OSPF adjacency clarification

[tjskier69]

Hi guys,

just need to sort out the amount of adjacencies created on the same LAN

segment in my understanding.

What I need to understand correctly is this

Say we have 5 routers without the DR/BDRm I calculate n(n-1) where n is the

number of routers, would be 20 adjacencies. Ok

Now if we introduce the the DR/BDR mechanism we would then have 2(n-2) (for

the 3 of the 5 routers) = 6, + 2(n-1) (for the DR & BDR) = 8 making a total

of 14, have I got this right?

Any clarification would be appreciated, thanks!

[pkhatri]

Hi,

Firstly, an adjacency is a bi-directional concept. So if you have an adjacency between A and B, it's better to count it as one adjacency rather than two adjacencies (one for A and one for B).

Therefore, in a network of 5 routers with no DR/BDR, the number of adjacencies is: n(n-1)/2 = 10.

In a network of 5 routers with a DR/BDR, the number of adjacencies is: 2(n-2)+1 since each router has an adjacency with the DR and the BDR. In addition, the DR is adjacent to the BDR. This gives you 7 adjacencies for 5 router network.

So your calcuations are pretty much correct - except that you have counted each adjancency twice.

Hope that helps - pls do rate the post if it does.

Paresh

[tjskier69]

Thanks Paresh,

yes I suppose you can see it from 2 perspectives,

1. How many adjacencies are there

2. How many adjacencies do all the routers collectively have.

So your answer then would be no.1 and mine no.2 yes?

Important distinction depending on what question you get asked in an exam I suppose.

Tony

[pkhatri]

Hi Tony,

It's not really about perspective. In an exam situation I would go for the answer I have given. Consider the case where you have only two routers with OSPF running between them. According to your perspective, you would have two adjacencies between them, which is not really correct. If you told someone that you had two adjacencies between two routers, they would immediately assume that both router had two links between them, each running OSPF. So the correct answer is that there is one adjacency in this case.

Paresh