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Comparing OSPF with distance vector routing protocols

Comparing OSPF with distance vector routing protocols
2.2.3 This page will explain how OSPF compares to distance vector 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 routing tables periodically as distance vector protocols do. Therefore, link-state routers use less bandwidth for routing table maintenance.
RIP is appropriate for small networks, and the best path is based on the lowest number of hops. OSPF is appropriate for large, scalable internetworks, and the best path is determined by the speed of the link. RIP and other distance vector protocols use simple algorithms to compute best paths. The SPF algorithm is complex. Routers that implement distance vector protocols need less memory and less powerful processors than those that implement OSPF.
OSPF selects routes based on cost, which is related to speed. The higher the speed, the lower the OSPF cost of the link.
OSPF selects the fastest loop-free path from the SPF tree as the best path in the network.
OSPF guarantees loop-free routing. Distance vector protocols may cause routing loops.
If links are unstable, flooding of link-state information can lead to unsynchronized link-state advertisements and inconsistent decisions among routers.
OSPF addresses the following issues:
  • Speed of convergence
  • Support for Variable Length Subnet Mask (VLSM)
  • Network size
  • Path selection
  • Grouping of members
In large networks RIP convergence can take several minutes since the routing table of each router is copied and shared with directly connected routers. After initial OSPF convergence, maintaining a converged state is faster because only the changes in the network are flooded to other routers in an area.
OSPF supports VLSMs and therefore is referred to as a classless protocol. RIP v1 does not support VLSMs, however, RIP v2 does support VLSMs.
RIP considers a network that is more than 15 routers away to be unreachable because the number of hops is limited to 15. This limits RIP to small topologies. OSPF has no size limits and is suitable for intermediate to large networks.
RIP selects a path to a network by adding one to the hop count reported by a neighbor. It compares the hop counts to a destination and selects the path with the smallest distance or hops. This algorithm is simple and does not require a powerful router or a lot of memory. RIP does not take into account the available bandwidth in best path determination.
OSPF selects a path using cost, a metric based on bandwidth. All OSPF routers must obtain complete information about the networks of every router to calculate the shortest path. This is a complex algorithm. Therefore, OSPF requires more powerful routers and more memory than RIP.
RIP uses a flat topology. Routers in a RIP region exchange information with all routers. OSPF uses the concept of areas. A network can be subdivided into groups of routers. In this way OSPF can limit traffic to these areas. Changes in one area do not affect performance in other areas. This hierarchical approach allows a network to scale efficiently.
The Interactive Media Activity will help students learn the differences between link-state and distance vector protocols.
The next page will discuss the shortest path algorithm

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