rfc1583.txt

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Network Working Group                                             J. Moy
Request for Comments: 1583                                 Proteon, Inc.
Obsoletes: 1247                                               March 1994
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.  Separate routes can be calculated
    for each IP Type of Service.  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.

    OSPF Version 2 was originally documented in RFC 1247. The
    differences between RFC 1247 and this memo are explained in Appendix
    E. The differences consist of bug fixes and clarifications, and are
    backward-compatible in nature. Implementations of RFC 1247 and of
    this memo will interoperate.

    Please send comments to [e-mail].








Moy                                                             [Page 1]

RFC 1583                     OSPF Version 2                   March 1994


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 .......................... 9
    2       The Topological Database .............................. 10
    2.1     The shortest-path tree ................................ 13
    2.2     Use of external routing information ................... 16
    2.3     Equal-cost multipath .................................. 20
    2.4     TOS-based routing ..................................... 20
    3       Splitting the AS into Areas ........................... 21
    3.1     The backbone of the Autonomous System ................. 22
    3.2     Inter-area routing .................................... 22
    3.3     Classification of routers ............................. 23
    3.4     A sample area configuration ........................... 24
    3.5     IP subnetting support ................................. 30
    3.6     Supporting stub areas ................................. 31
    3.7     Partitions of areas ................................... 32
    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 .............................. 41
    6       The Area Data Structure ............................... 42
    7       Bringing Up Adjacencies ............................... 45
    7.1     The Hello Protocol .................................... 45
    7.2     The Synchronization of Databases ...................... 46
    7.3     The Designated Router ................................. 47
    7.4     The Backup Designated Router .......................... 48
    7.5     The graph of adjacencies .............................. 49
    8       Protocol Packet Processing ............................ 50
    8.1     Sending protocol packets .............................. 51
    8.2     Receiving protocol packets ............................ 53
    9       The Interface Data Structure .......................... 55
    9.1     Interface states ...................................... 58
    9.2     Events causing interface state changes ................ 61
    9.3     The Interface state machine ........................... 62
    9.4     Electing the Designated Router ........................ 65
    9.5     Sending Hello packets ................................. 67
    9.5.1   Sending Hello packets on non-broadcast networks ....... 68
    10      The Neighbor Data Structure ........................... 69
    10.1    Neighbor states ....................................... 72
    10.2    Events causing neighbor state changes ................. 75
    10.3    The Neighbor state machine ............................ 77



Moy                                                             [Page 2]

RFC 1583                     OSPF Version 2                   March 1994


    10.4    Whether to become adjacent ............................ 83
    10.5    Receiving Hello Packets ............................... 83
    10.6    Receiving Database Description Packets ................ 86
    10.7    Receiving Link State Request Packets .................. 89
    10.8    Sending Database Description Packets .................. 89
    10.9    Sending Link State Request Packets .................... 90
    10.10   An Example ............................................ 91
    11      The Routing Table Structure ........................... 93
    11.1    Routing table lookup .................................. 96
    11.2    Sample routing table, without areas ................... 97
    11.3    Sample routing table, with areas ...................... 98
    12      Link State Advertisements ............................ 100
    12.1    The Link State Advertisement Header .................. 101
    12.1.1  LS age ............................................... 102
    12.1.2  Options .............................................. 102
    12.1.3  LS type .............................................. 103
    12.1.4  Link State ID ........................................ 103
    12.1.5  Advertising Router ................................... 105
    12.1.6  LS sequence number ................................... 105
    12.1.7  LS checksum .......................................... 106
    12.2    The link state database .............................. 107
    12.3    Representation of TOS ................................ 108
    12.4    Originating link state advertisements ................ 109
    12.4.1  Router links ......................................... 112
    12.4.2  Network links ........................................ 118
    12.4.3  Summary links ........................................ 120
    12.4.4  Originating summary links into stub areas ............ 123
    12.4.5  AS external links .................................... 124
    13      The Flooding Procedure ............................... 126
    13.1    Determining which link state is newer ................ 130
    13.2    Installing link state advertisements in the database . 130
    13.3    Next step in the flooding procedure .................. 131
    13.4    Receiving self-originated link state ................. 134
    13.5    Sending Link State Acknowledgment packets ............ 135
    13.6    Retransmitting link state advertisements ............. 136
    13.7    Receiving link state acknowledgments ................. 138
    14      Aging The Link State Database ........................ 139
    14.1    Premature aging of advertisements .................... 139
    15      Virtual Links ........................................ 140
    16      Calculation Of The Routing Table ..................... 142
    16.1    Calculating the shortest-path tree for an area ....... 143
    16.1.1  The next hop calculation ............................. 149
    16.2    Calculating the inter-area routes .................... 150
    16.3    Examining transit areas' summary links ............... 152
    16.4    Calculating AS external routes ....................... 154
    16.5    Incremental updates -- summary link advertisements ... 156
    16.6    Incremental updates -- AS external link advertisements 157
    16.7    Events generated as a result of routing table changes  157



