Dynamic Routing Protocols and Convergence
An important characteristic of a routing protocol is how quickly it converges when there is a change in the topology.
Convergence is when the routing tables of all routers are at a state of consistency. The network has converged when all routers have complete and accurate information about the network. Convergence time is the time it takes routers to share information, calculate best paths, and update their routing tables. A network is not completely operable until the network has converged; therefore, most networks require short convergence times.
Convergence is both collaborative and independent. The routers share information with each other but must independently calculate the impacts of the topology change on their own routes. Because they develop an agreement with the new topology independently, they are said to converge on this consensus.
After the network has converged, a link-state update is only sent when there is a change in the topology.
Link-state protocols work best in situations where
■ The network design is hierarchical, usually occurring in large networks.
■ The administrators have a good knowledge of the implemented link-state routing protocol.
■ Fast convergence of the network is crucial.