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Network Working Group                                             J. Moy
Internet Draft                                   Sycamore Networks, Inc.
Expiration Date: August 2001                               February 2001
File name: draft-ietf-ospf-ppp-flood-01.txt

              Flooding over parallel point-to-point links

Status of this Memo

    This document is an Internet-Draft and is in full conformance with
    all provisions of Section 10 of RFC2026.

    Internet-Drafts are working documents of the Internet Engineering
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    The OSPF routing protocol synchronizes its link-state database over
    all links. However, when multiple point-to-point links connect a
    pair of OSPF routers, it is only necessary to flood over one of the
    parallel links. This can be done in a backward-compatible fashion,
    without requiring negotiation between neighboring routers, as
    described in this memo.

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Table of Contents

    1        Overview ............................................... 2
    2        Implementation ......................................... 3
    2.1      Whether to become adjacent ............................. 3
    2.2      Lost adjacencies ....................................... 4
    2.3      Originating router-LSAs ................................ 4
    2.4      Next hop calculation ................................... 5
    2.5      MTU check .............................................. 5
    3        Backward compatibility ................................. 5
    4        Example ................................................ 6
    5        Notes .................................................. 6
             References ............................................. 7
             Security Considerations ................................ 7
             Authors' Addresses ..................................... 8

1.  Overview

    When multiple "equivalent" links connect a pair of OSPF routers,
    database synchronization (both initial via the Database Exchange
    process and ongoing via flooding, also called adjacency formation
    and maintenance) need only be performed over one of the links. The
    key reason for this is that remote routers only care that at least
    one link is advertised in the two routers' router-LSAs;
    advertisement of additional links is redundant.

    The definition of "equivalent" links is as follows. A set of links
    are equivalent if they (a) are all point-to-point links, (b) all
    connect the same pair of OSPF routers, and (c) all belong to the
    same OSPF area.

    The organization of this memo is as follows. Section 2 describes the
    implementation in detail. In a nutshell, the changes required to
    implement the reduction in adjacencies are: (Section 2.1) The router
    with the higher OSPF router ID chooses which of the equivalent links
    to form adjacencies over; the remaining equivalent links stay in
    state 2-Way. (Section 2.2) When an existing adjacency is lost, the
    router with the higher Router ID forms an adjacency over one of the
    other equivalent links. (Section 2.3) Router-LSAs advertise at most
    one Type 1 router link (point-to-point connection to another router)
    for the entire collection of equivalent links, with the advertised
    cost equal to the smallest cost of any of the 2-Way links.  (Section
    2.4) The routing calculation in the routers at either end of the
    equivalent links is modified to include the least cost 2-Way links
    as next hops. (Section 2.5) The MTU check is performed as part of
    Hello processing, since it is now required on 2-Way links as well as

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    Section 3 addresses backward compatibility with implementations of
    the OSPF specification [Ref1]. A simple example of the adjacency
    reduction is given in Section 4. Additional information concerning
    the adjacency reduction, including anomalies and possible
    enhancements, are provided in Section 5.

2.  Implementation

    This section discusses the implementation of the adjacency reduction
    in detail, identifying the sections of the base OSPF protocol [Ref1]
    which must be modified.

    2.1.  Whether to become adjacent

        The decision as to whether to become adjacent with a neighbor is
        covered by Section 10.4, "Whether to become adjacent", of the
        OSPF specification [Ref1]. That section must be modified to
        implement the following idea: "When there are multiple
        equivalent links attaching a pair of OSPF Routers, the Router
        with the higher OSPF Router ID decides which links will form

        In particular, if Section 10.4 of [Ref1] indicates that the
        router should form an adjacency with a neighbor (transitioning
        the neighbor from 2-Way to ExStart state), the router should
        execute additional steps as follows:

        (1) If the interface type is other than point-to-point, start
            forming the adjacency.

        (2) If the neighbor is asking to form an adjacency (that is,
            we're running the logic in Section 10.4 of [Ref1] because we
            have received a Database Description packet from the
            neighbor), start forming the adjacency. This is necessary
            for backward compatibility.

