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DR and BDR election in different kinds of network

"DR and BDR are elected on broadcast and nonbroadcast multi-access network", what is broadcast and nonbroadcast multi-access network? Example? Why DR and BDR are elected in that kind of network? Why not others? ex. Point-to-Point, Point-to-multipoint casting, Nonbroadcast and broadcast multipoint casting, multi-access broadcasting? What are they?


Re: DR and BDR election in different kinds of network


A good place to start with for understanding OSPF is OSPF design guide at cisco site

Most of your queries will be addressed by that document.

I suggest you to spend considerable amount of time for reading and understanding the concepts outlined in that document.

To address your query, you need to understand concepts like neighbour relationship and Adjacencies in OSPF.

Have a look at the following URL.

You can also search this forum where in similar such queries are addressed.

Have a look at this thread for explanation on NBMA.

You can find similar other Posts in this forum.

In nut shell,

Routers involved in OSPF should form adjacencies.( Refer URL for further details.

when two routers for adjacencies their topoogy informatio is synchronised.

In point to point network, there are only two routers involved, as they are directly connected to each other. So they can easily for adjancencies with each other.

On shared network topology(multiaccess) like ethernet, we can have many Routers participating in OSPF process. Say If there are N routers, all the N routers needs to form adjacencies with each other. So theoretically all routers will have to form, N * (N-1) Adjancencies. This is not an optimal way, hence the concept of DR/BDR is there in OSPF.

On a Multiaccess network, all the OSPF routers will elect one Router as a DR and another one as a BDR ( for fault tolerance). Then all the routers in that multiacces network will form adjacency only with DR and BDR. Meaning all the routers will exchange their topology detail with DR and BDR. The DR/BDR will then pass on that information to all others.

In this way, the total number of adjancencies that needs to be built is redunced, thus providing an optimal way to form adjacencies for OSPF.

Hope this helps.

Feel free to get back to us if you need further clarification.


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