Routers use routing protocols to build and maintain routing tables that contain route information. This aids in the process of path determination. Routing protocols fill routing tables with a variety of route information. This information varies depending on the routing protocol used. Routing tables contain the information necessary to forward data packets across connected networks. Layer 3 devices interconnect broadcast domains or LANs. A hierarchical addressing scheme is required for data transfer to occur.

Routers keep track of important information in their routing tables, including the following:

• Protocol type – The type of routing protocol that created the routing table entry
• Destination/next-hop associations – These associations tell a router that a particular destination is either directly connected to the router, or that it can be reached using another router called the “next-hop” on the way to the final destination. When a router receives an incoming packet, it checks the destination address and attempts to match this address with a routing table entry.
• Routing metric – Different routing protocols use different routing metrics. Routing metrics are used to determine the desirability of a route. For example, the Routing Information Protocol (RIP) uses hop count as its only routing metric. Interior Gateway Routing Protocol (IGRP) uses a combination of bandwidth, load, delay, and reliability metrics to create a composite metric value.
• Outbound interfaces – The interface that the data must be sent out on, in order to reach the final destination.

Routers communicate with one another to maintain their routing tables through the transmission of routing update messages. Some routing protocols transmit update messages periodically, while others send them only when there are changes in the network topology. Some protocols transmit the entire routing table in each update message, and some transmit only routes that have changed. By analyzing the routing updates from the neighboring routers, a router builds and maintains its routing table.

Web Links Routing Tables http://www.pku.edu.cn/academic/research/ computer-center/ tc/html/ TC0310.html