OSPF Designated Router election question

veritas_libertas

I'm not sure I'm fully understanding what I'm reading. I have some questions that I'm hoping someone can explain to me. I understand that the interface with the largest IP will be chosen as the DR, but than further in my reading I'm learning that the election is done by priority. Are we talking about an either or, is this locally on the router vs on the network between routers? Also, is the Backup DR the second highest IP?

Zartanasaurus

It's not an either/or, so much as an if then else type deal.

The process goes basically like this:
1) Make a list of all neighbors that are eligible IE in a 2-way state and not priority 0 (self included).
2) Make a sub-list of neighbors that aren't advertising themselves as the DR.
3) Elect BDR: Highest priority of router advertising itself as BDR = BDR. If tie, highest RID of advertising BDR = BDR. Else (IE none advertising self as BDR), highest priority = BDR. If tie, highest RID = BDR.
4) Elect DR: Highest priorty of router advertising self as DR = DR. If tie, highest RID of advertising DR = DR. Else BDR = DR.
5) Repeat as necessary.

This assumes they all start their process at the same time. Otherwise, it's just going to be the 2 routers that load first.

veritas_libertas

I'm not sure I'm following your explanation. You are saying that each router starts by looking at their adjacencies, and then each router advertises the highest IP router in their area?

Zartanasaurus

Routers advertise who they think the DR and BDR are in their hello packets. DRs and BDRs are chosen in an election which I described above.