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Why NBMA requires DR/BDR, point-to-multipoint NB doesn't ?

I am reading that "point-to-multipoint Nonbroadcast" topology one there is not a concept of DR/BDR.

On the NBMA there is the concept of electing a DR/BDR and one is supposed to do specify the DR election manually.

Can you clarify why the point-to-multipoint nonbroadcast dispenses the DR/BDR election ? Or perhaps it means that I should not bother to configure DR MANUALLY ? I totally understand that in the NBMA topology, the DR/BDR needs to be elected manually through the ip ospf priority command and neighbors need to be defined due to the lack of broadcast capabilities.

Since there is no broadcast capabilites on the 'point-to-multipoint non broadcast' why the same concept doesn't apply ?? On the point-to-multipoint non broadcast I see only the manual configuration of OSPF neighbors should suffice.


Re: Why NBMA requires DR/BDR, point-to-multipoint NB doesn't ?


Because multiaccess networks can support more than two routers, OSPF elects a DR to be the focal point of all link-state updates and LSAs. The role of the DR is critical, therefore a BDR is elected to "shadow" the DR. In the event that the DR fails, the BDR can smoothly take over.

Point-to-multipoint networks have the following properties:

Adjacencies are established between all neighboring routers. There is no DR or BDR for a point-to-multipoint network. No network LSA is originated for point-to-multipoint networks. Router priority is not configured for point-to-multipoint interfaces or for neighbors on point-to-multipoint networks.

When originating a router LSA, the point-to-multipoint interface is reported as a collection of point-to-point links to all the adjacent neighbors on the interface. This is together with a single stub link advertising the IP address of the interface with a cost of 0.




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