BDR Not elected as it should be...

Answered Question
Aug 16th, 2007

Hi, i have a problem with ospf not electing as it should:

I have 3 Routers without a priority:

London#:

FastEthernet0 192.168.1.1

Loopback0 192.168.31.11

Ottawa#:

FastEthernet0 192.168.1.2

Loopback0 192.168.31.22

Brasilia#:

FastEthernet0 192.168.1.3

Loopback0 192.168.31.33

So each router has the defaul priority of 1

The designated router is 192.168.31.33 as it should be.

The back-up designated router is 192.168.31.11 in stead of 192.168.31.22 ?!

As cisco states "The BDR is elected in the same way as the DR, if there is a tie in the priority the highest router id is elected to be the BDR."

I have this problem too.
0 votes
Correct Answer by mohammedmahmoud about 9 years 5 months ago

Hi,

Yes in your case you can play with "clear ip ospf x process" on the DR and BDR, or you can use "ip ospf priority 0" to force a router not to enter the election in order not to be the DR or the BDR.

HTH,

Mohammed Mahmoud.

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Overall Rating: 4.3 (3 ratings)
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mohammedmahmoud Thu, 08/16/2007 - 01:57

Hi Iwan,

There are a couple of probabilities, first make sure that there are not any other loopbacks configured and that the default priority value is not altered.

If the above concerns are fine, then you must take care of something, DR and BDR election is not an ongoing process, meaning that if Brasilia and London router came up first, they'll execute the election, and if Ottawa came up after a while it won't be the BDR instead of London, unless London fails, or Brasilia fails (and even in this second case London is going to be the DR and Ottawa is going to be the BDR although still this can make you confuse but when the DR the BDR is automatically converted to a DR and a new BDR is elected).

To summarize, the DR/BDR election is not an ongoing process, thus the first 2 router to be elected as DR/BDR will sustain there status even if a router with a higher priority is added, and the other rule is that if the DR fails, the BDR will be the DR and the other routers will elect another BDR (even if there exists a router among them with better priority than the current DR (that was the BDR)).

I hope that i've been informative.

HTH,

Mohammed Mahmoud.

Iwan_Pattyn Thu, 08/16/2007 - 05:09

Thanks, i understand.

One morre question:

Is there a way to force a re-election without havin to restart the routers or without having to use the reload command? e.g. clear ip ospf?

Correct Answer
mohammedmahmoud Thu, 08/16/2007 - 05:26

Hi,

Yes in your case you can play with "clear ip ospf x process" on the DR and BDR, or you can use "ip ospf priority 0" to force a router not to enter the election in order not to be the DR or the BDR.

HTH,

Mohammed Mahmoud.

Iwan_Pattyn Fri, 08/17/2007 - 08:00

K, understood and never to forget annymore!

I was able to test this in the new Packet Tracer 4.1. (unfortunatly i could not enter the "clear ip ospf" command...)

I switched all routers physical to off and restarted them one by one beginning with Brasilia and ending with London.

The election was correct this time. I even switched Brasilia off at a a time and pinged Brasilia, the dr and bdr changed also correct to Ottawa and London!

I must say that it is a downside of the ospf election process that it cannot re-assign the dr and bdr on a network change.

mohammedmahmoud Fri, 08/17/2007 - 08:16

Hi Iwan,

I am glade that you've sorted it out, but i want you to notice something, this logic is used to sustain stability in the network, as we won't like to have a new router entering the network to make an interruption to the network (since all the routers will have to reform new adjacencies with the new DR, and exchange new LSAs and do new SPF calculations) which mean temp down time, we won't desire this to happen unless we do it on purpose and not to be done automatically.

HTH,

Mohammed Mahmoud.

mheusing Thu, 08/16/2007 - 02:00

Hi Iwan,

Please note, that DR/BDR election is non preemptive. This means that an elected DR/BDR keeps its role even if a "better" router joins the LAN. Effectively this means any of your routers can take any role, depending on the order it connects to the LAN. In a lab environment you can test this simply by disconnecting your DR and reconnecting it after the DR/BDR election process has finished. You will see that your previous DR will not take over. This even holds, if the new router connecting to the LAN has a higher priority.

The described behaviour ensures LAN stability in that adjacencies only change if the DR or BDR go away.

Hope this helps! Please rate all posts.

Regards, Martin

Edison Ortiz Thu, 08/16/2007 - 04:24

London seems to be faster than Brasilia during the election process :)

If you want to take London out of the election process, you can change the priority on London to 0.

Richard Burts Thu, 08/16/2007 - 05:30

Yes the election results in the original post almost certainly reflect a timing issue of which router joinged the segment when. And the best way to get the desired election result is to set the priority to 0 (and then back to 1) of the current BDR. Changing the priority is much cleaner and better than trying to clear the ospf process or anything else.

HTH

Rick

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