Link-State Convergence
The convergence cycle used in Link-State Routing Protocols, such as OSFP and IS-IS, differs from that of the distance-vector protocols. When a router detects a link failure between itself and a neighbor, it tries to perform a Designated Router (DR) election process on the LAN interface, but fails to reach any neighbors. It then deletes the route from the routing table, builds a link-state advertisement (LSA) for OSFP or a link-state PDU (LSP) for IS-IS, and sends it out all other interfaces. Upon receipt of the LSA, the other neighbors that are up copy the advertisement and forward the LSA packet out all interfaces other than the one upon which it arrived. All routers, including the router that detected the failure, wait five seconds after receiving the LSA and run the shortest path first (SPF) algorithm. There after the router that detected the failure adds the new route to the routing table, while its neighbors update the metric in their routing table. After approximately 30 seconds, the failed router sends an LSA after aging out the topology entry from router that detected the failure. After five seconds, all routers run the SPF algorithm again and update their routing tables to the path to the failed link. Convergence time is the total of detection time, plus LSA flooding time, plus the five seconds wait before the second SPF algorithm is run.