holding time of the adjacency is
refreshed. Not refreshing the holding time preserves the intention The RR bit is used by a (re)starting router to signal to its
of the original holding time. Refreshing it may allow a longer neighbors that a (re)start is in progress, that an existing
grace period for the completion of the restart process. Whichever adjacency should be maintained even under circumstances when the
option is chosen, the "remaining time" transmitted according normal operation of the adjacency state machine would require the
to (b) below MUST reflect the actual time after which the adjacency to be reinitialized, and to request a set of CSNPs.
adjacency will now expire.
The RA bit is sent by the neighbor of a (re)starting router to
acknowledge the receipt of a restart TLV with the RR bit set.
When the neighbor of a (re)starting router receives an IIH with the
restart TLV having the RR bit set, if there exists on this interface
an adjacency in state "Up" with the same System ID, and in the case
of a LAN circuit, with the same source LAN address, then,
irrespective of the other contents of the "Intermediate System
Neighbors" option (LAN circuits), or the "Point-to-Point Adjacency
State" option (Point-to-Point circuits):
a) The state of the adjacency is not changed. It is an
implementation choice whether or not the holding time of the
adjacency is refreshed. Not refreshing the holding time preserves
the intention of the original holding time. Refreshing it may
allow a longer grace period for the completion of the (re)start
process. Whichever option is chosen, the "remaining time"
transmitted according to (b) below MUST reflect the actual time
after which the adjacency will now expire.
b) immediately (i.e. without waiting for any currently running timer 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 "re-start" TLV with corresponding interface an IIH including the restart TLV with the
the RR bit clear and the RA bit set, having updated the "Point-to- RR bit clear and the RA bit set, having updated the "Point-to-
Point Adjacency State" option to reflect any new values received Point Adjacency State" option to reflect any new values received
from the re-starting router. (This allows the restarting router to from the (re)starting router. (This allows a restarting router to
quickly acquire the correct information to place in its hellos.) quickly acquire the correct information to place in its hellos.)
The "Remaining Time" MUST be set to the current time (in seconds) The "Remaining Time" MUST be set to the current time (in seconds)
before the holding timer on this adjacency is due to expire. This before the holding timer on this adjacency is due to expire. This
IIH SHOULD be transmitted before any LSPs or SNPs transmitted as a IIH SHOULD be transmitted before any LSPs or SNPs transmitted as a
result of the receipt of the original IIH. result of the receipt 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 highest source MAC address breaking ties) among those routers
whose IIHs contain the restart TLV, excluding the transmitting whose IIHs contain the restart TLV, excluding the transmitting
router (note the actual DIS is NOT changed by this process.), router (note the actual DIS is NOT changed by this process.),
initiate the transmission over the corresponding interface of a initiate the transmission over the corresponding interface of a
complete set of CSNPs, and set SRMflags on the corresponding complete set of CSNPs, and set SRMflags on the corresponding
interface for all LSPs in the local LSP database. interface for all 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 re-initializing 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.
A router that does not support the re-start capability will ignore 4.2.2 Use of SA bit
the "re-start" TLV and re-initialize the adjacency as normal,
returning an IIH without the "re-start" TLV.
On starting, a router initializes the timer T3, starts timer T2 for The SA bit is used by a starting router to request that its neighbor
each LSPDB and for each interface (and in the case of a LAN circuit, suppress advertisement of the adjacency to the starting router in
for each level) starts a timer T1 and transmits an IIH containing the neighbors LSPs.
the "re-start" TLV with the RR bit set.
A router which is starting has no maintained forwarding function
state. This may or may not be the first time the router has started.
If this is not the first time the router has started, copies of LSPs
generated by this router in its previous incarnation may exist in
the LSP databases of other routers in the network. These copies are
likely to appear "newer" than LSPs initially generated by the
starting router due to the reinitialization of LSP fragment sequence
numbers by the starting router. This may cause temporary blackholes
to occur until the normal operation of the update process causes the
starting router to regenerate and flood copies of its own LSPs with
higher sequence numbers. The temporary blackholes can be avoided if
the starting router's neighbors suppress advertising an adjacency to
the starting router until the starting router has been able to
propagate newer versions of LSPs generated by previous incarnations.
