OSPF Incremental SPF

Hello Mohammad

There are several ways a router will be informed of a topology change.

If a router has one of its directly connected networks go down, that is, a cable is unplugged for example, the router will immediately know this destination is unavailable, and all destinations through which it used that interface will no longer be available. So the router removes the directly connected network from its routing table, and also begins sending out requests to neighbours to find alternative routes to the directly connected network as well as to the networks that were reached via this interface. Neighbours will receive this information and will respond with alternative routes if there are any.

Secondly, a router may be informed of remote routes that have become unavailable via route information packets that are sent from neighbouring routers. These packets contain information that states that these routers are no longer the next hop routers for the destination networks. The local router will then begin the process of searching for alternative routes to the destinations by communicating with all other neighbours. Once this process completes, the network converges again either with alternative routes to the destinations in question or by removing any destinations for which routes were not found.

I hope this has been helpful!