11.1.1 OSPF Features

OSPF is typically used to route packets within a single autonomous system (AS). OSPF maintains a network topology constructed from information about link states within the AS in a database on each router, and uses this database to calculate shortest routes. OSPF has the following advantages over RIP:

• Less routing traffic

  OSPF sends updates to other routers only when there is a change to the link state between routers. This generates far less traffic than routing protocols such as RIP, which exchange the entire routing table at fixed time intervals. In OSPF, each router distributes information about its own link state every 30 minutes.

• Elimination of routing loops

  Each router using OSPF maintains an identical database, which it uses to select suitable routes. Therefore, unlike RIP, OSPF does not produce routing loops.

• Cost-based route selection

  When there is more than one route to a given destination, OSPF selects the route with the lowest overall cost. Unlike RIP, OSPF allows route costs to be set in a flexible manner. This guarantees that the most desirable route is selected irrespective of the number of hops.

• Operation of large-scale networks

  OSPF can process routes whose total cost is 16777214 or less. Therefore, compared to RIP with metrics ranging from 1 to 15, it is suitable for operation of a network with a larger route with a large number of routers.

• Variable-length subnets

  Unlike RIP-1, OSPF can route traffic to subnetted networks because its routing information includes a subnet mask.