DR and BDR Elections

[steven.crutchley]

I have the following simple OSPF network:

R1, R2 and R3 are all connected to the switch on interface fa0/0. R1 has ospf priority 5, R3 has ospf priority 3 and R2 has the default of 1.

If I connect the routers up in the correct order they become the DR, BDR and DROther as shown in the diagram.

My Cisco book is unclear on when a DR/BDR election occurs.

If I disconnect R1 ... what should happen? From my GNS3 experiments I get the following:

R3, as I suspect, waits the dead timer and then takes over the as the DR. But R2 immediately takes over as the BDR.

This is perfect, but my understanding was that an election would only take place if a DR or BDR fails.... so I ask this.... does the BDR (R3) turning into the DR (after R1 is disconnected) count as a BDR failure, of sorts, promtping an election for a BDR which R2 conequently wins?

Also, one more question on this DR/BDR election process... given that elections only occur when failures occur... isn't the order in which you add the routers to the Transit network when it first starts up the ultimate determining factor in what will or will not become the DR or BDR? With the priorities (or RIDs) only coming into play when something fails?

[Richard Burts · Accepted solution]

Steven

You are (at least mostly) right. I would suggest this (slightly different) explanation of when there are elections. There is an election for DR when there are routers active in the network segment and there is no current DR and there is an election for DR when there has been a failure of the previously active DR. There is an election for BDR when there are routers active on the network segment and there is no current active BDR. You are correct that you could consider it a failure of the BDR (of some type of failure) when the BDR is promoted to DR.

And you are quite right that when the network is first initialized, that it is much more important the order (and timing) in which routers boot up. If the first router to boot up has an inferior priority (but not zero) and is sufficiently before the other router, then the router with inferior priority will become the DR. If several routers boot up with a short time of each other then priority will come into play. And you are quite right that the priority operation really describes what happens in a stable operating environment and not at network intiailzation time.

HTH

Rick

[sebastianpotok]

Hey guys, great post. However I have to say that I don't necessarily agree with the following, "

You are correct that you could consider it a failure of the BDR (of some type of failure) when the BDR is promoted to DR."

The BDR will be promoted to DR if current DR is not available or if priorities change and OSPF process is reset. So when BDR is promoted to DR, we should not consider it some type of BDR failure.

also, don't forget that OSPF routers don't preempt

Sent from Cisco Technical Support iPhone App

[Richard Burts]

If you accept my alternate explanation that there is an election for BDR when there are routers active in OSPF with non zero priority and there is no currently active BDR, then your position is quite right.

But if you start from the position explained by Steven (and shared by quite a few others) that "my understanding was that an election would only take place if a DR or BDR fails"  then how do you explain the election that takes place when the BDR is promoted to DR? If you interpret this as 'there is no BDR so there must have been a failure' then Steven's interpretation works. If you insist that the promotion is not a failure then how do you justify the election?

HTH

Rick