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1. Introduction

  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.








All routers run the exact same algorithm, in parallel.  From the
link-state database, each router constructs a tree of shortest
paths with itself as root.  This shortest-path tree gives the
route to each destination in the Autonomous System.  Externally
derived routing information appears on the tree as leaves.

When several equal-cost routes to a destination exist, traffic
is distributed equally among them.  The cost of a route is
described by a single dimensionless metric.

OSPF allows sets of networks to be grouped together.  Such a
grouping is called an area.  The topology of an area is hidden
from the rest of the Autonomous System.  This information hiding
enables a significant reduction in routing traffic.  Also,
routing within the area is determined only by the area's own
topology, lending the area protection from bad routing data.  An
area is a generalization of an IP subnetted network.

OSPF enables the flexible configuration of IP subnets.  Each
route distributed by OSPF has a destination and mask.  Two
different subnets of the same IP network number may have
different sizes (i.e., different masks).  This is commonly
referred to as variable length subnetting.  A packet is routed
to the best (i.e., longest or most specific) match.  Host routes
are considered to be subnets whose masks are "all ones"
(0xffffffff).

All OSPF protocol exchanges are authenticated.  This means that
only trusted routers can participate in the Autonomous System's
routing.  A variety of authentication schemes can be used; in
fact, separate authentication schemes can be configured for each
IP subnet.

Externally derived routing data (e.g., routes learned from an
Exterior Gateway Protocol such as BGP; see [Ref23]) is
advertised throughout the Autonomous System.  This externally
derived data is kept separate from the OSPF protocol's link
state data.  Each external route can also be tagged by the
advertising router, enabling the passing of additional
information between routers on the boundary of the Autonomous
System.

    1.2. Definitions of commonly used terms

    This section provides definitions for terms that have a specific
meaning to the OSPF protocol and that are used throughout the
text.  The reader unfamiliar with the Internet Protocol Suite is
referred to [Ref13] for an introduction to IP.


Router
    A level three Internet Protocol packet switch.  Formerly
    called a gateway in much of the IP literature.

Autonomous System
    A group of routers exchanging routing information via a
    common routing protocol.  Abbreviated as AS.

Interior Gateway Protocol
    The routing protocol spoken by the routers belonging to an
    Autonomous system.  Abbreviated as IGP.  Each Autonomous
    System has a single IGP.  Separate Autonomous Systems may be
    running different IGPs.

Router ID
    A 32-bit number assigned to each router running the OSPF
    protocol.  This number uniquely identifies the router within
    an Autonomous System.

Network
    In this memo, an IP network/subnet/supernet.  It is possible
    for one physical network to be assigned multiple IP
    network/subnet numbers.  We consider these to be separate
    networks.  Point-to-point physical networks are an exception
    - they are considered a single network no matter how many
    (if any at all) IP network/subnet numbers are assigned to
    them.

Network mask
    A 32-bit number indicating the range of IP addresses
    residing on a single IP network/subnet/supernet.  This
    specification displays network masks as hexadecimal numbers.








    For example, the network mask for a class C IP network is
    displayed as 0xffffff00.  Such a mask is often displayed
    elsewhere in the literature as 255.255.255.0.

Point-to-point networks
    A network that joins a single pair of routers.  A 56Kb
    serial line is an example of a point-to-point network.

Broadcast networks
    Networks supporting many (more than two) attached routers,
    together with the capability to address a single physical
    message to all of the attached routers (broadcast).
    Neighboring routers are discovered dynamically on these nets
    using OSPF's Hello Protocol.  The Hello Protocol itself
    takes advantage of the broadcast capability.  The OSPF
    protocol makes further use of multicast capabilities, if
    they exist.  Each pair of routers on a broadcast network is
    assumed to be able to communicate directly. An ethernet is
    an example of a broadcast network.

Non-broadcast networks
    Networks supporting many (more than two) routers, but having
    no broadcast capability.  Neighboring routers are maintained
    on these nets using OSPF's Hello Protocol.  However, due to
    the lack of broadcast capability, some configuration
    information may be necessary to aid in the discovery of
    neighbors.  On non-broadcast networks, OSPF protocol packets
    that are normally multicast need to be sent to each
    neighboring router, in turn. An X.25 Public Data Network
    (PDN) is an example of a non-broadcast network.

    OSPF runs in one of two modes over non-broadcast networks.
    The first mode, called non-broadcast multi-access or NBMA,
    simulates the operation of OSPF on a broadcast network. The
    second mode, called Point-to-MultiPoint, treats the non-
    broadcast network as a collection of point-to-point links.
    Non-broadcast networks are referred to as NBMA networks or
    Point-to-MultiPoint networks, depending on OSPF's mode of
    operation over the network.









