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Jan 27th, 2014

I want to know if my idea behine adjacencies in OSPF are correct. In a broadcast multiaccess network, is the calculation 2n-3?

This the the way I came up with the formula:

With a DR and BDR on a network consisting of n (>=2) routers, there are 2n-3 adjacencies. There are n-2 DROthers. They form an adjacency with both the DR and the BDR. That gives us (n-2)+(n-2) = 2n-4 adjacencies. But there is also an adjacency between the DR and BDR. So that means we have 2n-4+1 = 2n-3 adjacencies.

The answer is not 2(n-1)=2n-2 because you've counted the adjacency between the DR and the BDR twice. So you have to subtract 1 from 2n-2, which gives you 2n-2-1=2n-3 again.

Is this correct?

Thanks

It is correct according to my maths (which is never very reliable )

Jon

Overall Rating: 5 (1 ratings)

## Replies

Jon Marshall Mon, 01/27/2014 - 11:59
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It is correct according to my maths (which is never very reliable )

Jon