When the neighbor of a starting router receives an IIH with the
restart TLV having the SA bit set, if there exists on this interface
an adjacency in state "Up" with the same System ID, and in the case
of a LAN circuit, with the same source LAN address, then
advertisement of the adjacency to the starting router in LSPs should
be suppressed. Until an IIH with the SA bit clear has been received,
the adjacency advertisement should continue to be suppressed. If the
adjacency transitions to the UP state, the new adjacency should not
be advertised until an IIH with the SA bit clear has been received.
4.3 Adjacency (re)acquisition
Adjacency (re)acquisition is the first step in (re)initialization.
Both restarting and starting routers will make use of the RR bit in
the restart TLV, though at different stages of the (re)start
procedure.
4.3.1 Adjacency reacquisition during restart
The restarting router explicitly notifies its neighbor that the
adjacency is being reacquired, and hence that it should not
reinitialize the adjacency. This is achieved by setting the RR bit
in the restart TLV. When the neighbor of a restarting router
receives an IIH with the restart TLV having the RR bit set, if there
exists on this interface an adjacency in state "Up" with the same
System ID, and in the case of a LAN circuit, with the same source
LAN address, then the procedures described in 4.2.1 are followed.
A router that does not support the restart capability will ignore
the restart TLV and reinitialize the adjacency as normal, returning
an IIH without the restart TLV.
On restarting, a router initializes the timer T3, starts timer T2
for each LSPDB and for each interface (and in the case of a LAN
circuit, for each level) starts a timer T1 and transmits an IIH
containing the restart TLV with the RR bit set.
On a Point-to-Point circuit the "Point-to-Point Adjacency State" On a Point-to-Point circuit the "Point-to-Point Adjacency State"
SHOULD be set to "Init", because the receipt of the acknowledging SHOULD be set to "Init", because the receipt of the acknowledging
IIH (with RA set) MUST cause the adjacency to enter "Up" state IIH (with RA set) MUST cause the adjacency to enter "Up" state
immediately. 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 re-start. In particular, for any same as that used prior to the restart. In particular, for any
circuits for which the re-starting router was previously DIS, the circuits for which the restarting router was previously DIS, the use
use of a different LAN-ID would necessitate the generation of a new of a different LAN-ID would necessitate the generation of a new set
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. LAN-ID across the restart, this churn can be prevented. To enable a
restarting router to learn the LAN-ID used prior to restart, the
LAN-ID specified in an IIH w RR set MUST be ignored.
Transmission of "normal" IIHs is inhibited until the conditions 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 re-initialization). On expiry of the timer T1, it is adjacency reinitialization). On expiry of the timer T1, it is
restarted and the IIH is re-transmitted as above. restarted and the IIH is retransmitted as above.
On receipt of an IIH by the restarting router, a local adjacency is On receipt of an IIH by the restarting router, a local adjacency is
established as usual, and if the IIH contains a "re-start" TLV with established as usual, and if the IIH contains a restart TLV with the
the RA bit set, the receipt of the acknowledgement over that RA bit set, the receipt of the acknowledgement over that interface
interface is noted. is noted.
T3 is set to the minimum of its current value and the value of the T3 is set to the minimum of its current value and the value of the
"Remaining Time" field in the received IIH. "Remaining Time" field in the received IIH.
Receipt of an IIH not containing the "re-start" option is also Receipt of an IIH not containing the restart TLV is also treated as
treated as an acknowledgement, since it indicates that the neighbor an acknowledgement, since it indicates that the neighbor is not
is not re-start capable. In this case the neighbor will have re- restart capable. In this case the neighbor will have reinitialized
initialized the adjacency as normal, which in the case of a Point- the adjacency as normal, which in the case of a Point-to-Point link
to-Point link will guarantee that SRMflags have been set on its will guarantee that SRMflags have been set on its database, thus
database, thus ensuring eventual LSPDB synchronization. In the case ensuring eventual LSPDB synchronization. In the case of a LAN
of a LAN interface, the usual operation of the update process will interface, the usual operation of the update process will also
also ensure that synchronization is eventually achieved. However, ensure that synchronization is eventually achieved. However, since
since no CSNP is guaranteed to be received over this interface, T1 no CSNP is guaranteed to be received over this interface, T1 is
is cancelled immediately without waiting for a CSNP. Synchronization cancelled immediately without waiting for a CSNP. Synchronization
may therefore be deemed complete even though there are some LSPs may therefore be deemed complete even though there are some LSPs
which are held (only) by this neighbor (see section 4.3). which are held(only) by this neighbor (see section 4.4).
