OSPF Version 2
RFC 2178

Network Working Group                                             J. Moy
Request for Comments: 2178                  Cascade Communications Corp.
Obsoletes: 1583                                                July 1997
Category: Standards Track

                             OSPF Version 2

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.

Abstract

   This memo documents version 2 of the OSPF protocol. OSPF is a link-
   state routing protocol.  It is designed to be run internal to a
   single Autonomous System.  Each OSPF router maintains an identical
   database describing the Autonomous System's topology.  From this
   database, a routing table is calculated by constructing a shortest-
   path tree.

   OSPF recalculates routes quickly in the face of topological changes,
   utilizing a minimum of routing protocol traffic.  OSPF provides
   support for equal-cost multipath.  An area routing capability is
   provided, enabling an additional level of routing protection and a
   reduction in routing protocol traffic.  In addition, all OSPF routing
   protocol exchanges are authenticated.

   The differences between this memo and RFC 1583 are explained in
   Appendix G. All differences are backward-compatible in nature.
   Implementations of this memo and of RFC 1583 will interoperate.

   Please send comments to ospf@gated.cornell.edu.

Table of Contents

    1        Introduction ........................................... 5
    1.1      Protocol Overview ...................................... 5
    1.2      Definitions of commonly used terms ..................... 6
    1.3      Brief history of link-state routing technology ........  9
    1.4      Organization of this document ......................... 10
    1.5      Acknowledgments ....................................... 11
    2        The link-state database: organization and calculations  11
    2.1      Representation of routers and networks ................ 11

Moy                         Standards Track                     [Page 1]
RFC 2178                     OSPF Version 2                    July 1997

    2.1.1    Representation of non-broadcast networks .............. 13
    2.1.2    An example link-state database ........................ 14
    2.2      The shortest-path tree ................................ 18
    2.3      Use of external routing information ................... 20
    2.4      Equal-cost multipath .................................. 22
    3        Splitting the AS into Areas ........................... 22
    3.1      The backbone of the Autonomous System ................. 23
    3.2      Inter-area routing .................................... 23
    3.3      Classification of routers ............................. 24
    3.4      A sample area configuration ........................... 25
    3.5      IP subnetting support ................................. 31
    3.6      Supporting stub areas ................................. 32
    3.7      Partitions of areas ................................... 33
    4        Functional Summary .................................... 34
    4.1      Inter-area routing .................................... 35
    4.2      AS external routes .................................... 35
    4.3      Routing protocol packets .............................. 35
    4.4      Basic implementation requirements ..................... 38
    4.5      Optional OSPF capabilities ............................ 39
    5        Protocol data structures .............................. 40
    6        The Area Data Structure ............................... 42
    7        Bringing Up Adjacencies ............................... 44
    7.1      The Hello Protocol .................................... 44
    7.2      The Synchronization of Databases ...................... 45
    7.3      The Designated Router ................................. 46
    7.4      The Backup Designated Router .......................... 47
    7.5      The graph of adjacencies .............................. 48
    8        Protocol Packet Processing ............................ 49
    8.1      Sending protocol packets .............................. 49
    8.2      Receiving protocol packets ............................ 51
    9        The Interface Data Structure .......................... 54
    9.1      Interface states ...................................... 57
    9.2      Events causing interface state changes ................ 59
    9.3      The Interface state machine ........................... 61