Troubleshooting guide

BGP Overview

BGP is an Exterior Gateway Protocol (EGP) that is used within the Internet and multinational

organizations. EGP is one of two different types of dynamic routing protocols. The other protocol is

Interior Gateway Protocol (IGP). The difference between the two protocols is that IGPs (for example,

Routing Information Protocol (RIP), Open Shortest Path First (OSPF)) operate within an autonomous

system (AS), whereas EGPs allow routes to be exchanged between different autonomous systems.

Typically, an AS is defined by the boundaries of an organization. As an EGP, BGP routers must regulate

traffic between networks controlled by organizations with different policies. BGP is designed to allow

administrators to customize policies for route exchange. The following are some characteristics of BGP

that make it an appropriate protocol for connecting different autonomous systems:

• BGP can filter both the routes it receives and those that it sends according to bit length, thereby

minimizing the number of routes exchanged.

• BGP uses policies to determine best routes rather than per-hop counts used in RIP or link states used in

OSPF. Each AS can set their own policy.

• BGP routers communicate only with manually configured neighbors.

• You can configure different policies for route exchange with different neighbors.

BGP Advantages

Static routing, OSPF, and RIP are simple to configure, have low overhead, and are well suited for

medium-to-small networks. However, BGP offers several advantages, particularly in more complex

environments:

• Unlike routers using static routing, routers running BGP can automatically respond to connections that

are down and changes in network topology. Multiple protocol layer switching (MPLS) networks allow

an organization to change its IP addressing scheme without notifying the service provider.

• BGP can handle complex applications in which the private network connects to multiple service

provider routers or multiple service providers. BGP can be configured to balance loads among these

connections.

• BGP is the native protocol implemented by service providers, which decreases problems caused by

redistributing other routing protocols into BGP.

• BGP is policy based; therefore, organizations can maintain tight control over the routes transmitted and

accepted.

This configuration guide focuses on the common BGP applications of CE routers. The most popular

scenarios are illustrated in

Example Configurations on page 35.

Autonomous Systems

An AS is a group of networks administered by the same authority. Usually, an AS is the same as an

organization. If the AS connects directly to the Internet, then the organization must acquire a unique

number from the Internet Assigned Numbers Authority (IANA). However, many organizations connect to

the Internet through a service provider who has already been assigned an AS number for connecting to the

Internet. The service provider’s AS is subdivided into areas and includes any organizations connecting to

the Internet through them.