INTERNET DRAFT IS-IS restart Jan 2004
Network Working Group M. Shand
Internet Draft L. Ginsberg
Expiration Date: July 2004 Cisco Systems
January 2004
Restart signaling for IS-IS
draft-ietf-isis-restart-05.txt
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC 2026.
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Copyright Notice Copyright (C) The Internet Society (2003). All
Rights Reserved.
Abstract
The 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
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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
highly undesirable. This draft describes mechanisms to avoid or
minimize the disruption due to both of these causes.
Table of Contents
1. Conventions used in this document..............................3
2. Overview.......................................................3
3. Approach.......................................................4
3.1 Timers.......................................................4
3.2 Restart TLV..................................................4
3.2.1 Use of RR and RA bits.....................................5
3.2.2 Use of SA bit.............................................7
3.3 Adjacency (re)acquisition....................................8
3.3.1 Adjacency reacquisition during restart....................8
3.3.2 Adjacency acquisition during start.......................10
3.3.3 Multiple levels..........................................11
3.4 Database synchronization....................................12
3.4.1 LSP generation and flooding and SPF computation..........12
3.4.1.1. Restarting..........................................13
3.4.1.2. Starting............................................14
4. State Tables..................................................16
4.1 Running Router..............................................16
4.2 Restarting Router...........................................17
4.3 Starting Router.............................................18
5. Security Considerations.......................................19
6. IANA Considerations...........................................19
7. Normative References..........................................20
8. Acknowledgments...............................................20
9. Authors' Addresses............................................20
10. Full Copyright Statement.....................................21
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1. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in 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.
2. Overview
When an adjacency is reinitialized as a result of a neighbor
restarting, a router does three things:
1. It causes its own LSP(s) to be regenerated, thus triggering
SPF runs throughout the area (or in the case of Level 2,
throughout the domain).
2. It sets SRMflags on its own LSP database on the adjacency
concerned.
3. In the case of a Point-to-Point link it transmits a (set of)
CSNP(s) over the adjacency.
In the case of a restarting router process, the first of these is
highly undesirable, but the second is essential in order to ensure
synchronization of the LSP database.
The third action above minimizes the number of LSPs which must be
exchanged and, if made reliable, provides a means of determining
when the LSP databases of the neighboring routers have been
synchronized. This is desirable whether the router is being
restarted or not (so that the overload bit can be cleared in the
router's own LSP, for example).
This draft describes a mechanism for a restarting router to signal
that it is restarting to its neighbors, and allow them to
reestablish their adjacencies without cycling through the down
state, while still correctly initiating database synchronization.
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This draft additionally describes a mechanism for a restarting
router to determine when it has achieved LSP database
synchronization with its neighbors and a mechanism to optimize LSP
database synchronization and minimize transient routing disruption
when a router starts.
It is assumed that the three-way handshake [5] is being used on
Point-to-Point circuits.
3. Approach
3.1 Timers
Three additional timers, T1, T2 and T3 are required to support the
functionality defined in this document.
An instance of the timer T1 is maintained per interface, and
indicates the time after which an unacknowledged (re)start attempt
will be repeated. A typical value might be 3 seconds.
An instance of the timer T2 is maintained for each LSP database
present in the system i.e. for a Level1/2 system, there will be an
instance of the timer T2 for Level 1 and an instance for Level 2.
This is the maximum time that the system will wait for LSPDB
synchronization. A typical value might be 60 seconds.
A single instance of the timer T3 is maintained for the entire
system. It indicates the time after which the router will declare
that it has failed to achieve database synchronization (by setting
the overload bit in its own LSP). This is initialized to 65535
seconds, but is set to the minimum of the remaining times of
received IIHs containing a restart TLV with RA set and an indication
that the neighbor has an adjacency in the UP state to the restarting
router.
NOTE: The timer T3 is only used by a restarting router.
