When an EIGRP NSF-capable router initially re-boots after an NSF restart, it has no neighbor and its topology table is empty.
The router is notified by the standby (now active) switch when it needs to bring up the interfaces, reacquire neighbors, and
rebuild the topology and routing tables. The restarting router and its peers must accomplish these tasks without interrupting
the data traffic directed toward the restarting router. EIGRP peer routers maintain the routes learned from the restarting
router and continue forwarding traffic through the NSF restart process.
To prevent an adjacency reset by the neighbors, the restarting router uses a new Restart (RS) bit in the EIGRP packet header
to indicate a restart. The RS bit is set in the hello packets and in the initial INIT update packets during the NSF restart
period. The RS bit in the hello packets allows the neighbors to be quickly notified of the NSF restart. Without seeing the
RS bit, the neighbor can only detect an adjacency reset by receiving an INIT update or by the expiration of the hello hold
timer. Without the RS bit, a neighbor does not know if the adjacency reset should be handled using NSF or the normal startup
method.
When the neighbor receives the restart indication, either by receiving the hello packet or the INIT packet, it recognizes
the restarting peer in its peer list and maintains the adjacency with the restarting router. The neighbor then sends it topology
table to the restarting router with the RS bit set in the first update packet indicating that it is NSF-aware and is helping
out the restarting router. The neighbor does not set the RS bit in their hello packets, unless it is also a NSF restarting
neighbor.
Note |
A router may be NSF-aware but may not be helping the NSF restarting neighbor because booting from a cold start.
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If at least one of the peer routers is NSF-aware, the restarting router would then receive updates and rebuild its database.
The restarting router must then find out if it had converged so that it can notify the routing information base (RIB). Each
NSF-aware router is required to send an end of table (EOT) marker in the last update packet to indicate the end of the table
content. The restarting router knows it has converged when it receives the EOT marker. The restarting router can then begin
sending updates.
An NSF-aware peer would know when the restarting router had converged when it receives an EOT indication from the restarting
router. The peer then scans its topology table to search for the routes with the restarted neighbor as the source. The peer
compares the route timestamp with the restart event timestamp to determine if the route is still available. The peer then
goes active to find alternate paths for the routes that are no longer available through the restarted router.
When the restarting router has received all EOT indications from its neighbors or when the NSF converge timer expires, EIGRP
notifies the RIB of convergence. EIGRP waits for the RIB convergence signal and then floods its topology table to all awaiting
NSF-aware peers.
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