IP Routing: From Basic Principles to Link State Protocols
You will learn about the following in this chapter:
Basic routing principles
Distance Vector protocols
Link State protocols
Nonroutable Versus Routable Protocols
Some protocols can be routed and others cannot. Nonroutable protocols, such as DEC's LAT (Local Area Transport) and NetBEUI (Network Basic Input/Output System), do not contain network layer addressing information. Without the network layer addressing information, routers lack the information they need to determine how to decide for which network or host traffic is destined. Therefore, these protocols need to be bridged or switched using layer 2 information such as a MAC address.
As a general rule nonroutable protocols are broadcast-based, which adds overhead and traffic to a network. For this reason, you usually install a router to isolate the traffic from propagating through an entire internetwork.
However, on some occasions, you may find it necessary to allow this traffic to be forwarded to other segments. You can accomplish this by replacing the router with a layer 2 device (like a bridge or switch) or by configuring the intervening router to forward the nonroutable traffic without determining the specific route this traffic will take.
Routers configured to bridge traffic are referred to as Brouters, allowing the router to bridge traffic using the layer 2 address like a bridge. Routers can transparently bridge, source route, or combine the two methods, depending on your needs and the architectures supported, such as Ethernet or Token-Ring. How a router bridges traffic is determined by the layer 2 implementation you have in place.
Routable protocols are protocols that can be forwarded by a router. Any protocol that has layer 3 logical network addressing can be routed, like IP, IPX, and AppleTalk. Because IP, IPX, and AppleTalk all supply the network layer addressing information and any protocol, application or service that runs on top of these protocols may also be routed. Routers forward traffic based on network layer logical destination addresses; without it routing would not be possible. We discussed IP and logical network layer IP addresses in Chapter 2, "IP and IP Addressing."