3.2 Restart TLV
A new TLV is defined to be included in IIH PDUs. The presence of
this TLV indicates that the sender supports the functionality
defined in this document and it carries flags that are used to
convey information during a (re)start. All IIHs transmitted by a
router that supports this capability MUST include this TLV.
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Type 211
Length # of octets in the value field (1 to (3 + ID Length))
Value
No. of octets
+-----------------------+
| Flags | 1
+-----------------------+
| Remaining Time | 2
+-----------------------+
| Restarting Neighbor ID| ID Length
+-----------------------+
Flags (1 octet)
0 1 2 3 4 5 6 7
+--+--+--+--+--+--+--+--+
| Reserved |SA|RA|RR|
+--+--+--+--+--+--+--+--+
RR - Restart Request
RA - Restart Acknowledgment
SA - Suppress adjacency advertisement
(Note: Remaining fields are required when RA bit is set)
Remaining Time (2 octets)
Remaining holding time (in seconds)
Restarting Neighbor System ID (ID Length octets)
The system ID of the neighbor to which an RA refers. Note:
Implementations based on earlier versions of this document
may not include this field in the TLV when RA is set. In
this case a router which is expecting an RA on a LAN
circuit SHOULD assume that the acknowledgement is directed
at the local system.
3.2.1 Use of RR and RA bits
The RR bit is used by a (re)starting router to signal to its
neighbors that a (re)start is in progress, that an existing
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adjacency SHOULD be maintained even under circumstances when the
normal operation of the adjacency state machine would require the
adjacency to be reinitialized, to request a set of CSNPs, and to
request setting of SRMflags.
The RA bit is sent by the neighbor of a (re)starting router to
acknowledge the receipt of a restart TLV with the RR bit set.
When the neighbor of a (re)starting router receives an IIH with the
restart TLV having the RR bit set, if there exists on this interface
an adjacency in state "Up" with the same System ID, and in the case
of a LAN circuit, with the same source LAN address, then,
irrespective of the other contents of the "Intermediate System
Neighbors" option (LAN circuits), or the "Point-to-Point Three-Way
Adjacency" option (Point-to-Point circuits):
a) The state of the adjacency is not changed. If this is the first
IIH with the RR bit set that this system has received associated
with this adjacency then the adjacency is marked as being in
"Restart mode" and the adjacency holding time is refreshed -
otherwise the holding time is not refreshed. The "remaining time"
transmitted according to (b) below MUST reflect the actual time
after which the adjacency will now expire. Receipt of a normal
IIH with RR bit reset will clear the "Restart mode" state. This
procedure allows the restarting router to cause the neighbor to
maintain the adjacency long enough for restart to successfully
complete while also preventing repetitive restarts from
maintaining an adjacency indefinitely. Whether an adjacency is
marked as being in "Restart mode" or not has no effect on
adjacency state transitions.
b) immediately (i.e. without waiting for any currently running timer
interval to expire, but with a small random delay of a few 10s of
milliseconds on LANs to avoid "storms"), transmit over the
corresponding interface an IIH including the restart TLV with the
RR bit clear and the RA bit set, in the case of Point-to-Point
adjacencies having updated the "Point-to-Point Three-Way
Adjacency" option to reflect any new values received from the
(re)starting router. (This allows a restarting router to quickly
acquire the correct information to place in its hellos.) The
"Remaining Time" MUST be set to the current time (in seconds)
before the holding timer on this adjacency is due to expire. If
the corresponding interface is a LAN interface, then the
Restarting Neighbor System ID SHOULD be set to the System ID of
the router from whom the IIH with RR bit set was received. This
is required to correctly associate the acknowledgement and
holding time in the case where multiple systems on a LAN restart
at approximately the same time. This IIH SHOULD be transmitted
before any LSPs or SNPs transmitted as a result of the receipt of
the original IIH.