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 System ID" or handshake information. The presence of "Neighbor System ID" or
"Neighbor Extended Local Circuit ID" information which does not "Neighbor Extended Local Circuit ID" information which does not
match the values currently in use by the local system is ignored match the values currently in use by the local system is ignored
(since the IIH may have been transmitted before the neighbor had (since the IIH may have been transmitted before the neighbor had
received the new values from the re-starting router), but the received the new values from the restarting router), but the
adjacency remains in the initializing state until the correct adjacency remains in the initializing state until the correct
information is received. information is received.
In the case of a LAN circuit the information in the Intermediate In the case of a LAN circuit the information in the Intermediate
Systems Neighbors option is recorded and used for the generation of Systems Neighbors option is recorded and used for the generation of
subsequent IIHs as normal. subsequent IIHs 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 pt-pt circuit) and an acknowledgement have been received
over the interface, the timer T1 is cancelled. over the interface, the timer T1 is cancelled.
Once T3 has expired or been cancelled, subsequent IIHs are Once T3 has expired or been cancelled, subsequent IIHs are
transmitted according to the normal algorithms, but including the transmitted according to the normal algorithms, but including the
"re-start" TLV with both RR and RA clear. restart TLV 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 re-initialized 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 4.3.1.2 the SPF would never be run. and according to clause 4.4.1.1 the SPF would never be run.
Therefore timer T1 is cancelled after some pre-determined number of Therefore timer T1 is cancelled after some pre-determined number of
expirations (which MAY be 1). (By this time any existing adjacency expirations (which MAY be 1). (By this time any existing adjacency
on a remote system would probably have expired anyway.) on a remote system would probably have expired anyway.)
A router which supports re-start SHOULD ensure that the holding time A router which supports restart SHOULD ensure that the holding time
of any IIHs it transmits is greater than the expected time to of any IIHs it transmits is greater than the expected time to
complete a re-start. However, where this is impracticable or complete a restart. However, where this is impracticable or
undesirable a router MAY transmit one or more normal IIHs undesirable a router MAY transmit one or more normal IIHs
(containing a restart option, but with RR and RA clear) after the (containing a restart TLV with RR and RA clear) after the initial
initial RR/RA exchange, but before synchronization has been RR/RA exchange, but before synchronization has been achieved, in
achieved, in order to extend the holding time of the neighbors order to extend the holding time of the neighbors adjacencies beyond
adjacencies, beyond that indicated in the remaining time field of that indicated in the remaining time field of the neighbors IIH with
the neighbors IIH with the RA bit set. the RA bit set.
4.2.1 Multiple levels 4.3.2 Adjacency acquisition during start
A router which is operating as both a level 1 and a level 2 router The starting router wants to ensure that in the event a neighboring
router has an adjacency to the starting router in the UP state (from
a previous incarnation of the starting router) that this adjacency
is reinitialized. The starting router also wants neighboring routers
to suppress advertisement of an adjacency to the starting router
until LSP database synchronization is achieved. This is achieved by
sending IIHs with the RR bit clear and the SA bit set in the restart
TLV. The RR bit remains clear and the SA bit remains set in
subsequent transmissions of IIHs until the adjacency has reached the
UP state and the initial T1 timer interval (see below) has expired.
Receipt of an IIH with RR bit clear will result in the neighboring
router utilizing normal operation of the adjacency state machine.
This will ensure that any old adjacency on the neighboring router
will be reinitialized.
On receipt of an IIH with SA bit set the behavior described in 4.2.2
is followed.
On starting, a router initializes the timer T3, and starts timer T2
for each LSPDB.
For each interface (and in the case of a LAN circuit, for each
level), when an adjacency reaches the UP state, the starting router
starts a timer T1 and transmits an IIH containing the restart TLV
with the RR bit clear and SA bit set. On expiry of the timer T1, it
is restarted and the IIH is retransmitted with both RR and SA bits
set(only the RR bit has changed state from earlier IIHs).
