OSPF

OSPF, or Open Shortest Path First, is an open standard link-state interior gateway protocol (IGP) used to route IP packets within a single autonomous system. It was designed to scale to large networks and to converge rapidly after topology changes. OSPF establishes a detailed link-state database by exchanging link-state advertisements (LSAs) and then runs Dijkstra's shortest-path first algorithm to compute the best paths to all destinations. The protocol is defined for IPv4 in OSPFv2 and for IPv6 in OSPFv3, with separate LSDBs and packet formats for each version.

OSPF domains are organized into areas with a single backbone area, area 0, which interconnects the other areas. Routers form adjacencies with neighbours using Hello packets, negotiate parameters, and then flood LSAs to build a consistent view of the topology. Each router has a unique router ID, derived from the highest IP address on an active interface or configured manually.

LSA types include Type 1 Router LSA, Type 2 Network LSA, Type 3 Summary LSA, Type 4 ASBR LSA, Type 5 AS External LSA, and Type 7 NSSA LSA in not-so-stubby areas. These LSAs are flooded within an area and are used by SPF to calculate routes. Areas can be standard, stub, or not-so-stubby (NSSA); virtual links allow connecting remote areas to the backbone.

Routing metrics: OSPF uses a cost metric derived from the interface's bandwidth; the default reference bandwidth is 100 Mbps but can be adjusted to reflect faster networks. Route calculation uses the SPF algorithm to populate a routing table. OSPF supports route summarization, authentication options, and multiple instances per link (in OSPFv3).