Open Shortest Path First (OSPF) is a link-state interior gateway protocol (IGP) and works on the cost metric to find the best route to reach a subnet. It supports variable length subnet mask (VLSM), sends routing updates to the multicast IP address, and supports fast convergence. The default administrative distance for OSPF is 110. Using OSPF, a router floods link state advertisements (LSAs) over the network to share every detail with every other router on the network to form link state databases (LSDBs) on each router and to eventually become OSPF neighbours. Every router applies Dijkstra Shortest Path First algorithm on the LSDBs to find the best path to a particular subnet.
To set up OSPF routing, on a router, we need to configure the OSPF process ID and the router ID. A router can also choose the numerically highest IP address of its physical interface or the loopback interface as the router ID. When choosing the router ID highest loopback interface address takes precedence over the physical interface address and the manually configured router ID takes precedence over all. We also have to divide the network into smaller areas to efficiently carry out the OSPF implementation. Using multiple areas in an OSPF configuration improves the scalability of the network and helps contain the problems of a particular area within the area. It also results in smaller routing table entries hence reducing the overhead for the devices. Moreover, OSPF uses wildcard masks, instead of subnet masks, in its configuration.
The first step in configuring OSPF on a router is the assign the router OSPF process ID using the router ospf process-id global configuration command. After that, configuring a router ID is optional, as the router can also get a router ID form its loopback interface or physical interface, but it is preferred to configure the router ID manually to easily identify the router on the network. It can be done using the router-id id-value router sub-command. Then use one or more network directly-connected-network-id wildcard-mask area area-id router sub-commands to implement OSPF routing. We can also remove the network commands by using no network network-id area area-id sub-command. OSPF can also be configured on an interface by using the ip ospf process-id area area-id interface command.
We can verify the OSPF operation by using the show ip ospf neighbor, show ip ospf database, and show ip route, show ip ospf interface or show ip ospf interface brief commands to display the matching information.
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