On receipt of an IIH with RR bit set(regardless of whether SA is set
or not) the behavior described in 4.2.1 is followed.
When an IIH is received by the starting router and the IIH contains
a restart TLV with the RA bit set, the receipt of the
acknowledgement over that interface is noted.
T3 is set to the minimum of its current value and the value of the
"Remaining Time" field in the received IIH.
Receipt of an IIH not containing the restart TLV is also treated as
an acknowledgement, since it indicates that the neighbor is not
restart capable. In this case the neighbor will have reinitialized
the adjacency as normal, which in the case of a Point-to-Point link
will guarantee that SRMflags have been set on its database, thus
ensuring eventual LSPDB synchronization. In the case of a LAN
interface, the usual operation of the update process will also
ensure that synchronization is eventually achieved. However, since
no CSNP is guaranteed to be received over this interface, T1 is
cancelled immediately without waiting for a CSNP. Synchronization
may therefore be deemed complete even though there are some LSPs
which are held(only) by this neighbor (see section 4.4).
When BOTH a complete set of CSNP(s) (for each active level, in the
case of a pt-pt circuit) and an acknowledgement have been received
over the interface, the timer T1 is cancelled. Subsequent IIHs sent
by the starting router have the RR and RA bits clear and the SA bit
set in the restart TLV.
Once T3 has expired or been cancelled, subsequent IIHs are
transmitted according to the normal algorithms, but including the
restart TLV with RR, RA, and SA bits clear.
Timer T1 is cancelled after some pre-determined number of
expirations (which MAY be 1).
During the period when T1 is active, according to the rules defined
in 4.3 the neighbor of the starting router may choose not to update
the holding time for an adjacency because the RR bit is set in the
received IIH. To prevent holding time expiration a starting router
MAY transmit one or more IIHs containing a restart TLV with RR and
RA bits clear and SA bit set after the initial RR/RA exchange.
When T2 is cancelled or expires transmission of "normal" IIHs (with
RR,RA, and SA bits clear) will begin.
4.3.3 Multiple levels
A router which is operating as both a Level 1 and a Level 2 router
on a particular interface MUST perform the above operations for each on 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 IIHs and perform the CSNP synchronizations independently for Level 2 IIHs and perform the CSNP synchronizations independently for
each level. each level.
On a pt-pt interface, only a single IIH (indicating support for both On a pt-pt interface, only a single IIH (indicating support for both
levels) is required, but it MUST perform the CSNP synchronizations levels) is required, but it MUST perform the CSNP synchronizations
independently for each level. independently for each level.
4.3 Database synchronization 4.4 Database synchronization
When a router is started or re-started it can expect to receive a 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 (set of) CSNP(s) over each interface. The arrival of the CSNP(s) is
now guaranteed, since the "re-start" IIH with the RR bit set will be now guaranteed, since an IIH with RR bit set will be retransmitted
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.
On starting, a router starts the timer T3 and an instance of timer When (re)starting, a router starts the timer T3 and an instance of
T2 for each LSPDB. In addition to normal processing of the CSNPs, timer T2 for each LSPDB as described in 4.3.1 or 4.3.2. In addition
the set of LSPIDs contained in the first complete set of CSNP(s) to normal processing of the CSNPs, the set of LSPIDs contained in
received over each interface is recorded, together with their the first complete set of CSNP(s) received over each interface is
remaining lifetime. If there are multiple interfaces on the recorded, together with their remaining lifetime. If there are
restarting router, the recorded set of LSPIDs is the union of those multiple interfaces on the (re)starting router, the recorded set of
received over each interface. LSPs with a remaining lifetime of zero LSPIDs is the union of those received over each interface. LSPs with
are NOT so recorded. a 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 the LSP had arrived 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 becomes empty, the timer T2 is cancelled. list of LSPIDs is empty and T1 has been cancelled for all the
interfaces that have an adjacency at this level, the timer T2 is
cancelled.
At this point the local database is guaranteed to contain all the 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) which were present in the neighbors' databases at the time
of re-starting. LSPs that arrived in a neighbor's database after the of (re)starting. LSPs that arrived in a neighbor's database after
time of re-starting may, or may not, be present, but the normal the 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.
