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Interface Support
MT IS-IS is supported on physical Ethernet interfaces, port-channel interfaces (static & dynamic using LACP), and VLAN interfaces.
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 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 specied
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 conguration) 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 species 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 specic 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 specic
amount of time manually.
Intermediate System to Intermediate System
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