Dynamic routing allows the network to adjust to changes in the topology automatically, without administrator involvement. A static route cannot dynamically respond to changes in the network. If a link fails, the static route is no longer valid if it is configured to use that failed link, so a new static route must be configured. If a new router or new link is added, that information must also be configured on every router in the network. In a very large or unstable network, these changes can lead to considerable work for network administrators. It can also take a long time for every router in the network to receive the correct information. In situations such as these, it might be better to have the routers receive information about networks and links from each other using a dynamic routing protocol. Dynamic routing protocols must do the following:
■ Find sources from which routing information can be received (usually neighboring routers)
■ Select the best paths toward all reachable destinations, based on received information
■ Maintain this routing information
■ Have a means of verifying routing information (periodic updates or refreshes)
When using a dynamic routing protocol, the administrator configures the routing protocol on each router. The routers then exchange information about the reachable networks and the state of each network. Routers exchange information only with other routers running the same routing protocol. When the network topology changes, the new information is dynamically propagated throughout the network, and each router updates its routing table to reflect the changes.
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