rfc2328.txt

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                        N11      RT9       /  |RT12
                         o--------o-------o   o--------o H1
                             3                |   10
                                              |2
                                              |
                                              o N10


                     Figure 5: The SPF tree for Router RT6

              Edges that are not marked with a cost have a cost of
              of zero (these are network-to-router links). Routes
              to networks N12-N15 are external information that is
                         considered in Section 2.3





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                   Destination   Next  Hop   Distance
                   __________________________________
                   N1            RT3         10
                   N2            RT3         10
                   N3            RT3         7
                   N4            RT3         8
                   Ib            *           7
                   Ia            RT10        12
                   N6            RT10        8
                   N7            RT10        12
                   N8            RT10        10
                   N9            RT10        11
                   N10           RT10        13
                   N11           RT10        14
                   H1            RT10        21
                   __________________________________
                   RT5           RT5         6
                   RT7           RT10        8


    Table 2: The portion of Router RT6's routing table listing local
                             destinations.

        this externally derived routing information is considered in the
        next section.


    2.3.  Use of external routing information

        After the tree is created the external routing information is
        examined.  This external routing information may originate from
        another routing protocol such as BGP, or be statically
        configured (static routes).  Default routes can also be included
        as part of the Autonomous System's external routing information.

        External routing information is flooded unaltered throughout the
        AS.  In our example, all the routers in the Autonomous System
        know that Router RT7 has two external routes, with metrics 2 and
        9.

        OSPF supports two types of external metrics.  Type 1 external
        metrics are expressed in the same units as OSPF interface cost



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RFC 2328                     OSPF Version 2                   April 1998


        (i.e., in terms of the link state metric).  Type 2 external
        metrics are an order of magnitude larger; any Type 2 metric is
        considered greater than the cost of any path internal to the AS.
        Use of Type 2 external metrics assumes that routing between
        AS'es is the major cost of routing a packet, and eliminates the
        need for conversion of external costs to internal link state
        metrics.

        As an example of Type 1 external metric processing, suppose that
        the Routers RT7 and RT5 in Figure 2 are advertising Type 1
        external metrics.  For each advertised external route, the total
        cost from Router RT6 is calculated as the sum of the external
        route's advertised cost and the distance from Router RT6 to the
        advertising router.  When two routers are advertising the same
        external destination, RT6 picks the advertising router providing
        the minimum total cost. RT6 then sets the next hop to the
        external destination equal to the next hop that would be used
        when routing packets to the chosen advertising router.

        In Figure 2, both Router RT5 and RT7 are advertising an external
        route to destination Network N12.  Router RT7 is preferred since
        it is advertising N12 at a distance of 10 (8+2) to Router RT6,
        which is better than Router RT5's 14 (6+8).  Table 3 shows the
        entries that are added to the routing table when external routes
        are examined:



                         Destination   Next  Hop   Distance
                         __________________________________
                         N12           RT10        10
                         N13           RT5         14
                         N14           RT5         14
                         N15           RT10        17


                 Table 3: The portion of Router RT6's routing table
                           listing external destinations.


        Processing of Type 2 external metrics is simpler.  The AS
        boundary router advertising the smallest external metric is



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RFC 2328                     OSPF Version 2                   April 1998


        chosen, regardless of the internal distance to the AS boundary
        router.  Suppose in our example both Router RT5 and Router RT7
        were advertising Type 2 external routes.  Then all traffic
        destined for Network N12 would be forwarded to Router RT7, since
        2 < 8.  When several equal-cost Type 2 routes exist, the
        internal distance to the advertising routers is used to break
        the tie.

        Both Type 1 and Type 2 external metrics can be present in the AS
        at the same time.  In that event, Type 1 external metrics always
        take precedence.

        This section has assumed that packets destined for external
        destinations are always routed through the advertising AS
        boundary router.  This is not always desirable.  For example,
        suppose in Figure 2 there is an additional router attached to
        Network N6, called Router RTX.  Suppose further that RTX does
        not participate in OSPF routing, but does exchange BGP
        information with the AS boundary router RT7.  Then, Router RT7
        would end up advertising OSPF external routes for all
        destinations that should be routed to RTX.  An extra hop will
        sometimes be introduced if packets for these destinations need
        always be routed first to Router RT7 (the advertising router).

