Re: OSPF DR and BDR election

In general the first router to join the segment becomes DR and the second router to join becomes BDR. If DR fails the BDR is promoted to DR and there is a new election for BDR. There are some odd corner cases where this wont happen though.

When the router joins the segment it listens for WAIT timer to expire. If during this interval it receives a hello from another router on the segment, it will accept the DR information in the packet and start to send hellos. If however no one is heard during WAIT, the router elects itself DR and inserts this in its own hello.

This mean that if for some reason communication is broken during WAIT interval, two routers can elect themselves as both DR, in which case there *will* be a preemptive election once they both hear each others hellos.

For example this could happen if your OSPF DEAD/WAIT interval is lower than your STP Forward Delay, WAIT would expire before the router can receive packets and it would elect itself as DR.

This is not the normal case though. For more info see https://tools.ietf.org/html/rfc2328#page-75

Brian McGahan, 4 x CCIE #8593 (R&S/SP/SC/DC), CCDE #2013::13
bmcgahan_at_INE.com<mailto:bmcgahan_at_INE.com>

Internetwork Expert, Inc.
http://www.INE.com<http://www.ine.com/>

On Feb 21, 2014, at 6:01 AM, "Carlos G Mendioroz" <tron_at_huapi.ba.ar<mailto:tron_at_huapi.ba.ar>> wrote:

I don't know where are you reading this, but I thought that there is
only BDR election process. Once you are BDR, then you can jump to DR if
the current DR fails.

So that is why, I guess. You don't opt, you have no choice :)
-Carlos

vishal bhugra @ 20/02/2014 20:28 -0300 dixit:
Hello Experts

While going through the DR and BDR election process. I gone through a line
wherein during the process of election the router will segregate the list
of routers who have mentioned themselves as BDR and those who have
mentioned themselves as DR.

After segregation out of the list of BDR, the elected BDR will be with the
highest priority and if not then highest Router ID.

Now my question is .. Why would a router will opt himself for BDR and not
for DR. Or we can say how a router will come to conclusion that it has to
go for be a BDR not the DR.

--
Carlos G Mendioroz  <tron_at_huapi.ba.ar<mailto:tron_at_huapi.ba.ar>>  LW7 EQI  Argentina
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