Reference Guide

492 | Intermediate System to Intermediate System
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Graceful Restart
Graceful Restart is supported on the S5000 platform for both Helper and Restart modes.
Graceful Restart is a protocol-based mechanism that preserves the forwarding table of the restarting router
and its neighbors for a specified period to minimize the loss of packets. A graceful-restart router does not
immediately assume that a neighbor is permanently down and so does not trigger a topology change.
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. When a router goes down without a Graceful
Restart, there is a potential to lose access to parts of the network due to the necessity of network topology
changes.
IS-IS Graceful Restart recognizes the fact that in a modern router, the control plane and data plane are
functionally separate. Restarting the control plane functionality (such as the failover of the active
stack-unit to the backup in a redundant configuration) should not necessarily interrupt data packet
forwarding. This behavior is supported because the forwarding tables previously computed by an active
stack-unit have been downloaded into the Forwarding Information Base on the data plane and are still
resident. For packets that have existing FIB/CAM entries, forwarding between ingress and egress ports can
continue uninterrupted while the control plane IS-IS process comes back to full functionality and rebuilds
its routing tables.
A new TLV (the Restart TLV) is introduced in the IIH PDUs, indicating that the router supports Graceful
Restart.
Timers
Three timers are used to support IS-IS Graceful Restart functionality. Once Graceful Restart is enabled,
these timers manage the Graceful Restart process.
• The T1 timer specifies the wait time before unacknowledged restart requests are generated. This is the
interval before the system sends a Restart Request (an IIH with RR bit set in Restart TLV) until the
CSNP is received from the helping router. The duration can be set to a specific amount of time
(seconds) or a number of attempts.
• The T2 timer is the maximum time that the system will wait for LSP database synchronization. This
timer applies to the database type (level-1, level-2 or both).
• The T3 timer sets the overall wait time after which the router determines that it has failed to achieve
database synchronization (by setting the overload bit in its own LSP). This timer can be based on
adjacency settings with the value derived from adjacent routers that are engaged in graceful restart
recovery (the minimum of all the Remaining Time values advertised by the neighbors) or by setting a
specific amount of time manually.