Case Study Default Router Failure and Communication Recovery
A host transfers a file from a remote server using FTP. The host sends the traffic to its on-link default router. The host continues to receive ACKs for data sent, so the host knows that its default router must be reachable. In mid-session, the router fails. The host stops receiving ACKs. The host can no longer verify forward-direction communication through hints from the TCP layer, so it changes the router's state to STALE. It still attempts to send packets, so the state changes to DELAY. After 5 seconds, the host still has not received positive confirmation of the router's reachability state, so it changes the state to PROBE and sends NS. The router does not respond and therefore gets deleted from the host's neighbor cache.
The host still tries to send packets, but it no longer has a next-hop entry to which to send them. So it sees, the prefix of the destination is off-link and that retrieves a default router from its stored list. It puts the router into its neighbor cache with an INCOMPLETE state, if it does not already exist, and attempts to resolve its link layer address by sending an NS When the new router responds with an NA, positive reachability is confirmed, and traffic begins flowing through the new router.
Continue reading here: Duplicate Address Detection
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