rfc2328.txt

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    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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        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.




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    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.





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            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.






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        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



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            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