Reason for electing dr/bdr on broadcast network

Unanswered Question
Aug 14th, 2007

Hi Guys,

One more from me,what is the reason for electing the DR and BDR on broadcast networks,i m little bit confused on this.

Thanks

Mahmood

I have this problem too.
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Jon Marshall Tue, 08/14/2007 - 03:04

Hi Mahmood

It's basically down to scalability. Without a BDR/DR every router router would need to exchange updates with every other router on an shared segement. With a DR/BDR all the other routers on that segment communicate via the DR and then the DR propogates out the information to the rest of the OSPF speaking routers on that segment.

HTH

Jon

Richard Burts Tue, 08/14/2007 - 03:28

Mahmood

I agree with Jon that it is a question of scalability and think that an example might help to understand it. Think of an Ethernet segment with 6 routers running OSPF. Without DR/BDR every router would have to build a complete adjacency with 5 routers and a total of 15 adjacencies would be using bandwidth on that segment. Or think of a segment with 10 routers where every router would have to negotiate and maintain 9 adjacencies. Where with DR/BDR most routers have to negotiate and maintain only 2.

HTH

Rick

rajinikanth Tue, 08/14/2007 - 03:24

Hi,

The idea behind DR /BDR is that routers have a central point of contact for information exchange. Instead of each router exchanging updates with every other router on the segment, every router exchanges information with the DR and BDR. The DR and BDR relay the information to everybody else. In mathematical terms, this cuts the information exchange from O(n*n) to O(n) where n is the number of routers on a multi-access segment.

Hope this helps

Raj

Joseph W. Doherty Tue, 10/02/2007 - 04:58

One other tip, DR and BDR cause a bit more load. Because of this, you can also config priorities for which router will be elected DR and/or BDR, or excluded altogether. Even when all the routers are similar platforms, you might want to distribute the load depending on other OSPF issues (such as if router is also a border router) or physical topology (as in what other subnets the router connects to).

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