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c) if the corresponding interface is a Point-to-Point interface, or
if the receiving router has the highest LnRouterPriority (with
highest source MAC address breaking ties) among those routers to
which the receiving router has an adjacency in state "Up" on this
interface whose IIHs contain the restart TLV, excluding
adjacencies to all routers which are considered in "Restart mode"
(note the actual DIS is NOT changed by this process), initiate
the transmission over the corresponding interface of a complete
set of CSNPs, and set SRMflags on the corresponding interface for
all LSPs in the local LSP database.
Otherwise (i.e. if there was no adjacency in the "UP" state to the
system ID in question), process the IIH as normal by reinitializing
the adjacency, and setting the RA bit in the returned IIH.
3.2.2 Use of SA bit
The SA bit is used by a starting router to request that its neighbor
suppress advertisement of the adjacency to the starting router in
the neighbor's LSPs.
A router which is starting has no maintained forwarding function
state. This may or may not be the first time the router has started.
If this is not the first time the router has started, copies of LSPs
generated by this router in its previous incarnation may exist in
the LSP databases of other routers in the network. These copies are
likely to appear "newer" than LSPs initially generated by the
starting router due to the reinitialization of LSP fragment sequence
numbers by the starting router. This may cause temporary blackholes
to occur until the normal operation of the update process causes the
starting router to regenerate and flood copies of its own LSPs with
higher sequence numbers. The temporary blackholes can be avoided if
the starting router's neighbors suppress advertising an adjacency to
the starting router until the starting router has been able to
propagate newer versions of LSPs generated by previous incarnations.
When a router receives an IIH with the restart TLV having the SA bit
set, if there exists on this interface an adjacency in state "Up"
with the same System ID, and in the case of a LAN circuit, with the
same source LAN address, then the router MUST suppress advertisement
of the adjacency to the neighbor in its own LSPs. Until an IIH with
the SA bit clear has been received, the neighbor advertisement MUST
continue to be suppressed. If the adjacency transitions to the UP
state, the new adjacency MUST NOT be advertised until an IIH with
the SA bit clear has been received.
Note that a router which suppresses advertisement of an adjacency
MUST NOT use this adjacency when performing its SPF calculation. In
particular, if an implementation follows the example guidelines
presented in [3] Annex C.2.5 Step 0:b) "pre-load TENT with the local
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adjacency database", the suppressed adjacency MUST NOT be loaded
into TENT.
3.3 Adjacency (re)acquisition
Adjacency (re)acquisition is the first step in (re)initialization.
Restarting and starting routers will make use of the RR bit in the
restart TLV, though each will use it at different stages of the
(re)start procedure.
3.3.1 Adjacency reacquisition during restart
The restarting router explicitly notifies its neighbor that the
adjacency is being reacquired, and hence that it SHOULD NOT
reinitialize the adjacency. This is achieved by setting the RR bit
in the restart TLV. When the neighbor of a restarting router
receives an IIH with the restart TLV having the RR bit set, if there
exists on this interface an adjacency in state "Up" with the same
System ID, and in the case of a LAN circuit, with the same source
LAN address, then the procedures described in 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 the timer
T2 for each LSPDB and for each interface (and in the case of a LAN
circuit, for each level) starts the timer T1 and transmits an IIH
containing the restart TLV with the RR bit set.
On a Point-to-Point circuit the restarting router SHOULD set the
"Adjacency Three-Way State" to "Init", because the receipt of the
acknowledging IIH (with RA set) MUST cause the adjacency to enter
"Up" state immediately.
On a LAN circuit the LAN-ID assigned to the circuit SHOULD be the
same as that used prior to the restart. In particular, for any
circuits for which the restarting router was previously DIS, the use
of a different LAN-ID would necessitate the generation of a new set
of pseudonode LSPs, and corresponding changes in all the LSPs
referencing them from other routers on the LAN. By preserving the
LAN-ID across the restart, this churn can be prevented. To enable a
restarting router to learn the LAN-ID used prior to restart, the
LAN-ID specified in an IIH with RR set MUST be ignored.
