Maintaining LSDB and building the IP Routing Table

Maintaining LSDB and building the IP Routing Table

In OSPF, each router builds a complete map of the network topology in the form of a Link State Database (LSDB) by exchanging Link State Advertisements (LSAs) with its neighbors. This LSDB is used to build the IP routing table, which contains the best routes to reach all destinations in the network.

The following is the process of maintaining the LSDB and building the IP routing table in OSPF:

Neighbor discovery: OSPF routers discover their neighbors and establish adjacencies by exchanging Hello packets. The Hello packets contain information such as the router ID, OSPF interface type, network mask, and router priority.

Exchange of LSAs: Once a neighbor adjacency is established, the routers exchange their LSAs, which contain information about the state of their links and interfaces. The types of LSAs include Router LSAs, Network LSAs, and Summary LSAs.

Calculation of the Shortest Path Tree (SPT): OSPF routers use the Dijkstra algorithm to calculate the Shortest Path Tree (SPT) based on the information in the LSDB. The SPT determines the best path to reach each network in the network topology.

Building the IP routing table: The SPT is used to build the IP routing table, which contains the best routes to reach all destinations in the network. The IP routing table is updated dynamically as changes occur in the network topology. Maintenance of the LSDB: OSPF routers continuously update their LSDBs as changes occur in the network. If a router detects a change in the state of a link or interface, it generates a new LSA and floods it to all of its OSPF neighbors.

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