4.1 OSPF Overview
4.1.1 Issues addressed by OSPF
OSPF uses link-state technology , as opposed to distance vector technology used by protocols such as RIP . Link-state routers maintain a common picture of the network and exchange link information upon initial discovery or network changes. Link-state routers do not broadcast their routing tables periodically like distance vector routing protocols do. While RIP is appropriate for small networks, OSPF was written to address the needs of large, scalable internetworks. OSPF addresses the following issues:
  • Speed of convergence - In large networks, RIP convergence can take several minutes, since the entire routing table of each router is copied and shared with directly connected neighboring routers. In addition, a distance vector routing algorithm may experience hold down and/or route-aging periods. With OSPF, convergence is faster because only the routing changes (not the entire routing table) are flooded rapidly to other routers in the OSPF network.
  • Support for Variable-Length Subnet Masking (VLSM) - RIPv1 is a classful protocol and does not support VLSM. In contrast, OSPF, a classless protocol, supports VLSM. (Note: RIPv2 supports VLSM.)
  • Network size - In a RIP environment, a network that is more than 15 hops away is considered unreachable. Such limitations restrict the size of a RIP network to small topologies. On the other hand, OSPF has virtually no reachability limitations and is appropriate for intermediate to large size networks.
  • Use of bandwidth - RIP broadcasts full routing tables to all neighbors every 30 seconds. This is especially problematic over slow WAN links because these updates consume bandwidth. Alternately, OSPF multicasts minimally sized link-state updates and sends the updates only when there is a network change.
  • Path Selection - RIP selects a path by measuring the hop count, or distance, to other routers. It does not take into consideration the available bandwidth on the link or delays in the network. In contrast, OSPF selects optimal routes using cost as a factor ("cost" is a metric based on bandwidth).
  • Grouping of members - RIP uses a flat topology and all routers are part of the same network. Thus, communication between routers at each end of the network must travel through the entire network. Unfortunately, changes in even one router will affect every device in the RIP network. OSPF, on the other hand, uses the concept of "areas" and can effectively segment a network into smaller clusters of routers. By narrowing the scope of communication within areas, OSPF limits traffic regionally and can prevent changes in one area from affecting performance in other areas. This use of areas allows a network to scale efficiently.

Although OSPF was written for large networks, implementing it requires proper design and planning, which is especially important if your network has more than 50 routers. At this size, it is important to configure your network to let OSPF reduce traffic and combine routing information whenever possible.