From: kym blair (kymblair@hotmail.com)
Date: Sun Oct 20 2002 - 02:33:17 GMT-3
Set the Priority to zero if you don't want a router to be eligible for
DR/BDR election.
>From: Jay Hennigan <jay@west.net>
>Reply-To: Jay Hennigan <jay@west.net>
>To: Hunt Lee <ciscoforme3@yahoo.com.au>
>CC: ccielab@groupstudy.com
>Subject: Re: OSPF DR/BDR Election
>Date: Sat, 19 Oct 2002 18:54:27 -0700 (PDT)
>
>On Sun, 20 Oct 2002, [iso-8859-1] Hunt Lee wrote:
>
> > Guys,
> >
> > I'm sure this question has been asked many times before, but can
> > someone kindly explain to me again? There are 3 routers in a hub &
> > spoke topology that I want to influence the DR election. And I'm doing
> > this with "(config-router)# neighbor <x.x.x.x> priority <y>" command -
> > I also tried "(config-if)# ip ospf priority <y>" command. I have been
> > reloading the routers (as well as taking down OSPF by "no router ospf
> > <x>" & putting OSPF back on, and every time the DR, BDR & DROther is
> > different.
> >
> > I have read Caslow & Jeff Doyle, they both agree that the router with
> > the highest priority (on an interface basis) will be the DR for that
> > subnet, while Parkhurst says that the router with the lowest non-zero
> > priority will be elected the DR. So who is correct?? And why is the
> > DR/BDR/DROther are different each time?
>
>Caslow and Doyle are correct. Highest priority number will become DR.
>
>They may be different because an election doesn't occur unless the DR
>or BDR goes offline. And, when the DR goes offline, the BDR will become
>the DR. It may have to do with the timing of the election re the timing
>of the interfaces coming up.
>
>Say you have r1, r2, r3, and r4 and their priorities are 1,2,3,and 4
>respectively.
>
>Let's say they're all on ethernet connected by a switch. Power-cycle
>the switch so that all adjacencies get torn down and rebuilt at the
>same time. R4 will be DR and r3 will be BDR.
>
>Now unplug r4. R3 will become DR, and r2 BDR.
>
>Now plug r4 back in. Nothing changes. No new election. R3 remains
>DR, r2 BDR.
>
>Now unplug R3. Because R2 is already BDR, it becomes DR. R4 becomes
>BDR.
>
>Where it can get interesting is the following scenario. Same priorities,
>but R1 and R3 are on one switch and R2 and R4 on another switch, and the
>two switches are connected by a crossover cable. Unplug the crossover.
>
>Note what happens. Now plug it back in. Note what happens again.
>
>--
>Jay Hennigan - CCIE #7880 - Network Administration - jay@west.net
>NetLojix Communications, Inc. - http://www.netlojix.com/
>WestNet: Connecting you to the planet. 805 884-6323
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