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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  TCP/IP Routing Protocols (Gateway Protocols)

TCP/IP Routing Protocols (Gateway Protocols) 1 2 3 TCP/IP Interior Routing Protocols (RIP, OSPF, GGP, HELLO, IGRP, EIGRP)

Routing protocols play an important part in the overall process of routing in an internetwork. It is therefore easiest to understand them in the scope of an overall discussion of routing. It's difficult to describe the individual TCP/IP routing protocols without some background information on how routing protocols work. For this reason, I feel it is worth taking a brief look at key routing protocol concepts here, so that you will have more luck making sense of the rest of the routing protocol topics in this section.

Let's start with a look at routing protocol architectures. In this context, the word architecture refers to the way that an internetwork is structured. Once we have a number of networks and routers we wish to connect together, there are any number of ways that we can do this. The architecture we choose is based on the way that routers are linked up, and this has an impact on the way that routing is done, and how routing protocols operate.

TCP/IP and the Internet were developed simultaneously, so TCP/IP routing protocols evolved as the Internet itself did. Early architecture of the Internet consisted of a small number of core routers that contained comprehensive information about the internetwork. When the Internet was very small, it was expanded by adding more routers to this core. However, each time the core was expanded, the amount of routing information that needed to be maintained grew.

Eventually, the core became too large, so a two-level hierarchy was formed to allow further expansion. Non-core routers were located on the periphery of the core and contained only partial routing information; they relied on the core routers for transmissions that went across the internetwork. A special routing protocol called the Gateway-to-Gateway Protocol (GGP) was used within the core of the internetwork, while another protocol called the Exterior Gateway Protocol (EGP) was used between non-core and core routers. The non-core routers were sometimes single, stand-alone routers that connected a single network to the core, or they could be sets of routers for an organization.

This architecture served for a while, but itself did not scale very well as the Internet grew. The problem was mainly due to the fact that there was only a single level to the architecture: every router in the core had to communicate with every other. Even with peripheral routers being kept outside the core, the amount of traffic in the core kept growing.

To resolve the limitations of the early core system, a new architecture was created that moved away from the centralized concept of a core towards an architecture that was better suited to a larger and growing internetwork. This decentralized architecture treats the Internet as a set of independent groups, which each group called an autonomous system (AS). An AS consists of a set of routers and networks controlled by a particular organization or administrative entity, which uses a single consistent policy for internal routing.

The power of this system is that routing on the internetwork as a whole occurs between ASes and not individual routers. Information is only shared between one or maybe a couple of routers in each AS, not every router in each AS. The details of routing within an AS are also hidden from the rest of the internetwork. This provides both flexibility for each AS to do routing as it sees fit (thus the name autonomous) and efficiency for the overall internetwork. Each AS has its own number, and the numbers are globally managed to make sure they are unique across an internetwork (such as the Internet).

TCP/IP Routing Protocols (Gateway Protocols) 1 2 3 TCP/IP Interior Routing Protocols (RIP, OSPF, GGP, HELLO, IGRP, EIGRP)