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| 3.1 |  | Routing |
| 3.1.4 |  | Distance vector routing protocols |

Routing protocols may be classified as either distance vector or link-state routing protocols. These classifications describe the algorithm, or formula, that routers use to calculate and exchange routing information. Distance vector routing protocols are based on the Bellman-Ford algorithm.

Routers configured to use a distance vector routing protocol typically send their complete routing table at regular intervals to neighbor routers. Simple distance vector protocols, such as RIP and IGRP, broadcast their routing tables on all configured interfaces. This broadcast is also referred to as multicasting. Routers that use these protocols do not actually identify their neighbors for direct communication.

A neighbor router receiving the broadcast update examines it and compares the information to its current routing table. Routes to new networks, or routes to known networks with better metrics, are inserted in the table. The neighbor then broadcasts its routing table, which includes any updated routes.

Distance vector routing protocols are concerned with the distance and vector, or direction, of destination networks. Before sending an update, each router adds its own distance value to the route metric. When a router receives an update, it maps the learned network to the receiving interface. The router then uses that interface to reach those destinations.

Simple distance vector routing protocols offer two primary advantages over link-state protocols. They are relatively easy to configure and they generally use less memory and processing power. RIP v1 has the added advantage of almost universal support among all routing software and is often used as a common denominator in mixed vendor or legacy routing environments.

Simple distance vector routing protocols do not scale as well as their link-state counterparts. RIP v1 and IGRP are classful routing protocols, which means they do not send subnet information in updates. They are unable to support scalability features such as Variable Length Subnet Masking (VLSM) or supernetting. In general, simple distance vector routing protocols converge more slowly than link-state protocols. Most complex and scalable internetworks require routing protocols that achieve convergence quickly when all routers agree on the state of the network topology. Therefore, distance vector protocols are usually not appropriate. RIP restricts networks from growing beyond 15 hops between any two destinations, which is too limiting for large networks. IGRP overcomes this limitation by supporting a 255-hop maximum. IGRP is a proprietary protocol developed by Cisco and does not support a multi-vendor routing environment.

Because of the limitations of simple distance vector routing protocols, network administrators often use link-state routing in complex internetworks.

| Interactive Media Activity Interactivity: Distance Vector Routing Upon completion of this activity, the student will learn about distance vector routing and also learn about route metrics.

| Web Links The IP Routing Protocols http://www.cisco.com/univercd/cc/td/doc/product/ software/ssr90/rpc_r/54043.htm