510 | Intermediate System to Intermediate Systemw w w . d e l l . c o m | s u p p o r t . d e l l . c o m AdjacenciesAdjacencies on point-to-point interfaces are formed as usual, where IS-IS routers do not implementMulti-Topology (MT) extensions. If a local router does not participate in certain MTs, it will not advertisethose MT IDs in its IIHs and so will not include that neighbor within its LSPs. If an MT ID is not detectedin the remote side's IIHs, the local router does not include that neighbor within its LSPs. The local routerwill not form an adjacency if both routers don't have at least one common MT over the interface.Graceful RestartGraceful Restart is supported on e platforms for both Helper and Restart modes.Graceful Restart is a protocol-based mechanism that preserves the forwarding table of the restarting routerand its neighbors for a specified period to minimize the loss of packets. A graceful-restart router does notimmediately 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 boththe restarting router and the neighbors of the restarting router. When a router goes down without a GracefulRestart, there is a potential to lose access to parts of the network due to the necessity of network topologychanges.IS-IS Graceful Restart recognizes the fact that in a modern router, the control plane and data plane arefunctionality separate. Restarting the control plane functionality (such as the failover of the active RPM tothe backup in a redundant configuration) should not necessarily interrupt data packet forwarding. Thisbehavior is supported because the forwarding tables previously computed by an active RPM have beendownloaded into the Forwarding Information Base on the line cards (the data plane) and are still resident.For packets that have existing FIB/CAM entries, forwarding between ingress and egress ports can continueuninterrupted while the control plane IS-IS process comes back to full functionality and rebuilds its routingtables.A new TLV (the Restart TLV) is introduced in the IIH PDUs, indicating that the router supports GracefulRestart.TimersThree timers are used to support IS-IS Graceful Restart functionality. Once Graceful Restart is enabled,these timers manage the the Graceful Restart process.• The T1 timer specifies the wait time before unacknowledged restart requests are generated. This is theinterval before the system sends a Restart Request (an IIH with RR bit set in Restart TLV) until theCSNP 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. Thistimer applies to the database type (level-1, level-2 or both).
