Autonomous Systems
An autonomous system (AS) is a set of routers that use the same routing policy while
running under a single technical administration. An AS runs interior gateway protocols
(IGPs) such as RIP, OSPF, and IS-IS within its boundaries. ASs use exterior gateway
protocols (EGPs) to exchange routing information with other ASs. BGP is an EGP.
The outside world views an AS as a single entity, even though it can be a collection of
IGPs working together to provide routing within its interior.
Each AS has an identification number provided by an Internet registry or by an Internet
service provider (ISP) that uniquely identifies it to the outside world.
BGP Speaker
A router that has been configured to run the BGP routing protocol is called a BGP speaker.
BGP Peers and Neighbors
Unlike some other routing protocols, BGP speakers do not automatically discover each
other and begin exchanging information. Instead, each BGP speaker must be explicitly
configured with a set of BGP peers with which it exchanges routing information. BGP
peers do not have to be directly connected to each other in order to share a BGP session.
Another term for BGP peer is BGP neighbor. A BGP peer group consists of two or more
BGP peers that share a common set of update policies.
In Figure 1 on page 5, router NY and router Chicago are peers. Router NY and router LA
are peers. Router NY and router Boston are peers. Router NY and router Philly are not
peers. Router Chicago and router LA are not peers.
NOTE: The figures in this chapter indicate a BGP session with a dotted line. A physical
link is represented by a solid line.
Figure 1: BGP Peers
BGP Session
When two BGP speakers have both been configured to be BGP peers of each other, they
will establish a BGP session to exchange routing information. A BGP session is simply a
5Copyright © 2010, Juniper Networks, Inc.
Chapter 1: Configuring BGP Routing
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