A Border Gateway Protocol 4 (BGP-4)
RFC 4271

Network Working Group                                    Y. Rekhter, Ed.
Request for Comments: 4271                                    T. Li, Ed.
Obsoletes: 1771                                            S. Hares, Ed.
Category: Standards Track                                   January 2006

                  A Border Gateway Protocol 4 (BGP-4)

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This document discusses the Border Gateway Protocol (BGP), which 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).  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.

   This document obsoletes RFC 1771.

Rekhter, et al.             Standards Track                     [Page 1]
RFC 4271                         BGP-4                      January 2006

Table of Contents

   1. Introduction ....................................................4
      1.1. Definition of Commonly Used Terms ..........................4
      1.2. Specification of Requirements ..............................6
   2. Acknowledgements ................................................6
   3. Summary of Operation ............................................7
      3.1. Routes: Advertisement and Storage ..........................9
      3.2. Routing Information Base ..................................10
   4. Message Formats ................................................11
      4.1. Message Header Format .....................................12
      4.2. OPEN Message Format .......................................13
      4.3. UPDATE Message Format .....................................14
      4.4. KEEPALIVE Message Format ..................................21
      4.5. NOTIFICATION Message Format ...............................21
   5. Path Attributes ................................................23
      5.1. Path Attribute Usage ......................................25
           5.1.1. ORIGIN .............................................25
           5.1.2. AS_PATH ............................................25
           5.1.3. NEXT_HOP ...........................................26
           5.1.4. MULTI_EXIT_DISC ....................................28
           5.1.5. LOCAL_PREF .........................................29
           5.1.6. ATOMIC_AGGREGATE ...................................29
           5.1.7. AGGREGATOR .........................................30
   6. BGP Error Handling. ............................................30
      6.1. Message Header Error Handling .............................31
      6.2. OPEN Message Error Handling ...............................31
      6.3. UPDATE Message Error Handling .............................32
      6.4. NOTIFICATION Message Error Handling .......................34
      6.5. Hold Timer Expired Error Handling .........................34
      6.6. Finite State Machine Error Handling .......................35
      6.7. Cease .....................................................35
      6.8. BGP Connection Collision Detection ........................35
   7. BGP Version Negotiation ........................................36
   8. BGP Finite State Machine (FSM) .................................37
      8.1. Events for the BGP FSM ....................................38
           8.1.1. Optional Events Linked to Optional Session
                  Attributes .........................................38
           8.1.2. Administrative Events ..............................42
           8.1.3. Timer Events .......................................46
           8.1.4. TCP Connection-Based Events ........................47
           8.1.5. BGP Message-Based Events ...........................49