draft-ietf-lsr-isis-rfc5306bis-08.txt   draft-ietf-lsr-isis-rfc5306bis-09.txt 
IS-IS for IP Internets L. Ginsberg IS-IS for IP Internets L. Ginsberg
Internet-Draft P. Wells Internet-Draft P. Wells
Obsoletes: 5306 (if approved) Cisco Systems, Inc. Obsoletes: 5306 (if approved) Cisco Systems, Inc.
Intended status: Standards Track September 19, 2019 Intended status: Standards Track September 19, 2019
Expires: March 22, 2020 Expires: March 22, 2020
Restart Signaling for IS-IS Restart Signaling for IS-IS
draft-ietf-lsr-isis-rfc5306bis-08 draft-ietf-lsr-isis-rfc5306bis-09
Abstract Abstract
This document describes a mechanism for a restarting router to signal This document describes a mechanism for a restarting router to signal
to its neighbors that it is restarting, allowing them to reestablish to its neighbors that it is restarting, allowing them to reestablish
their adjacencies without cycling through the down state, while still their adjacencies without cycling through the down state, while still
correctly initiating database synchronization. correctly initiating database synchronization.
This document additionally describes a mechanism for a router to This document additionally describes a mechanism for a router to
signal its neighbors that it is preparing to initiate a restart while signal its neighbors that it is preparing to initiate a restart while
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acknowledgement is directed at the local system if the TLV acknowledgement is directed at the local system if the TLV
is received with RA set and Restarting Neighbor System ID is received with RA set and Restarting Neighbor System ID
is not present. It is an implementation choice whether to is not present. It is an implementation choice whether to
continue to accept (on a LAN) a TLV with RA set and continue to accept (on a LAN) a TLV with RA set and
Restarting Neighbor System ID absent. Note that the omission Restarting Neighbor System ID absent. Note that the omission
of the Restarting Neighbor System ID only introduces ambiguity of the Restarting Neighbor System ID only introduces ambiguity
in the case where there are multiple systems on a LAN in the case where there are multiple systems on a LAN
simultaneously performing restart. simultaneously performing restart.
The RR and SA flags may both be set in the TLV under the conditions The RR and SA flags may both be set in the TLV under the conditions
described in Section 3.3.2. All other flag combinations are invalid described in Section 3.3.2. All other combinations where multiple
and MUST NOT be transmitted. Received TLVs which have invalid flag flags are set are invalid and MUST NOT be transmitted. Received TLVs
combinations set MUST be ignored. which have invalid flag combinations set MUST be ignored.
3.2.1. Use of RR and RA Bits 3.2.1. Use of RR and RA Bits
The RR bit is used by a (re)starting router to signal to its The RR bit is used by a (re)starting router to signal to its
neighbors that a (re)start is in progress, that an existing adjacency neighbors that a (re)start is in progress, that an existing adjacency
SHOULD be maintained even under circumstances when the normal SHOULD be maintained even under circumstances when the normal
operation of the adjacency state machine would require the adjacency operation of the adjacency state machine would require the adjacency
to be reinitialized, to request a set of CSNPs, and to request to be reinitialized, to request a set of CSNPs, and to request
setting of the SRMflags. setting of the SRMflags.
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router can determine whether it is safe to maintain the adjacency if router can determine whether it is safe to maintain the adjacency if
other topology changes occur prior to the completion of the restart. other topology changes occur prior to the completion of the restart.
Signalling a planned restart in the absence of maintained forwarding Signalling a planned restart in the absence of maintained forwarding
plane state is likely to lead to significant traffic loss and MUST plane state is likely to lead to significant traffic loss and MUST
NOT be done. NOT be done.
Neighbors of the router which has signaled planned restart SHOULD Neighbors of the router which has signaled planned restart SHOULD
maintain the adjacency in a planned restart state until it receives maintain the adjacency in a planned restart state until it receives
an IIH with the RR bit set, receives an IIH with both PR and RR bits an IIH with the RR bit set, receives an IIH with both PR and RR bits
clear, or the adjacency holding time expires - whichever occurs clear, or the adjacency holding time expires - whichever occurs
first. first. Neighbors which choose not to follow the recommended behavior
need to consider the impact on traffic delivery of not using the
restarting router for forwarding traffic during the restart period.
While the adjacency is in planned restart state some or all of the While the adjacency is in planned restart state some or all of the
following actions MAY be taken: following actions MAY be taken:
a. if additional topology changes occur, the adjacency which is in a. if additional topology changes occur, the adjacency which is in
planned restart state MAY be brought down even though the hold planned restart state MAY be brought down even though the hold
time has not yet expired. Given that the neighbor which has time has not yet expired. Given that the neighbor which has
signaled a planned restart is not expected to update its signaled a planned restart is not expected to update its
forwarding plane in response to signalling of the topology forwarding plane in response to signalling of the topology
changes (since it is restarting) traffic which transits that node changes (since it is restarting) traffic which transits that node
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If the SA bit is set in a false IIH, this could cause suppression of If the SA bit is set in a false IIH, this could cause suppression of
the advertisement of an IS neighbor, which could either continue for the advertisement of an IS neighbor, which could either continue for
an indefinite period or occur intermittently with the result being a an indefinite period or occur intermittently with the result being a
possible loss of reachability to some destinations in the network possible loss of reachability to some destinations in the network
and/or increased frequency of LSP flooding and SPF calculation. and/or increased frequency of LSP flooding and SPF calculation.
If the PR bit is set in a false IIH, neighbors who receive such an If the PR bit is set in a false IIH, neighbors who receive such an
IIH could modify the holding time of an existing adjacency IIH could modify the holding time of an existing adjacency
inappropriately. In the event of topology changes, the neighbor inappropriately. In the event of topology changes, the neighbor
might also choose to bring the adjacency down in the false belief might also choose to not flood the topology updates and/or bring the
that the forwarding plane of the router identified as the source of adjacency down in the false belief that the forwarding plane of the
the false IIH is not currently processing announced topology changes. router identified as the source of the false IIH is not currently
processing announced topology changes. This would result in
unnecessary forwarding disruption.
If the PA bit is set in a false IIH, a router that receives such an If the PA bit is set in a false IIH, a router that receives such an
IIH may falsely believe that the neighbor on the corresponding IIH may falsely believe that the neighbor on the corresponding
interface supports the planned restart procedures defined in this interface supports the planned restart procedures defined in this
document. If such a router is planning to restart it might then document. If such a router is planning to restart it might then
proceed to initiate a restart in the false expectation that the proceed to initiate a restart in the false expectation that the
neighbor has updated its holding time as requested. This may result neighbor has updated its holding time as requested. This may result
in the neighbor bringing down the adjacency while the receiving in the neighbor bringing down the adjacency while the receiving
router is restarting, causing unnecessary disruption to forwarding. router is restarting, causing unnecessary disruption to forwarding.
 End of changes. 4 change blocks. 
8 lines changed or deleted 12 lines changed or added

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