4.3.1 LSP generation and flooding and SPF computation 4.4.1 LSP generation and flooding and SPF computation
The operation of a router starting, as opposed to re-starting 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.
4.3.1.1. Starting for the first time 4.4.1.1. Restarting
In the case of a starting router, as soon as each adjacency is
established, and before any CSNP exchanges, the router's own zeroth
LSP is transmitted with the overload bit set. This prevents other
routers from computing routes through the router until it has
reliably acquired the complete set of LSPs. The overload bit remains
set in subsequent transmissions of the zeroth LSP (such as will
occur if a previous copy of the routers LSP is still present in the
network) while any timer T2 is running.
When all the T2 timers have been cancelled, the own LSP(s) MAY be
regenerated with the overload bit clear (assuming the router isn't
in fact overloaded, and there is no other reason, such as incomplete
BGP convergence, to keep the overload bit set), and flooded as
normal.
Other 'own' LSPs (including pseudonodes) are generated and flooded
as normal, irrespective of the timer T2. The SPF is also run as
normal and the RIB and FIB updated as routes become available.
4.3.1.2. Re-starting
In order to avoid causing unnecessary routing churn in other In order to avoid causing unnecessary routing churn in other
routers, it is highly desirable that the own LSPs generated by the routers, it is highly desirable that the 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 for propagation into the local LSPs is fully available. required for propagation into the local LSPs is fully available.
Ideally, the information should be loaded into the LSPs in a Ideally, the information should be loaded into the LSPs in a
deterministic way, such that the same information occurs in the same deterministic way, such that the same information occurs in the same
place in the same LSP (and hence the LSPs are identical to their place in the same LSP (and hence the LSPs are identical to their
previous versions). If this can be achieved, the new versions will previous versions). If this can be achieved, the new versions will
not even cause SPF to be run in other systems. However, provided the 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 same information is included in the set of LSPs (albeit in a
different order, and possibly different LSPs), the result of running different order, and possibly different LSPs), the result of running
the SPF will be the same and will not cause churn to the forwarding the SPF will be the same and will not cause churn to the forwarding
tables. tables.
In the case of a re-starting router, none of the router's own non- In the case of a restarting router, none of the router's LSPs are
pseudonode LSPs are transmitted, nor are the router's own forwarding transmitted, nor are the router's own forwarding tables updated
tables updated while the timer T3 is running. while 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) should not be flooded until the other level's non-pseudonode LSP(s) should not be flooded until the other level's
T2 timer has expired and its SPF has been run. This ensures that any T2 timer has expired and its SPF has been run. This ensures that any
inter-level information that should be propagated can be included in inter-level information that should 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 an own LSP -- and hence churn in remote
routers. Even if the local system has one or more own LSPs (which it routers. Even if the local system has one or more own LSPs (which it
has generated, but not yet transmitted) it is still not valid to 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 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 result of propagation between Level 1 and Level 2 (or vice versa) a
further own LSP will need to be generated when the LSP databases further own LSP will need to be generated when the LSP databases
have synchronized. have synchronized.
When the timer T2 expires, or is cancelled indicating that 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 synchronization for that level is complete, the SPF for that level
is run in order to derive any information which is required to be is 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 'own' LSPs can be generated and flooded. Any
'own' LSPs which were previously ignored, but which are not part of 'own' LSPs which were previously ignored, but which are not part of
the current set of 'own' LSPs (including pseudonodes) should then be the current set of 'own' LSPs (including pseudonodes) should then be
purged. Note that it is possible that a Designated Router change may purged. Note that it is possible that a Designated Router change may
have taken place, and consequently the router should purge those have taken place, and consequently the router should purge those
pseudonode LSPs which it previously owned, but which are now no 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, this If the timer T3 expires before all the T2 timers have expired or
indicates that the synchronization process is taking longer than been cancelled, this indicates that the synchronization process is
minimum holding time of the neighbors. The router's own LSP(s) for taking longer than minimum holding time of the neighbors. The
levels which have not yet completed their first SPF computation are router's own LSP(s) for levels which have not yet completed their
then flooded with the overload bit set to indicate that the router's first SPF computation are then flooded with the overload bit set to
LSPDB is not yet synchronized (and other routers should therefore indicate that the router's LSPDB is not yet synchronized (and other
not compute routes through this router). In order to prevent the routers should therefore not compute routes through this router).
