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  1. The Hour Man
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    #1

    Default OSPF network types

    I know that there are 5 OSPF network types:
    NBMA (RFC)
    Point-to-multipoint broadcast (RFC)
    Point-to-point (Cisco)
    Point-to-multipoint non-broadcast (Cisco)
    Broadcast (Cisco)

    I am little confuse about these network types. My understanding is:
    Let say that we have R1, R2, R3 in OSPF Broadcast network type. This is like OSPF in ethernet network or like Token ring and all the routers are in a 1 network subnet. Hello/Dead timer is 10/40. Also, DR/BDR is elected.

    The NBMA network is like the Broadcast, but it uses Frame-relay, ATM, MPLS. And the hello/dead timer is 30/120.
    Now, I believe that NBMA can be a full mesh or Hub and Spoke setup. Based on my understanding, to setup a full mesh NBMA network, I would need to use the neighbor command under router ospf 1 to define my neighbors and R1, R2 and R3 need to have the neighbor command configured.
    For Hub and Spoke NBMA, only R1 would use the neighbor command, and the the spoke routers (R2, R3) would use the command frame-relay map ip ip-add dlci broadcast under interface mode.

    About the point-to-multipoint NBMA, it is a combination of point-to-point and multipoint setup. For instance, we have R1, R2, R3 and R4 in point-to-multipoint NBMA, R1 and R2 is point-to-point and R1, R3, R4 are multipoint. Therefore, R1 and R2 use a /30 mask, and R1, R3, R4 use /29 mask. No DR/BDR election, and hello/dead timer is 30/120.

    The point-to-multipoint Broadcast is similar to the point-to-multipoint NBMA, but in a Ethernet network.


    Am I right?

    Thanks
    Last edited by thehourman; 05-29-2011 at 12:15 PM.
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  3. Senior Member
    Join Date
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    #2
    I think you basically have it.

    broadcast: basically, ethernet
    non-broadcast: whether or not the routers can see each other is controlled by whether or not you pay for the circuits. Can get very expensive to have a full-mesh set-up across frame relay, for example.
    point-to-point: dedicated circuit setup between two routers.
    point-to-multipoint: one router has multiple connections to other routers. (hub and spoke). This can be configured using "broadcast" or "non-broadcast" media.

    broadcast is typical of LAN, and non-broadcast is typical of WAN connectivity
    point-to-point and point-to-multipoint are typical ways of connecting WAN-connected routers, but you can utilize point-to-multipoint within a broadcast network, if you so choose.
    OSPF classifies different media into the following three types of networks by default:
    Broadcast networks (Ethernet, Token Ring, and FDDI)
    Nonbroadcast multiaccess (NBMA) networks (Switched Multimegabit Data Service (SMDS), Frame Relay, and X.25)
    Point-to-point networks (High-Level Data Link Control [HDLC] and PPP)
    You can configure your network as either a broadcast or an NBMA network.
    X.25 and Frame Relay provide an optional broadcast capability that can be configured in the map to allow OSPF to run as a broadcast network. Refer to the x25 map and frame-relay map command descriptions in the Cisco IOS Wide-Area Networking Command Reference publication for more detail.
    Configuring Your OSPF Network Type

    You have the choice of configuring your OSPF network type as either broadcast or NBMA, regardless of the default media type. Using this feature, you can configure broadcast networks as NBMA networks when, for example, you have routers in your network that do not support multicast addressing. You also can configure NBMA networks (such as X.25, Frame Relay, and SMDS) as broadcast networks. This feature saves you from needing to configure neighbors, as described in the section "Configuring OSPF for Nonbroadcast Networks" later in this module.
    Configuring NBMA networks as either broadcast or nonbroadcast assumes that there are virtual circuits (VCs) from every router to every router or fully meshed network. This is not true for some cases, for example, because of cost constraints, or when you have only a partially meshed network. In these cases, you can configure the OSPF network type as a point-to-multipoint network. Routing between two routers not directly connected will go through the router that has VCs to both routers. Note that you need not configure neighbors when using this feature.
    An OSPF point-to-multipoint interface is defined as a numbered point-to-point interface having one or more neighbors. It creates multiple host routes. An OSPF point-to-multipoint network has the following benefits compared to NBMA and point-to-point networks:
    Point-to-multipoint is easier to configure because it requires no configuration of neighbor commands, it consumes only one IP subnet, and it requires no designated router election.
    It costs less because it does not require a fully meshed topology.
    It is more reliable because it maintains connectivity in the event of VC failure.
    On point-to-multipoint, broadcast networks, there is no need to specify neighbors. However, you can specify neighbors with the neighbor router configuration command, in which case you should specify a cost to that neighbor.

    Source: Cisco IOS IP Routing: OSPF Configuration Guide, Release 12.4 - Configuring OSPF [Cisco IOS Software Releases 12.4 Mainline] - Cisco Systems

    You may knock it as only being a configuration guide, but they tend to give pretty good explanations within the configuration guides, so I think they're awesome .

    To browse to this document yourself in the future:

    1. Cisco Systems, Inc
    2. Mouse-over "Support" > Under "Product Support" > "Product Categories" > choose "Cisco IOS and NXOS Software"
    3. Under "Make a selection to continue" choose "Cisco IOS"
    4. Next, Choose "Cisco IOS Software Releases 12.4 Family"
    5. Next, Choose "Cisco IOS Software Release 12.4 Mainline"
    6. Next, choose "Configure" > "Configuration Guides"
    7. Scroll down to and choose "Cisco IOS IP Routing: OSPF Configuration Guide, Release 12.4"
    8. Choose "Configuring OSPF"

    I hope this helps.

    Sometimes the "google" result won't necessarily be the IOS version you want, so it helps to be able to browse to the guide you want.
    Currently Working: CCIE R&S
    LinkedIn: http://www.linkedin.com/in/lewislampkin (Please connect: Just say you're from TechExams.Net!)
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  4. The Hour Man
    Join Date
    Nov 2009
    Location
    MD
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    #3
    Holy smokes! Dude, thanks a lot.
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