5

Routing Protocols

This is one of the major areas over which the network architect has a great deal of influence. Decisions made by the network architect can dramatically impact the behaviour of the IP traffic as the network grows and the number and type of services increase. Poor decisions at the design stage can severely impact your ability to grow and could necessitate a complete redesign of the architecture, causing major inconvenience to your customers.

Routing, as a generic term, is the process of selecting the next hop via which a destination is best reached. Best, in this case, is a rather nebulous concept, usually associated with the shortest path measured in some arbitrary units. The idea of the best route can be modified by routing policy. The term routing also incorporates the concept of forwarding. Forwarding is the actual process of receiving a packet on one interface and then transmitting the packet out of an interface based on the destination address.

The next hop to any destination in the network can be explicitly configured on any router. This is known as static routing. Alternatively, routers can use dynamic routing protocols. Routing protocols are the means by which network layer reachability information (NLRI) is exchanged between two or more routers. This exchange is essential if packets are to be forwarded correctly to their destinations. It would be possible to statically configure all routing information on all routers throughout the entire Internet but, in reality, this would be totally impractical since static routing cannot route around failure and it would be administratively intolerable to have to manage static routing in a large network. Static routing has a valid place in the Internet but it certainly cannot replace dynamic routing protocols.

Routing protocols can be categorized in several ways, including link state vs. distance vector, intra-autonomous system1 vs. inter-autonomous system, classful vs. classless.

1 An autonomous system is a contiguous collection of networks and nodes under the control of a single administrative authority.

Designing and Developing Scalable IP Networks G. Davies

2004 Guy Davies ISBN: 0-470-86739-6