neighbor's adjacencies from expiring, IIHs with the normal interface Normal operation of the update process resumes and the local
value for the holding time are transmitted over all interfaces with forwarding tables are updated. In order to prevent the neighbor's
neither RR nor RA set in the restart TLV. This will cause the adjacencies from expiring, IIHs with the normal interface value for
neighbors to refresh their adjacencies. The own LSP(s) will continue the holding time are transmitted over all interfaces with neither RR
to have the overload bit set until timer T2 has been cancelled as in nor RA set in the restart TLV. This will cause the neighbors to
the case of starting for the first time described in section 4.3.1.1 refresh their adjacencies. The own LSP(s) will continue to have the
overload bit set until timer T2 has expired or been cancelled.
4.4.1.2. Starting
In the case of a starting router, as soon as each adjacency is
established, and before any CSNP exchanges, the router's own zeroth
LSP is transmitted with the overload bit set. This prevents other
routers from computing routes through the router until it has
reliably acquired the complete set of LSPs. The overload bit remains
set in subsequent transmissions of the zeroth LSP (such as will
occur if a previous copy of the routers LSP is still present in the
network) while any timer T2 is running.
When all the T2 timers have been cancelled, the own LSP(s) MAY be
regenerated with the overload bit clear (assuming the router isn't
in fact overloaded, and there is no other reason, such as incomplete
BGP convergence, to keep the overload bit set), and flooded as
normal.
Other 'own' LSPs (including pseudonodes) are generated and flooded
as normal, irrespective of the timer T2. The SPF is also run as
normal and the RIB and FIB updated as routes become available.
To avoid the possible formation of temporary blackholes the starting
router sets the SA bit in the restart TLV (as described in 4.3.2) in
all IIHs that it sends.
When all T2 timers have been cancelled, the starting router MUST
transmit IIHs with the SA bit clear.
5. Security Considerations 5. Security Considerations
This memo does not create any new security issues for the IS-IS This memo does not create any new security issues for the IS-IS
protocol. Security considerations for the base IS-IS protocol are protocol. Security considerations for the base IS-IS protocol are
covered in [2] and [3]. covered in [2] and [3].
6. References 6. References
1 Bradner, S., "The Internet Standards Process -- Revision 3", BCP 1 Bradner, S., "The Internet Standards Process -- Revision 3", BCP
9, RFC 2026, October 1996. 9, RFC 2026, October 1996.
2 Callon, R., "OSI IS-IS for IP and Dual Environment," RFC 1195, 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 3 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 10589:1992. (ISO 8473)," ISO/IEC 10589:2002, Second Edition.
4 Bradner, S., "Key words for use in RFCs to Indicate Requirement 4 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., "Three-Way Handshake for IS-IS Point-to-Point 5 Katz, D., "Three-Way Handshake for IS-IS Point-to-Point
Adjacencies", draft-ietf-isis-3way-03.txt, July 2000 Adjacencies", RFC 3373, September 2002
7. Acknowledgments 7. Acknowledgments
The author would like to acknowledge contributions made by Radia The authors would like to acknowledge contributions made by Radia
Perlman, Mark Schaefer, Naiming Shen, Nischal Sheth, Russ White, and Perlman, Mark Schaefer, Naiming Shen, Nischal Sheth, Russ White, and
Rena Yang. Rena Yang.
8. Author's Address 8. Authors' Addresses
Mike Shand Mike Shand
Cisco Systems Cisco Systems
4, The Square, 250 Longwater Avenue,
Stockley Park, Reading,
UXBRIDGE, Berkshire,
Middlesex RG2 6GB
UB11 1BN, UK UK
Phone: +44 208 824 8690 Phone: +44 208 824 8690
Email: mshand@cisco.com Email: mshand@cisco.com
Les Ginsberg
Cisco Systems
510 McCarthy Blvd.
Milpitas, Ca. 95035 USA
Email: ginsberg@cisco.com
 End of changes. 

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