OSPF Non-Broadcast Network Type over Frame-Relay

The different OSPF network types are a pain to understand for most CCNP students. I’m going to cover all of them in a series of lessons and in this one we’ll take a look at the OSPF non-broadcast network type. We will be using a point-to-multipoint frame-relay network for the demonstration:

ospf network type topology

If you select the non-broadcast network type then OSPF will assume you are running a multi-access network. Couple of key things to remember here:

  • Multi-access means we have to select a DR and BDR.
  • Non-broadcast means that OSPF expects us to configure neighbors ourselves.

Now take a look close look at my frame-relay network in the picture above and assume we are going to run the non-broadcast network type here. Is my network multi-access? Interesting question right?

Is there any connectivity between router Spoke1 and Spoke2? You can see I only have 2 PVCs and there is no connection between Spoke1 and Spoke2. This is where things can get funky! If we select the non-broadcast network type we are telling OSPF our network is multi-access but in reality it’s not…there is only connectivity between the Hub router and the Spoke routers, not between the 2 spoke routers.

When I explained the DR/BDR to you I told you that we only have a full adjacency with the DR/BDR and not with all other routers. What do you think will happen if Spoke1 is elected as the DR?

Since Spoke2 can’t reach Spoke1 it can never setup a full OSPF neighbor adjacency and we’ll run into connectivity issues. How do we solve this? We have to make sure the Hub router becomes the DR and Spoke1 or Spoke2 will never become DR or BDR!

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Forum Replies

  1. Hello Sims!

    The network topology that you are describing would not COMPLETELY simulate the frame relay situation in this lesson. What you are describing would require all communication between R1 and R3 to go through R2 just like in the lesson, however, this also means that the two interfaces on R2 that connect R1 and R3 would have to be on the same subnet, something that is not possible. So you would just be creating two point to point links on different subnets, however, in the frame relay example in the lesson, all three routers are in the same subnet.

    Wha

    ... Continue reading in our forum

  2. Hello Sims!

    Although I’m sure you know most of this, let me start by stating the network types for OSPF and stating an example for each:

    **Point to Point** - a serial connection between two routers
    **Broadcast** - two or more routers on the same subnet connected to an Ethernet switch

    The next two types of network are used to accommodate multi access technologies that don’t support broadcast. These technologies include Frame Relay and ATM.

    **Non Broadcast Multi Access** - This is used when you have multiple routers connected on the same subnet over a non-broadc

    ... Continue reading in our forum

  3. Hello Laz,

    Thank you so much for your guidance, this absolutely cleared my doubt.

    Regards,
    Nanu

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