Transmission of "normal" IIHs is inhibited until the conditions
described below are met (in order to avoid causing an unnecessary
adjacency initialization). On expiry of the timer T1, it is
restarted and the IIH is retransmitted as above.
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When a restarting router receives an IIH a local adjacency is
established as usual, and if the IIH contains a restart TLV with the
RA bit set (and on LAN circuits with a Restart Neighbor System ID
which matches that of the local system), the receipt of the
acknowledgement over that interface is noted. When the RA bit is set
and the state of the remote adjacency is UP then the timer T3 is set
to the minimum of its current value and the value of the "Remaining
Time" field in the received IIH.
On a Point-to-Point link, receipt of an IIH not containing the
restart TLV is also treated as an acknowledgement, since it
indicates that the neighbor is not restart capable. However, since
no CSNP is guaranteed to be received over this interface, the timer
T1 is cancelled immediately without waiting for a complete set of
CSNP(s). Synchronization may therefore be deemed complete even
though there are some LSPs which are held (only) by this neighbor
(see section 4.4). In this case we also want to be certain that the
neighbor will reinitialize the adjacency in order to guarantee that
SRMflags have been set on its database, thus ensuring eventual LSPDB
synchronization. This is guaranteed to happen except in the case
where the Adjacency Three-Way State in the received IIH is UP and
the Neighbor Extended Local Circuit ID matches the extended local
circuit ID assigned by the restarting router. In this case the
restarting router MUST force the adjacency to reinitialize by
setting the local Adjacency Three-Way State to DOWN and sending a
normal IIH.
In the case of a LAN interface, receipt of an IIH not containing the
restart TLV is unremarkable since synchronization can still occur so
long as at least one of the non-restarting neighboring routers on
the LAN supports restart. Therefore T1 continues to run in this
case. If none of the neighbors on the LAN are restart capable, T1
will eventually expire after the locally defined number of retries.
In the case of a Point-to-Point circuit, the "LocalCircuitID" and
"Extended Local Circuit ID" information contained in the IIH can be
used immediately to generate an IIH containing the correct 3-way
handshake information. The presence of "Neighbor Extended Local
Circuit ID" information which does not match the value currently in
use by the local system is ignored (since the IIH may have been
transmitted before the neighbor had received the new value from the
restarting router), but the adjacency remains in the initializing
state until the correct information is received.
In the case of a LAN circuit the source neighbor information (e.g.
SNPAAddress) is recorded and used for adjacency establishment and
maintenance as normal.
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When BOTH a complete set of CSNP(s) (for each active level, in the
case of a pt-pt circuit) and an acknowledgement have been received
over the interface, the timer T1 is cancelled.
Once the timer T3 has expired or been cancelled, subsequent IIHs are
transmitted according to the normal algorithms, but including the
restart TLV with both RR and RA clear.
If a LAN contains a mixture of systems, only some of which support
the new algorithm, database synchronization is still guaranteed, but
the "old" systems will have reinitialized their adjacencies.
If an interface is active, but does not have any neighboring router
reachable over that interface the timer T1 would never be cancelled,
and according to clause 3.4.1.1 the SPF would never be run.
Therefore timer T1 is cancelled after some pre-determined number of
expirations (which MAY be 1).
3.3.2 Adjacency acquisition during start
The starting router wants to ensure that in the event 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 3.2.2
is followed.
On starting, a router starts timer T2 for each LSPDB.
For each interface (and in the case of a LAN circuit, for each
level), when an adjacency reaches the UP state, the starting router
starts a timer T1 and transmits an IIH containing the restart TLV
with the RR bit clear and SA bit set. 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 3.2.1 is followed.
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When an IIH is received by the starting router and the IIH contains
a restart TLV with the RA bit set (and on LAN circuits with a
Restart Neighbor System ID which matches that of the local system),
the receipt of the acknowledgement over that interface is noted.