Interface
    The connection between a router and one of its attached
    networks.  An interface has state information associated
    with it, which is obtained from the underlying lower level
    protocols and the routing protocol itself.  An interface to
    a network has associated with it a single IP address and
    mask (unless the network is an unnumbered point-to-point
    network).  An interface is sometimes also referred to as a
    link.

Neighboring routers
    Two routers that have interfaces to a common network.
    Neighbor relationships are maintained by, and usually
    dynamically discovered by, OSPF's Hello Protocol.

Adjacency
    A relationship formed between selected neighboring routers
    for the purpose of exchanging routing information.  Not
    every pair of neighboring routers become adjacent.

Link state advertisement
    Unit of data describing the local state of a router or
    network. For a router, this includes the state of the
    router's interfaces and adjacencies.  Each link state
    advertisement is flooded throughout the routing domain. The
    collected link state advertisements of all routers and
    networks forms the protocol's link state database.
    Throughout this memo, link state advertisement is
    abbreviated as LSA.

Hello Protocol
    The part of the OSPF protocol used to establish and maintain
    neighbor relationships.  On broadcast networks the Hello
    Protocol can also dynamically discover neighboring routers.

Flooding
    The part of the OSPF protocol that distributes and
    synchronizes the link-state database between OSPF routers.

Designated Router
    Each broadcast and NBMA network that has at least two
    attached routers has a Designated Router.  The Designated






    Router generates an LSA for the network and has other
    special responsibilities in the running of the protocol.
    The Designated Router is elected by the Hello Protocol.

    The Designated Router concept enables a reduction in the
    number of adjacencies required on a broadcast or NBMA
    network.  This in turn reduces the amount of routing
    protocol traffic and the size of the link-state database.

Lower-level protocols
    The underlying network access protocols that provide
    services to the Internet Protocol and in turn the OSPF
    protocol.  Examples of these are the X.25 packet and frame
    levels for X.25 PDNs, and the ethernet data link layer for
    ethernets.

    1.3. Brief history of link-state routing technology

    OSPF is a link state routing protocol.  Such protocols are also
    referred to in the literature as SPF-based or distributed-
    database protocols.  This section gives a brief description of
    the developments in link-state technology that have influenced
    the OSPF protocol.

    The first link-state routing protocol was developed for use in
    the ARPANET packet switching network.  This protocol is
    described in [Ref3].  It has formed the starting point for all
    other link-state protocols.  The homogeneous ARPANET
    environment, i.e., single-vendor packet switches connected by
    synchronous serial lines, simplified the design and
    implementation of the original protocol.

    Modifications to this protocol were proposed in [Ref4].  These
    modifications dealt with increasing the fault tolerance of the
    routing protocol through, among other things, adding a checksum
    to the LSAs (thereby detecting database corruption).  The paper
    also included means for reducing the routing traffic overhead in
    a link-state protocol.  This was accomplished by introducing
    mechanisms which enabled the interval between LSA originations
    to be increased by an order of magnitude.







    A link-state algorithm has also been proposed for use as an ISO
    IS-IS routing protocol.  This protocol is described in [Ref2].
    The protocol includes methods for data and routing traffic
    reduction when operating over broadcast networks.  This is
    accomplished by election of a Designated Router for each
    broadcast network, which then originates an LSA for the network.

    The OSPF Working Group of the IETF has extended this work in
    developing the OSPF protocol.  The Designated Router concept has
    been greatly enhanced to further reduce the amount of routing
    traffic required.  Multicast capabilities are utilized for
    additional routing bandwidth reduction.  An area routing scheme
    has been developed enabling information
    hiding/protection/reduction.  Finally, the algorithms have been
    tailored for efficient operation in TCP/IP internets.

    1.4. Organization of this document

    The first three sections of this specification give a general
    overview of the protocol's capabilities and functions.  Sections
    4-16 explain the protocol's mechanisms in detail.  Packet
    formats, protocol constants and configuration items are
    specified in the appendices.

    Labels such as HelloInterval encountered in the text refer to
    protocol constants.  They may or may not be configurable.
    Architectural constants are summarized in Appendix B.
    Configurable constants are summarized in Appendix C.

    The detailed specification of the protocol is presented in terms
    of data structures.  This is done in order to make the
    explanation more precise.  Implementations of the protocol are
    required to support the functionality described, but need not
    use the precise data structures that appear in this memo.

    1.5. Acknowledgments

    The author would like to thank Ran Atkinson, Fred Baker, Jeffrey
    Burgan, Rob Coltun, Dino Farinacci, Vince Fuller, Phanindra
    Jujjavarapu, Milo Medin, Tom Pusateri, Kannan Varadhan, Zhaohui






    Zhang and the rest of the OSPF Working Group for the ideas and
    support they have given to this project.

    The OSPF Point-to-MultiPoint interface is based on work done by
    Fred Baker.

    The OSPF Cryptographic Authentication option was developed by
    Fred Baker and Ran Atkinson.