![]() c) SPF Recalculation Time : When new LSAs update the LSDB they trigger new SPF calculations to update the FIB. LSA propagation time also depends on the number of hops between the routers in the network and the processing delay at each router and transmission delay at each hop. This provides sufficient time to update the LSDB from the last event and run the Dikjstra alogorithm. OSPF specifies that LSAs cannot be generated within 5 seconds from the last LSA generation time. ![]() The time for generating new LSAs on a single failure is between 4 to 12 milliseconds (ms). The LSAs have to be propagated to all routers in the network (area). b) LSA Propagation : Subsequent to a failure detection, a router generates new LSAs. This is the layer-3 failure detection mechanism and has been adopted predominantly. a) Failure Detection : Missing 4 consecutive Hello packets from a neighbor indicates link or router failure on that link and hence is one mechanism for failure detection. c) Populating Routing Tables: Using the topology information in the LSDB, each router then locally computes the shortest paths from itself to all other routers in the network (or area), using the Shortest Path First (SPF) or Dijkstra algorithm to populate the routing tables or Forwarding Information Bases (FIB). Each router records all the recived link state information that was flooded in a LSDB. On link state establishment as routers come up, distribution of adjacency information to all routers is initiated by flooding Link State Advertisements (LSA). b) Maintaining Link State Databases: Hello protocol is also used for link state check between established adjacent neighbor routers. This is necessary to reduce the number of adjacent direct neighbors and the traffic to establish / maintain them. ![]() In the case of multi-point networks, OSPF will elect a DR and a BDR using the router prioity and the router ID. Once Hello packets are exchanged, each router recognizes if they are connected via a point-to-point network or a multi-point network such as Ethernet, where several routers are in the same subnet. Hello packets are sent on each interface using a multicast address to neighbor routers. ![]()
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