Dell Force10 S25-01-GE-24V FTOS 8.4.2.7 Configuration Guide for E-Series TeraS - Page 510

www.dell.com | support.dell.com Adjacencies Adjacencies on point-to-point interfaces are formed as usual, where IS-IS routers do not implement Multi-Topology (MT) extensions. If a local router does not participate in certain MTs, it will not advertise those MT IDs in its IIHs and so will 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 will not form an adjacency if both routers don't have at least one common MT over the interface. Graceful Restart e Graceful Restart is supported on platforms 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 functionality separate. Restarting the control plane functionality (such as the failover of the active 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 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 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. 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 the interval before the system sends a Restart Request (an IIH with RR bit set in Restart TLV) until the CSNP 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. This timer applies to the database type (level-1, level-2 or both). 510 | Intermediate System to Intermediate System