        (3) Otherwise, we're running Section 10.4 of [Ref1] because
            either (i) we've just received a bidirectional Hello from
            the neighbor, (ii) there was an error in the previous
            Database Exchange over this link or (iii) an adjacency over
            an equivalent link has been lost (see Section 2.2). In this

            (a) If the router has a smaller Router ID than the neighbor,
                leave the neighbor in state 2-Way. The neighbor will
                decide over which of the equivalent links adjacencies
                should form.

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            (b) If the router's Router ID is larger, examine all the
                equivalent links to the neighbor. If one or more of them
                are adjacent (neighbor state Full) or are in the process
                of becoming adjacent (neighbor state greater than or
                equal to ExStart) leave the neighbor state on the
                current link in state 2-Way. Otherwise, start forming
                the adjacency by transitioning the neighbor state to

    2.2.  Lost adjacencies

        If the router with the higher OSPF Router ID notices that the
        single adjacency in a collection of equivalent links has gone
        down, it must replace it by forming an adjacency one another of
        the equivalent links.

        To be more precise, Section 10.3 of [Ref1] must be modified as
        follows.  If a neighbor in state ExStart or greater transitions
        to a state of 2-Way or lower, and (a) the router has a larger
        OSPF Router ID than the neighbor, (b) the link associated with
        the failed adjacency is one of a collection of equivalent links,
        and (c) none of the other equivalent links are in state ExStart
        or greater, then the router must start forming an adjacency on
        one of the equivalent 2-Way links (if any) by transitioning that
        link's neighbor's state to ExStart, which starts the Database
        Exchange process on that link.

    2.3.  Originating router-LSAs

        Section of [Ref1], "Describing point-to-point
        interfaces in the router-LSA", is changed as follows. If one or
        more of the equivalent links is fully adjacent (neighbor state
        Full), a single Type 1 link (point-to-point connection to
        another router) is added to the router-LSA. The advertised
        metric is set equal to the smallest cost of any of the
        equivalent links which are in state 2-Way or greater. In this
        way, in addition to the main benefit of reducing flooding
        traffic, this memo also reduces the size of the router-LSA by
        suppressing redundant link advertisements.

        Type 3 links (connection to stub networks) continue to be added
        to the router-LSA as specified in Section of [Ref1]. Up
        to one of these links will be added for each of the equivalent

        Now, in addition to the events listed in Section 12.4 of [Ref1],
        the transition of a point-to-point link to/from neighbor state
        2-Way can cause a router-LSA to be reoriginated. Such a state

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        transition may change the cost that is advertised for the
        equivalent links' Type 1 link.

    2.4.  Next hop calculation

        We must change routing calculation in the routers at the end of
        the equivalent links, allowing 2-Way interfaces to be installed
        as next hops as long as at least one equivalent link is fully
        adjacent (neighbor state Full).

        To this effect, Section 16.1.1 of [Ref1] is changed as follows.
        When installing a next hop to a directly connected router,
        through a point-to-point interface, all least cost equivalent
        links to the neighbor in state 2-Way or greater should be added
        as equal-cost next hops.

        Even if it doesn't cause the contents of the link-state database
        to change, the transition of a point-to-point link to/from
        neighbor state 2-Way may change the next hops of routing table
        entries, forcing rerunning of the routing calculation.

    2.5.  MTU check

        Since you are now adding certain 2-way, but non-adjacent, links
        as next hops in the routing table entries (Section 2.4), the MTU
        mismatch detection must be implemented in OSPF Hello packets
        sent over point-to-point links. To this end, Hello packets sent
        over point-to-point links (Section 9.5 of [Ref1]) have their
        Designated Router field set to the MTU of the point-to-point
        interface.  Upon receiving an Hello on a point-to-point
        interface (Section 10.5 of [Ref1]), the new MTU field is
        examined. If it is greater than the interface's MTU, the Hello
        is discarded, preventing the neighbor relationship from forming
        and the interface from being installed as a next hop in the
        routing table (see Section G.9 of [Ref3] for more details on MTU

3.  Backward compatibility

    This memo is backward compatible with implementations of the OSPF
    specification in [Ref1]. No negotiation between neighbors is
    required. If the neighbor runs [Ref1] but not the enhancements in
    this memo, adjacencies will form over all links, because of Step 2
    in Section 2.1.