Moy                                                             [Page 3]

RFC 1583                     OSPF Version 2                   March 1994


    16.8    Equal-cost multipath ................................. 158
    16.9    Building the non-zero-TOS portion of the routing table 158
            Footnotes ............................................ 161
            References ........................................... 164
    A       OSPF data formats .................................... 166
    A.1     Encapsulation of OSPF packets ........................ 166
    A.2     The Options field .................................... 168
    A.3     OSPF Packet Formats .................................. 170
    A.3.1   The OSPF packet header ............................... 171
    A.3.2   The Hello packet ..................................... 173
    A.3.3   The Database Description packet ...................... 175
    A.3.4   The Link State Request packet ........................ 177
    A.3.5   The Link State Update packet ......................... 179
    A.3.6   The Link State Acknowledgment packet ................. 181
    A.4     Link state advertisement formats ..................... 183
    A.4.1   The Link State Advertisement header .................. 184
    A.4.2   Router links advertisements .......................... 186
    A.4.3   Network links advertisements ......................... 190
    A.4.4   Summary link advertisements .......................... 192
    A.4.5   AS external link advertisements ...................... 194
    B       Architectural Constants .............................. 196
    C       Configurable Constants ............................... 198
    C.1     Global parameters .................................... 198
    C.2     Area parameters ...................................... 198
    C.3     Router interface parameters .......................... 200
    C.4     Virtual link parameters .............................. 202
    C.5     Non-broadcast, multi-access network parameters ....... 203
    C.6     Host route parameters ................................ 203
    D       Authentication ....................................... 205
    D.1     AuType 0 -- No authentication ........................ 205
    D.2     AuType 1 -- Simple password .......................... 205
    E       Differences from RFC 1247 ............................ 207
    E.1     A fix for a problem with OSPF Virtual links .......... 207
    E.2     Supporting supernetting and subnet 0 ................. 208
    E.3     Obsoleting LSInfinity in router links advertisements . 209
    E.4     TOS encoding updated ................................. 209
    E.5     Summarizing routes into transit areas ................ 210
    E.6     Summarizing routes into stub areas ................... 210
    E.7     Flushing anomalous network links advertisements ...... 210
    E.8     Required Statistics appendix deleted ................. 211
    E.9     Other changes ........................................ 211
    F.      An algorithm for assigning Link State IDs ............ 213
            Security Considerations .............................. 216
            Author's Address ..................................... 216







Moy                                                             [Page 4]

RFC 1583                     OSPF Version 2                   March 1994


1.  Introduction

    This document is a specification of the Open Shortest Path First
    (OSPF) TCP/IP internet routing protocol.  OSPF is classified as an
    Interior Gateway Protocol (IGP).  This means that it distributes
    routing information between routers belonging to a single Autonomous
    System.  The OSPF protocol is based on link-state or SPF technology.
    This is a departure from the Bellman-Ford base used by traditional
    TCP/IP internet routing protocols.

    The OSPF protocol was developed by the OSPF working group of the
    Internet Engineering Task Force.  It has been designed expressly for
    the TCP/IP internet environment, including explicit support for IP
    subnetting, TOS-based routing and the tagging of externally-derived
    routing information.  OSPF also provides for the authentication of
    routing updates, and utilizes IP multicast when sending/receiving
    the updates.  In addition, much work has been done to produce a
    protocol that responds quickly to topology changes, yet involves
    small amounts of routing protocol traffic.

    The author would like to thank Fred Baker, Jeffrey Burgan, Rob
    Coltun, Dino Farinacci, Vince Fuller, Phanindra Jujjavarapu, Milo
    Medin, Kannan Varadhan and the rest of the OSPF working group for
    the ideas and support they have given to this project.

    1.1.  Protocol overview

        OSPF routes IP packets based solely on the destination IP
        address and IP Type of Service found in the IP packet header.
        IP packets are routed "as is" -- they are not encapsulated in
        any further protocol headers as they transit the Autonomous
        System.  OSPF is a dynamic routing protocol.  It quickly detects
        topological changes in the AS (such as router interface
        failures) and calculates new loop-free routes after a period of
        convergence.  This period of convergence is short and involves a
        minimum of routing traffic.

        In a link-state routing protocol, each router maintains a
        database describing the Autonomous System's topology.  Each
        participating router has an identical database.  Each individual
        piece of this database is a particular router's local state
        (e.g., the router's usable interfaces and reachable neighbors).
        The router distributes its local state throughout the Autonomous
        System by flooding.

        All routers run the exact same algorithm, in parallel.  From the
        topological database, each router constructs a tree of shortest
        paths with itself as root.  This shortest-path tree gives the