In an OSPF network, the network topology is distributed among the nodes of the autonomous system (AS) and is regularly updated. Because topology changes are flooded throughout the network, every node maintains the same copy of the network map in its local topological database. Packets are then routed based on the shared topology.
To activate OSPF on a network, you must enable the protocol on all interfaces within the network on which OSPF traffic is to travel. To enable OSPF on one or more interfaces, you must configure one or more interfaces on the Services Router within an OSPF area. Once the interfaces are configured, OSPF link-state advertisements (LSAs) are transmitted on all OSPF-enabled interfaces, and the network topology is shared throughout the network.
OSPF is enabled on a per-interface basis. Those interfaces are configured as OSPF enabled, and are assigned to an area. In a simple, single-area network, the area has the numeric identifier 0.0.0.0, which designates it as the backbone area. As the network grows, it is divided into multiple subnetworks or areas that are identified by numeric identifiers unique to the AS.
In a multiarea network, all areas must be directly connected to the backbone area by area border routers (ABRs). Because all areas are adjacent to the backbone area, OSPF routers send all traffic not destined for their own area through the backbone area. The ABRs in the backbone area are then responsible for transmitting the traffic through the appropriate ABR to the destination area.
Once the topology is shared across the network, OSPF uses it to route packets between network nodes. Each path between neighbors is assigned a cost based on the throughput, round-trip time, and reliability of the link. The sum of the costs across a particular path between hosts determines the overall cost of the path. Packets are then routed along the shortest path using the shortest path first (SPF) algorithm. If multiple equal-cost paths exist between a source and destination address, OSPF routes packets along each path alternately, in round-robin fashion.
OSPF allows you to manually assign a cost (or metric) to a particular path segment to control the flow of packets across the network.
If you are configuring OSPF across a demand circuit such as an ISDN link, you must enable dial-on-demand routing backup on the OSPF-enabled interface. Because demand circuits do not pass all traffic required to maintain an OSPF adjacency (hello packets, for example), you configure dial-on-demand routing so individual nodes in an OSPF network can maintain adjacencies despite the lack of LSA exchanges.
To configure an ISDN link, see Configuring ISDN. For information about configuring OSPF demand circuits, see Configuring Dial-on-Demand Routing Backup with OSPF Support (Optional).