1. Introduction

1. Introduction

The Border Gateway Protocol (BGP) is an inter-Autonomous System routing protocol.

The primary function of a BGP speaking system is to exchange network reachability information with other BGP systems. This network reachability information includes information on the list of Autonomous Systems (ASes) that reachability information traverses. This information is sufficient for constructing a graph of AS connectivity for this reachability, from which routing loops may be pruned and, at the AS level, some policy decisions may be enforced.

BGP-4 provides a set of mechanisms for supporting Classless Inter- Domain Routing (CIDR) [RFC1518, RFC1519]. These mechanisms include support for advertising a set of destinations as an IP prefix and eliminating the concept of network "class" within BGP. BGP-4 also introduces mechanisms that allow aggregation of routes, including aggregation of AS paths.

Routing information exchanged via BGP supports only the destination- based forwarding paradigm, which assumes that a router forwards a packet based solely on the destination address carried in the IP header of the packet. This, in turn, reflects the set of policy decisions that can (and cannot) be enforced using BGP. BGP can support only those policies conforming to the destination-based forwarding paradigm.

1.1. Definition of Commonly Used Terms

This section provides definitions for terms that have a specific meaning to the BGP protocol and that are used throughout the text.

Adj-RIB-In The Adj-RIBs-In contains unprocessed routing information that has been advertised to the local BGP speaker by its peers.

Adj-RIB-Out The Adj-RIBs-Out contains the routes for advertisement to specific peers by means of the local speaker's UPDATE messages.

Autonomous System (AS) The classic definition of an Autonomous System is a set of routers under a single technical administration, using an interior gateway protocol (IGP) and common metrics to determine how to route packets within the AS, and using an inter-AS routing protocol to determine how to route packets to other ASes. Since this classic definition was developed, it has become common for a single AS to

RFC 4271 BGP-4 January 2006

use several IGPs and, sometimes, several sets of metrics within an AS. The use of the term Autonomous System stresses the fact that, even when multiple IGPs and metrics are used, the administration of an AS appears to other ASes to have a single coherent interior routing plan, and presents a consistent picture of the destinations that are reachable through it.

BGP Identifier A 4-octet unsigned integer that indicates the BGP Identifier of the sender of BGP messages. A given BGP speaker sets the value of its BGP Identifier to an IP address assigned to that BGP speaker. The value of the BGP Identifier is determined upon startup and is the same for every local interface and BGP peer.

BGP speaker A router that implements BGP.

EBGP External BGP (BGP connection between external peers).

External peer Peer that is in a different Autonomous System than the local system.

Feasible route An advertised route that is available for use by the recipient.

IBGP Internal BGP (BGP connection between internal peers).

Internal peer Peer that is in the same Autonomous System as the local system.

IGP Interior Gateway Protocol - a routing protocol used to exchange routing information among routers within a single Autonomous System.

Loc-RIB The Loc-RIB contains the routes that have been selected by the local BGP speaker's Decision Process.

NLRI Network Layer Reachability Information.

Route A unit of information that pairs a set of destinations with the attributes of a path to those destinations. The set of

RFC 4271 BGP-4 January 2006

destinations are systems whose IP addresses are contained in one IP address prefix carried in the Network Layer Reachability Information (NLRI) field of an UPDATE message. The path is the information reported in the path attributes field of the same UPDATE message.

RIB Routing Information Base.

Unfeasible route A previously advertised feasible route that is no longer available for use.

1.2. Specification of Requirements

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].