OSPF NBMA Topology question

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Sep 4th, 2008
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Hello Friends,

I seem to be totally lost while i was going through the OSPF NBMA network types and desperately need your assistance to clear my concepts. From what i have known so far, OSPF supports the following network types

a. Point - Point (Serial connection)

b. Broadcast Multi Access (Ethernet, Token Ring)

c. Non - Broadcast Multi Access (FR, X.25, ATM)


Further, NBMA networks can be broken down to

Nonbroadcast (NBMA)

Point - Multipoint

Point - Multipoint nonbroadcast


Point to point

1. From my understanding, NBMA networks are FR, ATM, X.25 which do not allow multicast and broadcast traffic, is that right? Then how come configuring FR networks like point - multipoint allow multicast traffic to go to OSPF neighbor.

2. From practical network point of view, which is the most commonly used NBMA network type?

3. Is it the default behaviour of OSPF that it considers NBMA to be a broadcast network though it is not?

4. Under which circumstances would you configure Broadcast and

Point to point network types in NBMA?

5. I have gone through KNet, BSCI books but still am not clear of these concepts. If there is any blog or link which gives good explanation of these, do let me know.

Thanks in advance for your time and assistance.

Best Regards,


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francisco_1 Thu, 09/04/2008 - 05:48
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with network types like "Point-Multipoint None-Broadcast" and None-broadcast, when you enable the ospf process on routers using those network types on the interfaces you have to manually configure the neighbour peering between your routers since they are none broadcast type, the SLA is not broastcast between the routers through multicast, so you have to send it in unicast by manually setup the peering using the neigh command. See below.


routers ospf 1

neighbour (send as unicast)


routers ospf 1

neighbour (send as unicast)

None broadcast: Normally used between routers on the same IP Subnet. Neighbours must be manually configured. DR/BDR election. Used in Full mesh/Partial mesh.

Point-Multipoint-None broadcast: Neighbours must be manually configured. No DR/BDR election.

The difference between the NBMA and Broadcast like i mentioned on NBMA, you have to manually setup your neighbour and also the hello/dead timers are different.

Hope i was helpful providing you with the basics

devang_etcom Thu, 09/04/2008 - 05:59
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Think about all our Routing Protocols, they are sending updates on Broadcast or Multicast addresses, so if you have network that is not capable of supporting Broadcast or Multicast like NBMA there you need to enable such a mechanism so that peers can send the update using unicast method (Use "neighbor" command), but you want to go for the broadcast/multicast update then you will have to force that NBMA environment to support the broadcast.

Now point to point link is like you have all your peers in different subnets, Multipoint is again the collection of multiple point to point link.


Devang Patel


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