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4.  Example

    Suppose there are six point-to-point links connecting Routers A and
    B. Router A has the higher OSPF Router ID. The first two links
    (IfIndex 1 and 2 on the Router A end) belong to Area The
    last four (IfIndexes 3-6 in Router A) belong to Area There
    are then two sets of equivalent links, one for each area.

    In all cases, OSPF Hellos will always be sent over all links.
    Assuming the links are all operational, they will all attain a
    neighbor state of 2-Way.

    There are then three cases of interest.

    Case 1:
        A and B running enhancements defined in this memo. In this case,
        B will let A choose one link in each area over which to form an
        adjacency.  Suppose these are the links corresponding to
        IfIndexes 1 and 3. If the link corresponding to IfIndex 3 later
        fails, A will choose a different link (say IfIndex 4) over which
        to form an adjacency. Suppose that IfIndexes 5 and 6 have been
        assigned the smallest costs, each with cost 10. As long as
        IfIndexes 5 and 6 are bidirectional (in neighbor state 2-Way or
        greater), A's router-LSA for area will include a single
        Type 1 link to B with cost 10, and the outgoing interfaces for
        routing table entries through B will be the combination of
        IfIndexes 5 and 6. This will be true both before and after the
        failure of IfIndex 3.

    Case 2:
        Only A runs the enhancements in this memo. A will receive
        requests to form adjacencies on all links (that is, Database
        Description packets from B) and will cooperate by establishing
        adjacencies over all links.

    Case 3:
        Only B runs the enhancements in this memo. The mirror image of
        Case 2; adjacencies again form over all links.

5.  Notes

    Here is additional information on the enhancements provided by this

     (1)   The biggest code change required by this memo is to base the
           decision to form an adjacency on whether a Database
           Description packet has just been seen from the neighbor (Step
           2 of Section 2.1).  However, this distinction is useful for

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           other reasons; for example, in rate-limiting the number of
           concurrent Database Exchange sessions (see Section 8.3 of

     (2)   Why not include Point-to-MultiPoint links in the equivalent
           links definition? Because they can't be excluded from the
           router-LSA, as they are necessary for the next hop

     (3)   When the single adjacency goes down, packets will not be
           forwarded between the neighbors until a new adjacency is
           formed.  To get around this problem, you can introduce a new
           parameter, NumFloodingLinks, and require that that many
           adjacencies be formed within each set of equivalent links.
           This is equivalent to OSPF's Backup Designated Router on
           broadcast subnets.

     (4)   Whenever you are limiting the number of adjacencies, you
           should timeout adjacencies that are not progressing towards
           Full state. See Section 8.3 of [Ref2] for details.

     (5)   If a router running the enhancements in this memo restarts,
           and chooses not to form an adjacency over a given point-to-
           point link, its neighbor may mistakenly believe that an
           adjacency still exists: there may have been an adjacency
           before the restart, and either the router did not send an
           empty Hello Packet out the interface after restart, or the
           Hello was dropped for some reason. The router will eventually
           notice its neighbor's confusion when the neighbor sends a
           Link State Update packet over the former adjacency. At this
           time the router should tell the neighbor that the adjacency
           no longer exists by responding with an empty Hello Packet.


    [Ref1]  Moy, J., "OSPF Version 2", RFC 2328, April 1998.

    [Ref2]  Moy, J., "OSPF Complete Implementation", Addison-Wesley,
            October 2000.

    [Ref3]  Moy, J., "OSPF Version 2", RFC 2178, July 1997.

Security Considerations

    This memo does not create any new security issues for the OSPF
    protocol. Security considerations for the base OSPF protocol are
    covered in [Ref1].

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Authors Addresses

    J. Moy
    Sycamore Networks, Inc.
    150 Apollo Drive
    Chelmsford, MA 01824
    Phone: (978) 367-2505
    Fax:   (978) 256-4203
    email: jmoy@sycamorenet.com

Moy                                                             [Page 8]

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