Interface SupportMT IS-IS is supported on physical Ethernet interfaces, physical synchronous optical network technologies (SONET) interfaces, port-channel interfaces (static and dynamic using LACP), and virtual local area network (VLAN) interfaces.AdjacenciesAdjacencies 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 thatneighbor 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 itsLSPs. The local router does not form an adjacency if both routers do not have at least one common MT over the interface.Graceful RestartGraceful restart is a protocol-based mechanism that preserves the forwarding table of the restarting router and its neighbors for a specifiedperiod to minimize the loss of packets. A graceful-restart router does not immediately assume that a neighbor is permanently down and sodoes 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 theneighbors of the restarting router. When a router goes down without a graceful restart, there is a potential to lose access to parts of thenetwork 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 controlplane functionality (such as the failover of the active route processor module (RPM) to the backup in a redundant configuration) should notnecessarily interrupt data packet forwarding. This behavior is supported because the forwarding tables previously computed by an activeRPM have been downloaded into the forwarding information base (FIB) on the line cards (the data plane). For packets that have existingFIB/content addressable memory (CAM) entries, forwarding between ingress and egress ports can continue uninterrupted while thecontrol 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.TimersThree timers are used to support IS-IS graceful restart functionality. After you enable graceful restart, these timers manage the gracefulrestart 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 systemsends a Restart Request (an IIH with the RR bit set in Restart TLV) until the complete sequence number PDU (CSNP) is received fromthe 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 (bysetting the overload bit in its own LSP). You can base this timer on adjacency settings with the value derived from adjacent routers thatare engaged in graceful restart recovery (the minimum of all the Remaining Time values advertised by the neighbors) or by setting aspecific amount of time manually.Implementation InformationIS-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 beenexpanded to include two new type, length, values (TLVs) in the PDU that carry information required for IPv6 routing. The new TLVs are482 Intermediate System to Intermediate System
