OSPF Routing Algorithm

OSPF uses the shortest path first (SPF) algorithm, also referred to as the Dijkstra algorithm, to determine the route to reach each destination. All routers in an area run this algorithm in parallel, storing the results in their individual topological databases. Routers with interfaces to multiple areas run multiple copies of the algorithm. This section provides a brief summary of how the SPF algorithm works.

When a router starts, it initializes OSPF and waits for indications from lower-level protocols that the router interfaces are functional. The router then uses the OSPF hello protocol to acquire neighbors, doing this by sending hello packets to its neighbors and receiving their hello packets.

On broadcast or nonbroadcast multiaccess networks (physical networks that support the attachment of more than two routers), the OSPF hello protocol elects a designated router for the network. This router is responsible for sending link-state advertisements that describe the network, which reduces the amount of network traffic and the size of the routers’ topological databases.

The router then attempts to form adjacencies with some of its newly acquired neighbors. (On multiaccess networks, only the designated router and backup designated router form adjacencies with other routers.) Adjacencies determine the distribution of routing protocol packets: routing protocol packets are sent and received only on adjacencies, and topological database updates are sent only along adjacencies. When adjacencies have been established, pairs of adjacent routers synchronize their topological databases.

A router sends LSA packets to advertise its state periodically and when the router’s state changes. These packets include information about the router’s adjacencies, which allows detection of nonoperational routers.

Using a reliable algorithm, the router floods LSAs throughout the area, which ensures that all routers in an area have exactly the same topological database. Each router uses the information in its topological database to calculate a shortest-path tree, with itself as the root. The router then uses this tree to route network traffic.

The description of the SPF algorithm up to this point has explained how the algorithm works within a single area (intra-area routing). For internal routers to be able to route to destinations outside the area (interarea routing), the area border routers must inject additional routing information into the area. Because the area border routers are connected to the backbone, they have access to complete topological data about the backbone. They use this information to calculate paths to all destinations outside its area and then advertise these paths to the area’s internal routers.

AS boundary routers flood information about external ASs throughout the AS, except to stub areas. Area border routers are responsible for advertising the paths to all AS boundary routers.

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