On a Point-to-Point link, receipt of an IIH not containing the
restart TLV is also treated as an acknowledgement, since it
indicates that the neighbor is not restart capable. Since the
neighbor will have reinitialized the adjacency this guarantees that
SRMflags have been set on its database, thus ensuring eventual LSPDB
synchronization. However, since no CSNP is guaranteed to be received
over this interface, the timer T1 is cancelled immediately without
waiting for a complete set of CSNP(s). Synchronization may therefore
be deemed complete even though there are some LSPs which are held
(only) by this neighbor (see section 4.4).
In the case of a LAN interface, receipt of an IIH not containing the
restart TLV is unremarkable since synchronization can still occur so
long as at least one of the non-restarting neighboring routers on
the LAN supports restart. Therefore T1 continues to run in this
case. If none of the neighbors on the LAN are restart capable, T1
will eventually expire after the locally defined number of retries.
The usual operation of the update process will ensure that
synchronization is eventually achieved.
When BOTH a complete set of CSNP(s) (for each active level, in the
case of a 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.
Timer T1 is cancelled after some pre-determined number of
expirations (which MAY be 1).
When the T2 timer(s) are cancelled or expire transmission of
"normal" IIHs (with RR, RA, and SA bits clear) will begin.
3.3.3 Multiple levels
A router which is operating as both a Level 1 and a Level 2 router
on a particular interface MUST perform the above operations for each
level.
On a LAN interface, it MUST send and receive both Level 1 and
Level 2 IIHs and perform the CSNP synchronizations independently for
each level.
On a pt-pt interface, only a single IIH (indicating support for both
levels) is required, but it MUST perform the CSNP synchronizations
independently for each level.
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3.4 Database synchronization
When a router is started or restarted it can expect to receive a
(set of) CSNP(s) over each interface. The arrival of the CSNP(s) is
now guaranteed, since an IIH with RR bit set will be retransmitted
until the CSNP(s) are correctly received.
The CSNPs describe the set of LSPs that are currently held by each
neighbor. Synchronization will be complete when all these LSPs have
been received.
When (re)starting, a router starts an instance of timer T2 for each
LSPDB as described in 4.3.1 or 4.3.2. In addition to normal
processing of the CSNPs, the set of LSPIDs contained in the first
complete set of CSNP(s) received over each interface is recorded,
together with their remaining lifetime. In the case of a LAN
interface, a complete set of CSNPs MUST consist of CSNPs received
from neighbor(s) which are not restarting. If there are multiple
interfaces on the (re)starting router, the recorded set of LSPIDs is
the union of those received over each interface. LSPs with a
remaining lifetime of zero are NOT so recorded.
As LSPs are received (by the normal operation of the update process)
over any interface, the corresponding LSPID entry is removed (it is
also removed if the LSP had arrived before the CSNP containing the
reference). When an LSPID has been held in the list for its
indicated remaining lifetime, it is removed from the list. When the
list of LSPIDs is empty and the timer T1 has been cancelled for all
the interfaces that have an adjacency at this level, the timer T2 is
cancelled.
At this point the local database is guaranteed to contain all the
LSP(s) (either the same sequence number, or a more recent sequence
number) which were present in the neighbors' databases at the time
of (re)starting. LSPs that arrived in a neighbor's database after
the time of (re)starting may or may not be present, but the normal
operation of the update process will guarantee that they will
eventually be received. At this point the local database is deemed
to be "synchronized".
Since LSPs mentioned in the CSNP(s) with a zero remaining lifetime
are not recorded, and those with a short remaining lifetime are
deleted from the list when the lifetime expires, cancellation of the
timer T2 will not be prevented by waiting for an LSP that will never
arrive.
3.4.1 LSP generation and flooding and SPF computation
The operation of a router starting, as opposed to restarting is
somewhat different. These two cases are dealt with separately below.
