To aid the process of path determination, routing algorithms initialize and maintain ro uting tables , which contain route information. Route information varies depending on the routing algorithm used. Routing algorithms fill routing tables with a variety of information. When a router receives an incoming packet, it checks the destination address and attempts to associate this address with a next hop. Table Routing tables can also contain other information, such as information about the desirability of a path.
Routers compare metrics to determine optimal routes.
Metrics differ depending on the design of the routing algorithm being used. A variety of common metrics will be introduced and described later in this chapter. The ro uting update message is one such message. Routing updates generally consist of all or a portion of a routing table. By analyzing routing updates from all routers, a router can build a detailed picture of network topology.