Before we take a look at a how a path is determined, it’s important to be able to differentiate between a routed and routing protocol. Although the names are similar, there is a big difference between the two. A routed protocol is a Network Layer protocol that is used to move traffic between networks. IP, IPX, and AppleTalk are all examples of routed protocols. Routed protocols allow a host on one network to communicate with a host on another, with routers forwarding traffic between the source and destination networks. They are characterized by logical addressing (such as an IP or IPX address) that not only identifies a source or destination host, but also the network (or subnet) on which they reside. In contrast, a protocol like NetBEUI does not use any logical addressing, and isn’t routable. Why is that? Because when a router comes across a NetBEUI packet, it has no way of determining where the destination host resides, since a NetBEUI packet does not include a logical destination address, only a name. The protocol cannot be routed; this means that communication between NetBEUI hosts is limited to occurring within a single non-routed network. Obviously that limits NetBEUI’s usefulness on a large internetwork.
Routing protocols serve a different purpose. Instead of being used to send data between source and destination hosts, a routing protocol is used by routers to exchange routing information with one another. For example, if we want our routers to dynamically “learn” about networks from one another, we configure them with a common routing protocol such as RIP or IGRP. Routers use routing protocols to exchange information about the networks they are aware of. In other words, routing protocols allow routers to “talk” to one another. This doesn’t mean that we need to configure routing protocols on every internetwork – there are other options, such as statically defining paths to destination networks on each router. If that sounds like a lot of work, you’re right. Once a network moves beyond a few routers, you will definitely need to consider adding one or more routing protocols. A variety of routing protocols exist beyond RIP and IGRP, including OSPF, EIGRP, AURP and others. The reason for choosing one over another will be influenced by a number of factors, including the size of a network, required performance, and the routed protocol(s) in use.