Dell PowerEdge FN I/O Module Configuration Guide 9.10(0.0)

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.