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3.4.1.1. Restarting
In order to avoid causing unnecessary routing churn in other
routers, it is highly desirable that the own LSPs generated by the
restarting system are the same as those previously present in the
network (assuming no other changes have taken place). It is
important therefore not to regenerate and flood the LSPs until all
the adjacencies have been re-established and any information
required for propagation into the local LSPs is fully available.
Ideally, the information is loaded into the LSPs in a deterministic
way, such that the same information occurs in the same place in the
same LSP (and hence the LSPs are identical to their previous
versions). If this can be achieved, the new versions may not even
cause SPF to be run in other systems. However, provided the same
information is included in the set of LSPs (albeit in a different
order, and possibly different LSPs), the result of running the SPF
will be the same and will not cause churn to the forwarding tables.
In the case of a restarting router, none of the router's own LSPs
are transmitted, nor are the router's own forwarding tables updated
while the timer T3 is running.
Redistribution of inter-level information MUST be regenerated before
this router's LSP is flooded to other nodes. Therefore the Level-n
non-pseudonode LSP(s) MUST NOT be flooded until the other level's T2
timer has expired and its SPF has been run. This ensures that any
inter-level information which is to be propagated can be included in
the Level-n LSP(s).
During this period, if one of the router's own (including
pseudonodes) LSPs is received, which the local router does not
currently have in its own database, it is NOT purged. Under normal
operation, such an LSP would be purged, since the LSP clearly should
not be present in the global LSP database. However, in the present
circumstances, this would be highly undesirable, because it could
cause premature removal of an own LSP - and hence churn in remote
routers. Even if the local system has one or more own LSPs (which it
has generated, but not yet transmitted) it is still not valid to
compare the received LSP against this set, since it may be that as a
result of propagation between Level 1 and Level 2 (or vice versa) a
further own LSP will need to be generated when the LSP databases
have synchronized.
During this period a restarting router SHOULD send CSNPs as it
normally would. Information about the router's own LSPs MAY be
included, but if it is included it MUST be based on LSPs which have
been received, not on versions which have been generated (but not
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yet transmitted). This restriction is necessary to prevent premature
removal of an LSP from the global LSP database.
When the timer T2 expires or is cancelled indicating that
synchronization for that level is complete, the SPF for that level
is run in order to derive any information which is required to be
propagated to another level, but the forwarding tables are not yet
updated.
Once the other level's SPF has run and any inter-level propagation
has been resolved, the own LSPs can be generated and flooded. Any
own LSPs which were previously ignored, but which are not part of
the current set of own LSPs (including pseudonodes) MUST then be
purged. Note that it is possible that a Designated Router change may
have taken place, and consequently the router SHOULD purge those
pseudonode LSPs which it previously owned, but which are now no
longer part of its set of pseudonode LSPs.
When all the T2 timers have expired or been cancelled, the timer T3
is cancelled and the local forwarding tables are updated.
If the timer T3 expires before all the T2 timers have expired or
been cancelled, this indicates that the synchronization process is
taking longer than minimum holding time of the neighbors. The
router's own LSP(s) for levels which have not yet completed their
first SPF computation are then flooded with the overload bit set to
indicate that the router's LSPDB is not yet synchronized (and
therefore other routers MUST NOT compute routes through this
router). Normal operation of the update process resumes and the
local forwarding tables are updated. In order to prevent the
neighbor's adjacencies from expiring, IIHs with the normal interface
value for the holding time are transmitted over all interfaces with
neither RR nor RA set in the restart TLV. This will cause the
neighbors to refresh their adjacencies. The own LSP(s) will continue
to have the overload bit set until timer T2 has expired or been
cancelled.
3.4.1.2. Starting
In the case of a starting router, as soon as each adjacency is
established, and before any CSNP exchanges, the router's own zeroth
LSP is transmitted with the overload bit set. This prevents other
routers from computing routes through the router until it has
reliably acquired the complete set of LSPs. The overload bit remains
set in subsequent transmissions of the zeroth LSP (such as will
occur if a previous copy of the 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
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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.
