Adjacencies on Non Broadcast Multi Access NBMA
Special care should be taken when configuring OSPF over multi-access non-broadcast medias such as Frame Relay, X.25, ATM. The protocol considers these media like any other broadcast media such as Ethernet. NBMA clouds are usually built in a hub and spoke topology. PVCs or SVCs are laid out in a partial mesh and the physical topology does not provide the multi access that OSPF believes is out there. The selection of the DR becomes an issue because the DR and BDR need to have full physical connectivity with all routers that exist on the cloud. Also, because of the lack of broadcast capabilities, the DR and BDR need to have a static list of all other routers attached to the cloud. This is achieved using the neighbor command:
neighbor ip-address [priority number] [poll-interval seconds]
where the "ip-address" and "priority" are the IP address and the OSPF priority given to the neighbor. A neighbor with priority 0 is considered ineligible for DR election. The "poll-interval" is the amount of time an NBMA interface waits before polling (sending a Hello) to a presumably dead neighbor. The neighbor command applies to routers with a potential of being DRs or BDRs (interface priority not equal to 0). The following diagram shows a network diagram where DR selection is very important:
In the above diagram, it is essential for RTA's interface to the cloud to be elected DR. This is because RTA is the only router that has full connectivity to other routers. The election of the DR could be influenced by setting the ospf priority on the interfaces. Routers that do not need to become DRs or BDRs will have a priority of 0 other routers could have a lower priority.
I will not dwell too much on the use of the "neighbor" command as this is becoming obsolete with the introduction of new means of setting the interface Network Type to whatever we want irrespective of what the underlying physical media is. This is explained in the following section.
Continue reading here: Selecting Interface Network Types
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