Service Manual

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 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 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 route processor module
(RPM) 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 (FIB) 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. After you enable graceful restart, these
timers manage the graceful restart process.
There are three times, T1, T2, and T3.
• 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 the RR bit set in Restart TLV) until the
complete sequence number PDU (CSNP) is received from the helping router. You can set the duration
to a specific amount of time (seconds) or a number of attempts.
• The T2 timer is the maximum time that the system waits 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). You can base this timer 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 the neighbors advertise) or by setting a specific
amount of time manually.
Implementation Information
IS-IS implementation supports one instance of IS-IS and six areas.
You can configure the system as a Level 1 router, a Level 2 router, or a Level 1-2 router. For IPv6, the IPv4
implementation has been expanded to include two types, length, values (TLVs) in the PDU that carries
Intermediate System to Intermediate System 577