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4. State Tables
This section presents state tables which summarize the behaviors
described in this document. Other behaviors, in particular adjacency
state transitions and LSP database update operation, are NOT
included in the state tables except where this document modifies the
behaviors described in [3] and [5].
The states named in the columns of the tables below are a mixture of
states that are specific to a single adjacency (ADJ suppressed, ADJ
Seen RA, ADJ Seen CSNP) and states which are indicative of the state
of the protocol instance (Running, Restarting, Starting, SPF Wait).
Three state tables are presented from the point of view of a running
router, a restarting router, and a starting router.
4.1 Running Router
Event | Running | ADJ suppressed
==============================================================
RX RR | Maintain ADJ State |
| Send RA |
| Set SRM,send CSNP |
| (Note 1) |
| Update Hold Time, |
| set Restart Mode |
| (Note 2) |
-------------+----------------------+-------------------------
RX RR clr | Clr Restart mode |
-------------+----------------------+-------------------------
RX SA | Suppress IS neighbor |
| TLV in LSP(s) |
| Goto ADJ Suppressed |
-------------+----------------------+-------------------------
RX SA clr | |Unsuppress IS neighbor
| | TLV in LSP(s)
| |Goto Running
==============================================================
Note 1: CSNPs are sent by routers in accordance with Section 3.2.1c
Note 2: If Restart Mode clear
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4.2 Restarting Router
Event | Restarting | ADJ Seen | ADJ Seen | SPF Wait
| | RA | CSNP |
===================================================================
Router | Send IIH/RR | | |
restarts | ADJ Init | | |
| Start T1,T2,T3 | | |
------------+--------------------+-----------+-----------+------------
RX RR | Send RA | | |
------------+--------------------+-----------+-----------+------------
RX RA | Adjust T3 | | Cancel T1 |
| Goto ADJ Seen RA | | Adjust T3 |
----------- +--------------------+-----------+-----------+------------
RX CSNP set| Goto ADJ Seen CSNP | Cancel T1 | |
------------+--------------------+-----------+-----------+------------
RX IIH w/o | Cancel T1 (Point- | | |
Restart TLV| to-point only) | | |
------------+--------------------+-----------+-----------+------------
T1 Expires | Send IIH/RR |Send IIH/RR|Send IIH/RR|
| Restart T1 | Restart T1| Restart T1|
------------+--------------------+-----------+-----------+------------
T1 Expires | Send IIH/ | Send IIH/ | Send IIH/ |
nth time | normal | normal | normal |
------------+--------------------+-----------+-----------+------------
T2 expires | Trigger SPF | | |
| Goto SPF Wait | | |
------------+--------------------+-----------+-----------+------------
T3 expires | Set OL | | |
| Flood local LSPs | | |
| Update fwd plane | | |
------------+--------------------+-----------+-----------+------------
LSP DB Sync| Cancel T2, and T3 | | |
| Trigger SPF | | |
| Goto SPF wait | | |
------------+--------------------+-----------+-----------+------------
All SPF | | | | Clear OL
done | | | | Update fwd
| | | | plane
| | | | Flood local
| | | | LSPs
| | | | Goto Runing
======================================================================
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4.3 Starting Router
Event | Starting | ADJ Seen RA| ADJ Seen CSNP
=============================================================
Router | Send IIH/SA | |
starts | Start T1,T2 | |
-------------+-------------------+------------+---------------
RX RR | Send RA | |
-------------+-------------------+------------+---------------
RX RA | Goto ADJ Seen RA | | Cancel T1
-------------+-------------------+------------+---------------
RX CSNP Set | Goto ADJ Seen CSNP| Cancel T1 |
-------------+-------------------+------------+---------------
RX IIH w | Cancel T1 | |
no Restart | (Point-to-Point | |
TLV | only) | |
-------------+-------------------+------------+---------------
ADJ UP | Start T1 | |
| Send local LSPs | |
| w OL | |
-------------+-------------------+------------+---------------
T1 Expires | Send IIH/RR |Send IIH/RR | Send IIH/RR
| and SA | and SA | and SA
| Restart T1 |Restart T1 | Restart T1
-------------+-------------------+------------+---------------
T1 Expires | Send IIH/SA |Send IIH/SA | Send IIH/SA
nth time | | |
-------------+-------------------+------------+---------------
T2 expires | Clear OL | |
| Send IIH normal | |
| Goto Running | |
-------------+-------------------+------------+---------------
LSP DB Sync | Cancel T2 | |
| Clear OL | |
| Send IIH normal | |
==============================================================
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5. Security Considerations
Any new security issues raised by the procedures in this document
depend upon the ability of an attacker to inject a false but
apparently valid IIH, the ease/difficulty of which has not been
altered.
