Reference Guide

Adjacencies
Adjacencies on point-to-point interfaces are formed as usual, where IS-IS routers do not implement MT extensions.
If a local router does not participate in certain MTs, it does not advertise those MT IDs in its IS-IS hellos (IIHs) and so
does not include that neighbor within its LSPs. If an MT ID is not detected in the remote side’s IIHs, the local router does
not include that neighbor within its LSPs. The local router does not form an adjacency if both routers do not have at least
one common MT over the interface.
Graceful Restart
Graceful restart is supported on the 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 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 RPM have been downloaded into the
forwarding information base (FIB) on the line cards (the data plane) and are still resident. For packets that have existing
FIB/content addressable memory (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 advertised by the neighbors) 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 new type, length, values (TLVs) in the PDU that carry information
required for IPv6 routing. The new TLVs are IPv6 Reachability and IPv6 Interface Address . Also, a new IPv6 protocol
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