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:
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!