If the RR bit is set in a false IIH, neighbors who receive such an
IIH will continue to maintain an existing adjacency in the UP state
and may (re)send a complete set of CSNPs. While the latter action is
wasteful, neither action causes any disruption in correct protocol
operation.
If the RA bit is set in a false IIH, a (re)starting router which
receives such an IIH may falsely believe that there is a neighbor on
the corresponding interface which supports the procedures described
in this document. In the absence of receipt of a complete set of
CSNPs on that interface, this could delay the completion of
(re)start procedures by requiring the timer T1 to time out the
locally defined maximum number of retries. This behavior is the same
as would occur on a LAN where none of the (re)starting router's
neighbors support the procedures in this document and is covered in
Sections 3.3.1 and 3.3.2.
If an SA bit is set in a false IIH, this could cause suppression of
the advertisement of an IS neighbor which could either continue for
an indefinite period or occur intermittently with the result being
possible loss of reachability to some destinations in the network
and/or increased frequency of LSP flooding and SPF calculation.
The possibility of IS-IS PDU spoofing can be reduced by the use of
authentication as described in [2] and [3], and especially the use
of cryptographic authentication as described in [6].
6. IANA Considerations
This document defines the following new ISIS TLV that needs to be
reflected in the ISIS TLV code-point registry:
Type Description IIH LSP SNP
---- ----------------------------------- --- --- ---
211 Restart TLV y n n
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7. Normative References
1 Bradner, S., "The Internet Standards Process -- Revision 3", BCP
9, RFC 2026, October 1996.
2 Callon, R., "OSI IS-IS for IP and Dual Environment," RFC 1195,
December 1990.
3 ISO, "Intermediate system to Intermediate system routeing
information exchange protocol for use in conjunction with the
Protocol for providing the Connectionless-mode Network Service
(ISO 8473)," ISO/IEC 10589:2002, Second Edition.
4 Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997
5 Katz, D., and Saluja, R., "Three-Way Handshake for IS-IS Point-
to-Point Adjacencies", RFC 3373, September 2002
6 Li, T., and Atkinson, R., "Intermediate System to Intermediate
System (IS-IS) Cryptographic Authentication", RFC 3567, July
2003
7 Narten, T. and Alvestrand, H., "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26 , RFC 2434, October 1998
8. Acknowledgments
The authors would like to acknowledge contributions made by Jeff
Parker, Radia Perlman, Mark Schaefer, Naiming Shen, Nischal Sheth,
Russ White, and Rena Yang.
9. Authors' Addresses
Mike Shand
Cisco Systems
250 Longwater Avenue,
Reading,
Berkshire,
RG2 6GB
UK
Phone: +44 208 824 8690
Email: mshand@cisco.com
Les Ginsberg
Cisco Systems
510 McCarthy Blvd.
Milpitas, Ca. 95035 USA
Email: ginsberg@cisco.com
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10. Full Copyright Statement
Copyright (C) The Internet Society (2003). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph
are included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
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