        To deal with this situation, the OSPF protocol allows an AS
        boundary router to specify a "forwarding address" in its AS-
        external-LSAs.  In the above example, Router RT7 would specify
        RTX's IP address as the "forwarding address" for all those
        destinations whose packets should be routed directly to RTX.

        The "forwarding address" has one other application.  It enables
        routers in the Autonomous System's interior to function as
        "route servers".  For example, in Figure 2 the router RT6 could
        become a route server, gaining external routing information
        through a combination of static configuration and external
        routing protocols.  RT6 would then start advertising itself as
        an AS boundary router, and would originate a collection of OSPF
        AS-external-LSAs.  In each AS-external-LSA, Router RT6 would
        specify the correct Autonomous System exit point to use for the
        destination through appropriate setting of the LSA's "forwarding
        address" field.




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RFC 2328                     OSPF Version 2                   April 1998


    2.4.  Equal-cost multipath

        The above discussion has been simplified by considering only a
        single route to any destination.  In reality, if multiple
        equal-cost routes to a destination exist, they are all
        discovered and used.  This requires no conceptual changes to the
        algorithm, and its discussion is postponed until we consider the
        tree-building process in more detail.

        With equal cost multipath, a router potentially has several
        available next hops towards any given destination.


3.  Splitting the AS into Areas

    OSPF allows collections of contiguous networks and hosts to be
    grouped together.  Such a group, together with the routers having
    interfaces to any one of the included networks, is called an area.
    Each area runs a separate copy of the basic link-state routing
    algorithm.  This means that each area has its own link-state
    database and corresponding graph, as explained in the previous
    section.

    The topology of an area is invisible from the outside of the area.
    Conversely, routers internal to a given area know nothing of the
    detailed topology external to the area.  This isolation of knowledge
    enables the protocol to effect a marked reduction in routing traffic
    as compared to treating the entire Autonomous System as a single
    link-state domain.

    With the introduction of areas, it is no longer true that all
    routers in the AS have an identical link-state database.  A router
    actually has a separate link-state database for each area it is
    connected to.  (Routers connected to multiple areas are called area
    border routers).  Two routers belonging to the same area have, for
    that area, identical area link-state databases.

    Routing in the Autonomous System takes place on two levels,
    depending on whether the source and destination of a packet reside
    in the same area (intra-area routing is used) or different areas
    (inter-area routing is used).  In intra-area routing, the packet is
    routed solely on information obtained within the area; no routing



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RFC 2328                     OSPF Version 2                   April 1998


    information obtained from outside the area can be used.  This
    protects intra-area routing from the injection of bad routing
    information.  We discuss inter-area routing in Section 3.2.


    3.1.  The backbone of the Autonomous System

        The OSPF backbone is the special OSPF Area 0 (often written as
        Area 0.0.0.0, since OSPF Area ID's are typically formatted as IP
        addresses). The OSPF backbone always contains all area border
        routers. The backbone is responsible for distributing routing
        information between non-backbone areas. The backbone must be
        contiguous. However, it need not be physically contiguous;
        backbone connectivity can be established/maintained through the
        configuration of virtual links.

        Virtual links can be configured between any two backbone routers
        that have an interface to a common non-backbone area.  Virtual
        links belong to the backbone.  The protocol treats two routers
        joined by a virtual link as if they were connected by an
        unnumbered point-to-point backbone network.  On the graph of the
        backbone, two such routers are joined by arcs whose costs are
        the intra-area distances between the two routers.  The routing
        protocol traffic that flows along the virtual link uses intra-
        area routing only.


    3.2.  Inter-area routing

        When routing a packet between two non-backbone areas the
        backbone is used.  The path that the packet will travel can be
        broken up into three contiguous pieces: an intra-area path from
        the source to an area border router, a backbone path between the
        source and destination areas, and then another intra-area path
        to the destination.  The algorithm finds the set of such paths
        that have the smallest cost.

        Looking at this another way, inter-area routing can be pictured
        as forcing a star configuration on the Autonomous System, with
        the backbone as hub and each of the non-backbone areas as
        spokes.




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