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

Published: January 2006
Status: Standards Track
Authors: Y. Rekhter, Ed., T. Li, Ed., S. Hares, Ed.
Obsoletes: RFC 1771

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.

Table of Contents​

  • 1. Introduction
    • 1.1. Definition of Commonly Used Terms
    • 1.2. Specification of Requirements
  • 2. Acknowledgements
  • 3. Summary of Operation
    • 3.1. Routes: Advertisement and Storage
    • 3.2. Routing Information Base
  • 4. Message Formats
    • 4.1. Message Header Format
    • 4.2. OPEN Message Format
    • 4.3. UPDATE Message Format
    • 4.4. KEEPALIVE Message Format
    • 4.5. NOTIFICATION Message Format
  • 5. Path Attributes
    • 5.1. Path Attribute Usage
      • 5.1.1. ORIGIN
      • 5.1.2. AS_PATH
      • 5.1.3. NEXT_HOP
      • 5.1.4. MULTI_EXIT_DISC
      • 5.1.5. LOCAL_PREF
      • 5.1.6. ATOMIC_AGGREGATE
      • 5.1.7. AGGREGATOR
  • 6. BGP Error Handling
    • 6.1. Message Header Error Handling
    • 6.2. OPEN Message Error Handling
    • 6.3. UPDATE Message Error Handling
    • 6.4. NOTIFICATION Message Error Handling
    • 6.5. Hold Timer Expired Error Handling
    • 6.6. Finite State Machine Error Handling
    • 6.7. Cease
    • 6.8. BGP Connection Collision Detection
  • 7. BGP Version Negotiation
  • 8. BGP Finite State Machine (FSM)
    • 8.1. Events for the BGP FSM
      • 8.1.1. Optional Events Linked to Optional Session Attributes
      • 8.1.2. Administrative Events
      • 8.1.3. Timer Events
      • 8.1.4. TCP Connection-Based Events
      • 8.1.5. BGP Message-Based Events
    • 8.2. Description of FSM
      • 8.2.1. FSM Definition
        • 8.2.1.1. Terms "active" and "passive"
        • 8.2.1.2. FSM and Collision Detection
        • 8.2.1.3. FSM and Optional Session Attributes
        • 8.2.1.4. FSM Event Numbers
        • 8.2.1.5. FSM Actions that are Implementation Dependent
      • 8.2.2. Finite State Machine
  • 9. UPDATE Message Handling
    • 9.1. Decision Process
      • 9.1.1. Phase 1: Calculation of Degree of Preference
      • 9.1.2. Phase 2: Route Selection
        • 9.1.2.1. Route Resolvability Condition
        • 9.1.2.2. Breaking Ties (Phase 2)
      • 9.1.3. Phase 3: Route Dissemination
      • 9.1.4. Overlapping Routes
    • 9.2. Update-Send Process
      • 9.2.1. Controlling Routing Traffic Overhead
        • 9.2.1.1. Frequency of Route Advertisement
        • 9.2.1.2. Frequency of Route Origination
      • 9.2.2. Efficient Organization of Routing Information
        • 9.2.2.1. Information Reduction
        • 9.2.2.2. Aggregating Routing Information
    • 9.3. Route Selection Criteria
    • 9.4. Originating BGP routes
  • 10. BGP Timers

Appendices​

Additional Sections​

Resources​

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