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4. Functional Summary Connected: An Internet Encyclopedia
4. Functional Summary

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Up: RFC 1583
Prev: 3.7. Partitions of areas
Next: 4.1. Inter-area routing

4. Functional Summary

4. Functional Summary A separate copy of OSPF's basic routing algorithm runs in each area. Routers having interfaces to multiple areas run multiple copies of the algorithm. A brief summary of the routing algorithm follows.

When a router starts, it first initializes the routing protocol data structures. The router then waits for indications from the lower- level protocols that its interfaces are functional.

A router then uses the OSPF's Hello Protocol to acquire neighbors. The router sends Hello packets to its neighbors, and in turn receives their Hello packets. On broadcast and point-to-point networks, the router dynamically detects its neighboring routers by sending its Hello packets to the multicast address AllSPFRouters. On non-broadcast networks, some configuration information is necessary in order to discover neighbors. On all multi-access networks (broadcast or non-broadcast), the Hello Protocol also elects a Designated router for the network.

The router will attempt to form adjacencies with some of its newly acquired neighbors. Topological databases are synchronized between pairs of adjacent routers. On multi-access networks, the Designated Router determines which routers should become adjacent.

Adjacencies control the distribution of routing protocol packets. Routing protocol packets are sent and received only on adjacencies. In particular, distribution of topological database updates proceeds along adjacencies.

A router periodically advertises its state, which is also called link state. Link state is also advertised when a router's state changes. A router's adjacencies are reflected in the contents of its link state advertisements. This relationship between adjacencies and link state allows the protocol to detect dead routers in a timely fashion.

Link state advertisements are flooded throughout the area. The flooding algorithm is reliable, ensuring that all routers in an area have exactly the same topological database. This database consists of the collection of link state advertisements received from each router belonging to the area. From this database each router calculates a shortest-path tree, with itself as root. This shortest-path tree in turn yields a routing table for the protocol.

Next: 4.1. Inter-area routing

Connected: An Internet Encyclopedia
4. Functional Summary


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