From: Jay Hennigan (jay@west.net)
Date: Sat Oct 19 2002 - 22:54:27 GMT-3
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
This archive was generated by hypermail 2.1.4 : Tue Nov 05 2002 - 08:35:51 GMT-3