Content

| 6.1 |  | OSPF Overview |
| 6.1.2 |  | OSPF terminology |

As a link-state protocol, OSPF operates differently than the distance vector routing protocols. Link-state routers identify and communicate with their neighbors so that they can gather firsthand information from other routers in the network. The OSPF terminology is depicted in Figure . A brief description of each term is given.

The information gathered from OSPF neighbors is not a complete routing table. Instead, OSPF routers tell each other about the status of their connections, or links to the internetwork. In other words, OSPF routers advertise their link states. Figure shows how the routers process this information and build link-state databases. Figure , is essentially a picture of who is connected to what. All routers in a given area, as seen in Figure , should have identical link-state databases. Independently, each router then runs the Shortest Path First (SPF) algorithm, also known as the Dijkstra algorithm, on the link-state database to determine the best routes to a destination. The SPF algorithm adds up the cost, which is a value usually based on bandwidth, of each link between the router and the destination. This is shown in Figure . The router then chooses the lowest cost path to add to its routing table, also known as a forwarding database .

Figure shows how OSPF routers keep track of their neighbors in their adjacencies database. To simplify the exchange of routing information among several neighbors on the same network, OSPF routers may elect a Designated Router (DR) and a Backup Designated Router (BDR) as shown in Figure , to serve as focal points for routing updates.

| Web Links OSPF Design Guide http://www.cisco.com/warp/public/104/1.html