OSPF: NBMA mode on NBMA network type

Unanswered Question
Aug 3rd, 2008

Hello,

Let's say we have an NBMA network (e.g. Frame Relay) and we configure NBMA mode on it.

I found in CBT Nuggets that there must be full-mesh connectivity in an NBMA mode in order to have IP communication between all routers.

However, Cisco Student Guide says that we must have connectivity between each non-DR and DR (PVCs in Frame Relay). In addition, it states that NBMA mode is for full-mesh and partial mesh topologies.


any idea?

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Richard Burts Sun, 08/03/2008 - 04:44

My thoughts are that OSPF NBMA was designed for full mesh networks and for it to really work right (and reduce manual efforts) it should have full mesh. But it can work in partial mesh - but doing so means that you will manually have to make sure that certain things will happen.


Perhaps an example will clarify this:

lets assume that you have an OSPF network segment that is NBMA and that you want routerA to be the DR. So you set the priority of routerA to be better than any other router on the NBMA. If it were full mesh then every router on the NBMA would receive a Hello from routerA and every router would know that routerA should become the DR. But if the network were partial mesh there might be some routers that do not have a PVC to routerA and therefore they do not receive a Hello from routerA and they would not recognize that routerA needs to be the DR. So you have to manually assure that all routers in the partial mesh do have a PVC to the router that you want to become DR.


There is a related aspect of this that depends on a basic behavior of OSPF, which is that any non-DR router will send its LSAs using a multicast address for "all DR routers" and then the DR sends a multicast to "all OSPF routers" on that subnet. If there is some non-DR router that does not have a PVC to the DR then its multicast LSA will not get to the DR, and therefore its LSA will not be advertised to the rest of the routers in the subnet. Therefore it is important that every router have a PVC to the DR and important that the DR have a PVC to every member of the subnet. This is easy in full mesh, but is a potential problem in partial mesh. So in partial mesh you will need to manually control the election of DR so that only the router(s) that you want (which have PVCs to all other routers) can become DR.


HTH


Rick

lamav Sun, 08/03/2008 - 17:20

From my experience, i found it easiest and most straightforward to simply create point-to-point links from the hub to the spoke routers in an NBMA environment. This way you dont have to worry about DR and BDR elections or the need to be fully meshed.


For example, you can have a hub router sitting at the data center's edge, with an ATM T3 with logical subinterfaces/PVCs to the spoke routers out in the field.


I'm not sure I ever understaood what the benefit would be to implementing an NBMA network as an OSPF NBMA topology.


HTH


Victor

Richard Burts Sun, 08/03/2008 - 18:40

Victor's response gives me an opportunity to re-read the original post and to think some more about this thread. I have these comments:


to the original poster

I am paying more attention to this part of your post:

I found in CBT Nuggets that there must be full-mesh connectivity in an NBMA mode

and I believe that the use of "must be full mesh" is an exaggeration. Certainly full mesh is what was intended when OSPF NBMA was developed. And full mesh makes it easier (and removes the requirement for manual efforts). But in fact full mesh is not required. Partial mesh can work for OSPF NBMA - if you have set things up right.


Victor

Certainly I would agree that point to point links are better when running OSPF on a Frame Relay network. And if I were setting up an OSPF network to run on Frame Relay I would strongly advocate for it to be point to point. But as a consultant I usually am not able to tell a customer that they should change their network architecture. If they chose to use NBMA (for whatever reason seemed compelling to them) then I need to understand the implications of this and need to be able to make it work.


HTH


Rick

lamav Sun, 08/03/2008 - 19:59

Rick:


I understand exactly what you mean. The poster asked a specific question regarding OSPF NBMA requirements and you answered it, lucidly, too, I might add.


I was just thinking about NBMA and OSPF and made my comments simply because they came to mind.


Good night :-)


Victor

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