Alcatel Carrier Internetworking Solutions 060187-10 REV D Ospf Overview, Ospf Hello Protocol

Configuring OSPF OSPF Overview

OSPF Overview

Open Shortest Path First routing (OSPF) is a shortest path first (SPF), or link-state, protocol. OSPF is an interior gateway protocol (IGP) that distributes routing information between routers in a Single Autono- mous System (AS). OSPF chooses the least-cost path as the best path.

Each participating router distributes its local state (i.e., the router’s usable interfaces, local networks, and reachable neighbors) throughout the AS by flooding. In a link-state protocol, each router maintains a data- base describing the entire topology. This database is built from the collected link state advertisements of all routers. Each multi-access network that has at least two attached routers has a designated router and a backup designated router. The designated router floods a link state advertisement for the multi-access network.

When a router starts, it uses the OSPF Hello Protocol to discover neighbors. The router sends Hello pack- ets 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 Hello packets to a multicast address. On nonbroadcast and point-to-multipoint networks, some configuration information is necessary in order to configure neighbors. On all networks (broadcast or nonbroadcast), the Hello Protocol also elects a desig- nated router for the network.

Hello. Please respond...	Hello. Please respond...
Are you a neighbor...	Are you a neighbor...
My link state is...	My link state is...

OSPF Hello Protocol

The router will attempt to form full adjacencies with all of its newly acquired neighbors. Only some pairs, however, will be successful in forming full adjacencies. Topological databases are synchronized between pairs of fully adjacent routers.

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.

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 downed routers in a timely fashion.

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

OmniSwitch 6600 Family Advanced Routing Configuration Guide March 2005 page 1-7