rfc2178.txt

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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 [e-mail].

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



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    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
    9.4      Electing the Designated Router ........................ 64
    9.5      Sending Hello packets ................................. 66
    9.5.1    Sending Hello packets on NBMA networks ................ 67
    10       The Neighbor Data Structure ........................... 68
    10.1     Neighbor states ....................................... 70
    10.2     Events causing neighbor state changes ................. 75
    10.3     The Neighbor state machine ............................ 76
    10.4     Whether tocome adjacent    ............................ 82
    10.5     Receiving Hello Packets ............................... 83
    10.6     Receiving Database Description Packets ................ 85
    10.7     Receiving Link State Request Packets .................. 88
    10.8     Sending Database Description Packets .................. 89
    10.9     Sending Link State Request Packets .................... 90
    10.10    An Example ............................................ 91



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    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 ...................... 97
    12       Link State Advertisements (LSAs) ......................100
    12.1     The LSA Header ........................................100
    12.1.1   LS age ............................................... 101
    12.1.2   Options .............................................. 101
    12.1.3   LS type .............................................. 102
    12.1.4   Link State ID ........................................ 102
    12.1.5   Advertising Router ................................... 104
    12.1.6   LS sequence number ................................... 104
    12.1.7   LS checksum .......................................... 105
    12.2     The link state database .............................. 105
    12.3     Representation of TOS ................................ 106
    12.4     Originating LSAs ..................................... 107
    12.4.1   Router-LSAs .......................................... 110
    12.4.1.1 Describing point-to-point interfaces ................. 112
    12.4.1.2 Describing broadcast and NBMA interfaces ............. 113
    12.4.1.3 Describing virtual links ............................. 113
    12.4.1.4 Describing Point-to-MultiPoint interfaces ............ 114
    12.4.1.5 Examples of router-LSAs .............................. 114
    12.4.2   Network-LSAs ......................................... 116
    12.4.2.1 Examples of network-LSAs ............................. 116
    12.4.3   Summary-LSAs ......................................... 117
    12.4.3.1 Originating summary-LSAs into stub areas ............. 119
    12.4.3.2 Examples of summary-LSAs ............................. 119
    12.4.4   AS-external-LSAs ..................................... 120
    12.4.4.1 Examples of AS-external-LSAs ......................... 121
    13       The Flooding Procedure ............................... 122
    13.1     Determining which LSA is newer ....................... 126
    13.2     Installing LSAs in the database ...................... 127
    13.3     Next step in the flooding procedure .................. 128
    13.4     Receiving self-originated LSAs ....................... 130
    13.5     Sending Link State Acknowledgment packets ............ 131
    13.6     Retransmitting LSAs .................................. 133
    13.7     Receiving link state acknowledgments ................. 134
    14       Aging The Link State Database ........................ 134
    14.1     Premature aging of LSAs .............................. 135
    15       Virtual Links ........................................ 135
    16       Calculation of the routing table ..................... 137
    16.1     Calculating the shortest-path tree for an area ....... 138
    16.1.1   The next hop calculation ............................. 144
    16.2     Calculating the inter-area routes .................... 145
    16.3     Examining transit areas' summary-LSAs ................ 146
    16.4     Calculating AS external routes ....................... 149
    16.4.1   External path preferences ............................ 151
    16.5     Incremental updates -- summary-LSAs .................. 151



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    16.6     Incremental updates -- AS-external-LSAs .............. 152
    16.7     Events generated as a result of routing table changes  153
    16.8     Equal-cost multipath ................................. 154
             Footnotes ............................................ 155
             References ........................................... 158
    A        OSPF data formats .................................... 160
    A.1      Encapsulation of OSPF packets ........................ 160
    A.2      The Options field .................................... 162
    A.3      OSPF Packet Formats .................................. 163
    A.3.1    The OSPF packet header ............................... 164
    A.3.2    The Hello packet ..................................... 166
    A.3.3    The Database Description packet ...................... 168
    A.3.4    The Link State Request packet ........................ 170
    A.3.5    The Link State Update packet ......................... 171
    A.3.6    The Link State Acknowledgment packet ................. 172
    A.4      LSA formats .......................................... 173
    A.4.1    The LSA header ....................................... 174
    A.4.2    Router-LSAs .......................................... 176
    A.4.3    Network-LSAs ......................................... 179
    A.4.4    Summary-LSAs ......................................... 180
    A.4.5    AS-external-LSAs ..................................... 182
    B        Architectural Constants .............................. 184
    C        Configurable Constants ............................... 186
    C.1      Global parameters .................................... 186
    C.2      Area parameters ...................................... 187
    C.3      Router interface parameters .......................... 188
    C.4      Virtual link parameters .............................. 190
    C.5      NBMA network parameters .............................. 191
    C.6      Point-to-MultiPoint network parameters ............... 191
    C.7      Host route parameters ................................ 192
    D        Authentication ....................................... 193
    D.1      Null authentication .................................. 193
    D.2      Simple password authentication ....................... 193
    D.3      Cryptographic authentication ......................... 194
    D.4      Message generation ................................... 196
    D.4.1    Generating Null authentication ....................... 196
    D.4.2    Generating Simple password authentication ............ 197
    D.4.3    Generating Cryptographic authentication .............. 197
    D.5      Message verification ................................. 198
    D.5.1    Verifying Null authentication ........................ 199
    D.5.2    Verifying Simple password authentication ............. 199
    D.5.3    Verifying Cryptographic authentication ............... 199
    E        An algorithm for assigning Link State IDs ............ 201
    F        Multiple interfaces to the same network/subnet ....... 203
    G        Differences from RFC 1583 ............................ 204
    G.1      Enhancements to OSPF authentication .................. 204
    G.2      Addition of Point-to-MultiPoint interface ............ 204
    G.3      Support for overlapping area ranges .................. 205



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    G.4      A modification to the flooding algorithm ............. 206
    G.5      Introduction of the MinLSArrival constant ............ 206
    G.6      Optionally advertising point-to-point links as subnets 207
    G.7      Advertising same external route from multiple areas .. 207
    G.8      Retransmission of initial Database Description packets 209
    G.9      Detecting interface MTU mismatches ................... 209
    G.10     Deleting the TOS routing option ...................... 209
             Security Considerations .............................. 210
             Author's Address ..................................... 211

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 CIDR
   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.

1.1.  Protocol overview

   OSPF routes IP packets based solely on the destination IP address
   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.  This database is
   referred to as the link-state database. 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.




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