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Webcast-Catalyst9k

Racing to become the DR....

So I understand that highest priority wins the DR election and that highest router ID is used to break ties. I also understand that a new router coming online with a higher priority than the rest won't knock down the current DR from its perch.

But when I was confirming all of this in my lab the other day (with all at priority 1 except Router G at priority 200) I noticed from the debug output that some elected themselves, or their high-router-ID neighbors as DR but then elected Router G as DR after reaching 2WAY state with it.

I set up four unconnected routers with OSPF on their token-ring interfaces, a high loopback address for each. I plugged them into my TR hub and then powered it on; so everyone would come online at the same time.

I'm just wondering about how neighbor discovery is ordered, if at all. With all except Router G at the same priority, it seems like if Routers E and F reach the 2WAY state with each other before anyone else on the segment, F (with its higher loopback address) will be elected DR, even if high-priority Router G comes online only microseconds later. Is this a luck of the draw, or is there some mechanism by which the DR election might be tarried till all neighbors are discovered?

2 REPLIES
Hall of Fame Super Bronze

Re: Racing to become the DR....

On a broadcast network, the hello interval default to 10 seconds. When an OSPF interface becomes active, it sends its first hello packet containing information such as: Router-ID, Authentication, Area information, MTU, Priority, etc. It also sets the DR and BDR information to 0.0.0.0 and the wait timer to be equal to the dead-interval timer (40 seconds).

During those 40 seconds, if another router does not respond with a higher priority or if the priority are equal, a higher RouterID - the router becomes the DR of that segment.

This step usually takes place after the 2-WAY process.

HTH,

__

Edison.

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Bronze

Re: Racing to become the DR....

It appears the neigbors once they initialise will wait until all others have started up and ready with their state/priority.

Once 2way state is reached...it goes on to know the priority value, after which the highest priority guy is elected.

But during the period until all reach 2 way state..there could be minor delays due to which some of them get fully initialised may try to be DR for the moment.

Pls rate/mark if this helps!!!

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