Introduction

BGPv4 (RFC 4271) is the defacto internet exterior gateway protocol used between ISPs.

The characteristics of BGP are:

• Controls route propagation and the selection of optimal routes, rather than route discovery and calculation, which makes BGP different from interior gateway protocols such as OSPF and RIP.

• Uses TCP to enhance reliability.

• Supports CIDR.

• Reduces bandwidth consumption by advertising only incremental updates, which allows advertising large amounts of routing information on the Internet.

• Eliminates routing loops completely by adding AS path information to BGP routes.

• Provides policies to implement flexible route filtering and selection.

• Provides scalability.

A router that advertises BGP messages is called a BGP speaker. The BGP speaker establishes peer relationships with other BGP speakers to exchange routing information. When a BGP speaker receives a new route or a route better than the current one from another AS, it advertises the route to all the other BGP peers in the local AS.

BGP can be configured to run on a router in the following two modes:

• iBGP (internal BGP)

• eBGP (external BGP)

When a BGP speaker peers with another BGP speaker that resides in the same autonomous system, the session is referred to as an iBGP session. When a BGP speaker peers with a BGP speaker that resides in a different autonomous system, the session is referred to as an eBGP session.