Understanding the Function of Protocols

The function of a protocol is largely determined by the layer of the TCP/IP model at which it operates. The layer on which it operates is dependent on the type of information that it uses in the process of carrying out its job. For example, if a protocol uses Transport layer information (port numbers) to perform its job, it will be a Transport layer protocol. If it uses MAC addresses, it will be a Network Access layer protocol.

However, there is another way to classify networking protocols such as TCP/IP. There are routed protocols and routing protocols. In this section, routing and routed protocols are discussed.

Defining Routed Protocols

A routed protocol is a networking protocol that is capable of being routed. This means that the protocol was designed for transferring information to networks other than the network in which the source device resides. That requires its design to include some method of identifying devices and groups of devices that can be used in the routing process.

Routing services are provided to routed protocols in one of two ways. In static routing, routes are manually configured on the routers at the command line. In dynamic routing, routers on the network are configured to use a routing protocol to determine routes.

Not all networking protocols are capable of being routed. Therefore, they are not routed protocols, even though they are networking protocols. An example is NetBEUI, an early Microsoft protocol. It was designed as a protocol for small workgroups of computers that were located on the same physical segment. It uses no numbering system of any kind, and the computers are identified by names that each machine broadcasts out to the other machines when required. The protocol can't be routed because there is no information for the router to use to determine where it came from and where it needs to go.

Other protocols such as TCP/IP, IPX/SPX, and AppleTalk were designed with routing in mind. Each has a numbering system that can be used by routers to determine the source and destination and the path required to get the packet from point A to point B.

Defining Routing Protocols

A routing protocol obtains and uses the information required to route the packets of the routed protocol. For example, in an IP network, the routing protocol will use routing tables that are constructed in terms of IP addressing information to determine how to get the IP packets to their destination. Cisco routers are capable of routing IP, IPX, and AppleTalk even though they use different numbering systems. In today's networks, it is hard to find IPX and AppleTalk being routed, however.

Routing protocols were developed as an alternative to manually creating the routing tables in the routers from the command-line interface of the router. Configuring routes from the command line is called static routing. There are several reasons why routing protocols are preferable to static routing. First, routing protocols have the capability to react to changes in the network, such as a malfunction of a link in the network. Second, they can choose the best route to a destination when multiple routes to the same destination exist. Finally, they have the capability to learn the routes and populate the routing tables without manual programming by the administrator. The rest of this chapter is devoted to routed protocols.