PART 2
Routing Protocol Basics

Although this book is primarily concerned with exterior gateway protocols—routing between different autonomous systems—it makes sense to look at internal gateway protocols as a first step because, conceptually and in practice, the two will affect each other's behavior. Thus, Chapter 4 begins with a consideration of protocols intended for routing within an autonomous system before moving into exterior gateway protocols. Chapter 4 concludes with an overview of the particular exterior gateway protocol, BGP, which we will focus on. Chapter 5 contains a more in-depth exploration of BGP's attribute manipulation and the use of route filtering in influencing the BGP decision process. Understanding the basics of BGP, as described in Part 2, is necessary before we can put the protocols's capabilities to use in practical routing design problems throughout the rest of the book.

Chapter 4—Interdomain Routing Basics

Chapter 5—Tuning BGP Capabilities

This chapter covers the following key topics:

•  Overview of IGPs
A brief consideration of interior gateway protocols as a point of contrast for this chapter's more in-depth consideration of exterior gateway protocols.

•  Autonomous Systems
An autonomous system is a set of routers sharing the same routing policies. Various configurations for autonomous systems are possible, depending on how many exit points to outside networks are desired and whether the system should permit through traffic.

•  How BGP Works
An overview of how the Border Gateway Protocol (version 4) operates, including its message header format, and how and what it negotiates with neighboring routers. The formats and purposes of BGP's four main message types—OPEN, NOTIFICATION, KEEPALIVE, and UPDATE—are covered.

Chapter 4
Interdomain Routing Basics

The Internet is a collection of autonomous systems that define the administrative authority and the routing policies of different organizations. Autonomous systems run Interior Gateway Protocols (IGPs), such as RIP, IGRP, EIGRP, OSPF, and ISIS, within their boundaries and interconnect via an Exterior Gateway Protocol (EGP) called the Border Gateway Protocol (BGP).

Routers are devices that direct traffic between hosts. Routers build routing tables that contain their collected information on all the best paths to all the destinations they know how to reach. They both announce and receive route information to and from other routers. This information goes into the routing tables.

Overview of Routers and Routing

Routers develop a hop-by-hop mechanism by keeping track of "next hop" information that enables a data packet to find its destination through the network. A router that does not have a direct physical connection to the destination checks its routing table and forwards the packet to another next hop router that is closer to that destination. The process repeats until the traffic finds its way through the network to its final destination.

EGPs, such as BGP, were introduced because IGPs do not scale in networks that go beyond the enterprise level. IGPs were never designed for the purpose of global internetworking because they do not have the necessary hooks to segregate enterprises into different administrations that are technically and politically independent from one another. This chapter touches upon basic IGP functionality and then explains the specifics of BGP.

Figure 4-1 describes three routers, RTA, RTB, and RTC, connecting three local area networks, 192.10.1.0, 192.10.5.0, and 192.10.6.0, via serial links. Each serial link is repesented by its own network number, which results in three additional networks, 192.10.2.0, 192.10.3.0, and 192.10.4.0. Each network has a metric associated with it indicating the level of overhead (cost) of transmitting traffic on that particular link. The link between RTA and RTB, for example, has a cost of 2,000, much higher than the cost of 60 of the link between RTA and RTC. In practice, the link between RTA and RTB is a 56 Kbps link with much bigger delays than the T1 link between RTA and RTC and the T1 link between RTC and RTB combined.

Figure 4-1  Basic routing behavior.

Routers RTA, RTB, and RTC would exchange network information via some interior gateway protocol and build their respective IP routing tables. Figure 4-1 shows examples of RTA's IP routing table for two different scenarios; the routers are exchanging routing information via RIP in one scenario and OSPF in another.

As an example of how traffic is passed between end stations, if host 192.10.1.2 is trying to reach host 192.10.6.2, it will first send the traffic to RTA. RTA will look in its IP routing table for any network that matches this destination and would find that network 192.10.6.0 is reachable via next hop 192.10.3.2 (RTC) out on Serial line 2 (S2). RTC would receive the traffic and would try to look for the destination in its IP routing table (not shown). RTC would discover that the host is directly connected to its Ethernet 0 interface (E0) and would send the traffic to 192.10.6.2.

In the preceding example, the routing is the same whether RTA is using the RIP or OSPF scenario. RIP and OSPF, however, fall into different categories of IGP protocols, namely distance vector protocols and link state protocols, respectively. For a different routing example in figure 4-1, the results might be different depending on whether you are looking at the RIP or OSPF scenario. It is useful at this point to consider characteristics of both IGP protocol categories, to see how protocols generally have evolved to meet increasingly sophisticated routing demands.