4.1 OSPF Overview
4.1.1 Issues addressed by OSPF
OSPF was developed by the Internet Engineering Task Force (IETF) in 1988. The current version, OSPF Version 2, was defined in RFC 1247 and refined in RFCs 1583, 2178, and 2328 (current specification). OSPF uses link-state technology , as opposed to distance vector technology used by protocols such as Routing Information Protocol (RIP) . Link-state routers maintain a common picture of the network and exchange link information upon initial discovery or 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, scaleable internetworks. The issues it addresses are as follows:
  • 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. Also, 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) --- RIP Version 1 is known as a "classful" protocol and does not support VLSM. It uses a default subnet based on a particular class of IP address. For example, an address of 192.10.10.1 would be assigned a 24 bit-subnet mask of 255.255.255.0 because it is the default subnet for a Class C IP address. In contrast, OSPF, a "classless" protocol, does support variable subnet masking through use of VLSM. Thus, the same 192.10.10.1 address could be variably subnetted in OSPF with any non-standard mask, such as a 16 bit mask of 255.255.0.0. (Note: RIP Version 2 does support VLSM.)

  • Network size --- A RIP network that spans more than 15 hops (or the number of devices the packets travel through) 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. However, this is especially problematical over slow WAN links because these updates consume large amounts of bandwidth. Alternately, OSPF multicasts minimally sized link-state updates and sends the updates only when there is a change in the network.

  • Path Selection --- RIP measures the hop count, or distance, to other routers.
     However, 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). For example, if there are two routes to a destination – one a T1 high speed link, the other a 56 Kbps link – RIP might choose either path if the hop count is equal. However, OSPF would select the faster T1 link based on the faster connection speed.

  • "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 impact 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 impacting performance in other areas. This concept of areas can allow 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.