[Cu-wireless] OSPF and non-broadcast networks

David Young dyoung at ojctech.com
Fri May 17 16:24:01 CDT 2002

Below, I have snipped some text from RFC 2328, which defines OSPFv2.
Zebra implements OSPFv2. It seems to me that in pods, we should run OSPF
in a point-to-multipoint mode. Does anyone else have a different reading?

Zebra appears to support point-to-multipoint mode through its command
"ip ospf network point-to-multipoint".


*** snip snip ***

 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.

*** snip snip ***

 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.

*** snip snip ***

 2.1.1.  Representation of non-broadcast networks

            As mentioned previously, OSPF can run over non-broadcast
            networks in one of two modes: NBMA or Point-to-MultiPoint.
            The choice of mode determines the way that the Hello

Moy                         Standards Track                    [Page 15]
RFC 2328                     OSPF Version 2                   April 1998

            protocol and flooding work over the non-broadcast network,
            and the way that the network is represented in the link-
            state database.

            In NBMA mode, OSPF emulates operation over a broadcast
            network: a Designated Router is elected for the NBMA
            network, and the Designated Router originates an LSA for the
            network. The graph representation for broadcast networks and
            NBMA networks is identical. This representation is pictured
            in the middle of Figure 1a.

            NBMA mode is the most efficient way to run OSPF over non-
            broadcast networks, both in terms of link-state database
            size and in terms of the amount of routing protocol traffic.
            However, it has one significant restriction: it requires all
            routers attached to the NBMA network to be able to
            communicate directly. This restriction may be met on some
            non-broadcast networks, such as an ATM subnet utilizing
            SVCs. But it is often not met on other non-broadcast
            networks, such as PVC-only Frame Relay networks. On non-
            broadcast networks where not all routers can communicate
            directly you can break the non-broadcast network into
            logical subnets, with the routers on each subnet being able
            to communicate directly, and then run each separate subnet
            as an NBMA network (see [Ref15]). This however requires
            quite a bit of administrative overhead, and is prone to
            misconfiguration. It is probably better to run such a non-
            broadcast network in Point-to-Multipoint mode.

            In Point-to-MultiPoint mode, OSPF treats all router-to-
            router connections over the non-broadcast network as if they
            were point-to-point links. No Designated Router is elected
            for the network, nor is there an LSA generated for the
            network. In fact, a vertex for the Point-to-MultiPoint
            network does not appear in the graph of the link-state

            Figure 1b illustrates the link-state database representation
            of a Point-to-MultiPoint network. On the left side of the
            figure, a Point-to-MultiPoint network is pictured. It is
            assumed that all routers can communicate directly, except
            for routers RT4 and RT5. I3 though I6 indicate the routers'

Moy                         Standards Track                    [Page 16]
RFC 2328                     OSPF Version 2                   April 1998

            IP interface addresses on the Point-to-MultiPoint network.
            In the graphical representation of the link-state database,
            routers that can communicate directly over the Point-to-
            MultiPoint network are joined by bidirectional edges, and
            each router also has a stub connection to its own IP
            interface address (which is in contrast to the
            representation of real point-to-point links; see Figure 1a).

            On some non-broadcast networks, use of Point-to-MultiPoint
            mode and data-link protocols such as Inverse ARP (see
            [Ref14]) will allow autodiscovery of OSPF neighbors even
            though broadcast support is not available.

                +---+      +---+
                |RT3|      |RT4|              |RT3|RT4|RT5|RT6|
                +---+      +---+        *  --------------------
                I3|    N2    |I4        *  RT3|   | X | X | X |
            +----------------------+    T  RT4| X |   |   | X |
                I5|          |I6        O  RT5| X |   |   | X |
                +---+      +---+        *  RT6| X | X | X |   |
                |RT5|      |RT6|        *   I3| X |   |   |   |
                +---+      +---+            I4|   | X |   |   |
                                            I5|   |   | X |   |
                                            I6|   |   |   | X |

                    Figure 1b: Network map components
                       Point-to-MultiPoint networks

             All routers can communicate directly over N2, except
                routers RT4 and RT5. I3 through I6 indicate IP
                           interface addresses

David Young             OJC Technologies
dyoung at onthejob.net     Engineering from the Right Brain
                        Urbana, IL * (217) 278-3933

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