Routing is the process of moving information packets and messages across a network from a source host to a destination host. Along the way, at least one router is encountered. Routing involves two basic activities: determining optimal routing paths and transporting message packets through an network.

Routing takes place at the Network Layer—Layer 3 in the OSI 7 layers model. Routed protocols such as IP and IS-IS are the layer 3 protocols, which include the source and destination addresses of the packets. Routing protocols such as OSPF and BGP are used to evaluate what path will be the best for a packet to travel.

Routing tables contain information used by switching software to select the best route. Destination/next hop associations tell a router that a particular destination can be reached optimally by sending the packet to a particular router representing 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 associate this address with a next hop. Routing tables may also contain other information, such as data about the desirability of a path.

Information in the routing tables can be static, with routes manually entered by the network administrator, or dynamic, where routers communicate to exchange connection and route information using various routing protocols. Routers communicate with one another and maintain their routing tables through the transmission of a variety of messages. By analyzing routing updates from all other routers, a router can build a detailed picture of network topology. A link-state advertisement, another example of a message sent between routers, informs other routers of the state of the sender's links. Link information can also be used to build a complete picture of the network topology to enable routers to determine optimal routes to network destinations. A routing table can be as simple as a few lines in the smallest routers, but can grow to massive size and complexity in the very large routers that handle the bulk of Internet messages.

To aid the process of path determination, routing algorithms initialize and maintain routing tables, which contain route information. Route information varies depending on the routing algorithm used. Routing algorithms may use many different metrics to determine the best route. Sophisticated routing algorithms can base route selection on multiple metrics such as Path length, Path Reliability, Packet Delay, Bandwidth, Load and Communication cost.

The Routers in a network and the Routing Process:

Routing: Process, Algorithm and Routing Table

Related Terms: Router, Switch, Switching, Network Bridging

Reference Links:
http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/routing.htm: Routing Basics
http://computer.howstuffworks.com/router.htm: How routers work