Chapter 17. BGP

17.0. Introduction

The Border Gateway Protocol (BGP) is a routing protocol currently defined in RFC 4271. It was historically specified in RFC 1771 and RFC 1654. BGP is typically used as an exterior routing protocol to exchange routing information between Autonomous Systems (ASs). A BGP AS is a unified network that is administered by a single entity or organization. Examples of BGP ASs include an Internet Service Provider’s (ISP’s) global backbone, and the corporate network of an enterprise. Because BGP enables disparate ASs to exchange routing information and, thus, communicate with each other, BGP is often referred to as the “glue” that binds the modern Internet together.

BGP AS numbers range from 0–65535 in value. The following values are reserved by the Internet Assigned Numbers Authority (IANA):




Reserved by the IANA


Reserved as private AS numbers



The following Internet registries manage and distribute AS numbers around the world:

BGP is a destination-based path-vector routing protocol wherein a BGP route update provides a router with knowledge of the path of ASs traversed from one AS to another.

When BGP is employed to exchange routes between two disparate ASs, the connection is termed an external BGP, or EBGP, connection. On the other ...

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