OSPF DR/BDR Selection

Unanswered Question
Apr 17th, 2008

Hello Tutor,

My query is with regards to the DR and BDR selection process in OSPF.

We all know how the selection process works , with regards to the highest priority or ip address etc.

When routers send across hello packets across to each other during the selection process, within those packets the routers mark whether they want to be DRs or BDRs.

Now, on what basis do these routers mark in their own Hello packets whether they want to be DRs or BDRs? (As read in Jeff Doyle).

Hoping to hear from ya.

Thank you.

-/ Kiran Cherian.

( I have completed my CCNP certification, just was not sure where i could post this question).

I have this problem too.
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Kevin Dorrell Thu, 04/17/2008 - 04:42

A router marks whether it wants to be DR by using the priority field. The OSPF priority is set as an interface command. The default is priority 1, and the higher priority wins. If you want a router never to become DR or BDR (for example, a spoke in a hub-and-spoke), then give it priority 0.

The IP address is used as a tie-breaker if two routers have the same priority.

In practice, the DR and BDR are usually the first two non-zero-priority routers to come up. That is because the election of DR and BDR is not pre-emptive. In other words, once two routers are talking and they have got as far as deciding that one should be the DR and the other the BDR, then any third router will not become DR or BDR regardless of its priority ... until one of them fails, in which case it's a bye-election.

Does that make sense?

Kevin Dorrell

Luxembourg

mohammedmahmoud Thu, 04/17/2008 - 06:18

Hi Kevin,

Very nicely explained indeed, just to add a tiny detail, using "ip ospf priority 0" is the strongest tool if you wish to have an instantaneous action, it doesn't require resetting the OSPF process.

BR,

Mohammed Mahmoud.

sundar.palaniappan Thu, 04/17/2008 - 06:27

Just want to add my 2 cents to the discussion.

When DR/BDR already exist and if DR goes down BDR would be automatically promoted to DR status and a new BDR will be elected based on the priority at that point.

HTH

Sundar

Kevin Dorrell Thu, 04/17/2008 - 06:30

Hi Mohammed,

Let's see if I understand you correctly ... are you saying you can slap a priority 0 on an interface in the fly, and it will resign whatever DR/BDR is is holing at the moment? Whereas the non-zero values are non-preemptive? If so, that is very useful to know. Thanks.

Kevin Dorrell

Luxembourg

Richard Burts Thu, 04/17/2008 - 06:36

Kevin

Yes you can assign a priority of 0 on the fly and the router will immediately resign as DR or BDR (if it is currently DR or BDR on that interface). This is the easiest and most low impact way to alter the DR BDR relationships.

HTH

Rick

mohammedmahmoud Thu, 04/17/2008 - 06:37

Hi Kevin,

Yes exactly, kindly check below:

R5(config)#do sh ip osp ne

Neighbor ID Pri State Dead Time Address Interface

44.44.44.44 0 FULL/ - - 192.168.45.4 OSPF_VL0

44.44.44.44 1 FULL/BDR 00:00:34 192.168.45.4 GigabitEthernet0/1

R5(config)#int g0/1

R5(config-if)#ip osp

R5(config-if)#ip ospf pri

R5(config-if)#ip ospf priority 0

R5(config-if)#do sh ip osp ne

Neighbor ID Pri State Dead Time Address Interface

44.44.44.44 0 FULL/ - - 192.168.45.4 OSPF_VL0

44.44.44.44 1 FULL/DR 00:00:35 192.168.45.4 GigabitEthernet0/1

R5(config-if)#

R5(config-if)#

R5(config-if)#do sh ip osp ne

Neighbor ID Pri State Dead Time Address Interface

44.44.44.44 0 FULL/ - - 192.168.45.4 OSPF_VL0

44.44.44.44 1 FULL/DR 00:00:39 192.168.45.4 GigabitEthernet0/1

BR,

Mohammed Mahmoud.

k.cherian Thu, 04/17/2008 - 06:58

Hi Mohammad,Kevin;

Thank you for the explanation. In Jeff Doyle's book, he explains that during the selection process, when the routers are exchanging packet, some of them poll and set priority (in the packets) to be DRs and some to be BDRs.

My exact doubt is, how do the routers locally decide they want to be DRs and BDRs?

Rest all of the selection process is ok.

Thank you.

-/ Kiran Cherian

Richard Burts Thu, 04/17/2008 - 07:50

Kiran

The election is actually for BDR and the DR is always done by promoting the BDR to DR. So a router decides whether it is willing to be elected BDR. In making its decision the router considers its priority (any non-zero priority means the router would be willing to be elected) and whether it already has a role on the segmeent (if the router is DR it is not willing to be elected BDR).

HTH

Rick

k.cherian Thu, 04/17/2008 - 21:19

Hi Rick,

Thank you for the explanation. It's cleared the dust a a little, but i guess i need to try it out and see how it exactly works.

Thank you for the help.Much appreciated.

-/ K.C.

ezsurf0902 Thu, 04/17/2008 - 21:46

Just to ride on the DR/BDR issue, please help me to clear my doubt.

in a IP segment, it is true that it can have as many of DR and BDR?

thanks

mohammedmahmoud Thu, 04/17/2008 - 22:08

Hi,

Each multi-access segment (ex: Ethernet Segment), will have 1 DR and 1 BDR. Each router on the segment forms a Full adjacency with the DR/BDR. Keep in mind that a router might be a DR/BDR on one of its attached multi-access networks, and it might not be the DR/BDR on another of its attached multi-access networks. In other words, the DR/BDR is a property of a router's interface, not the entire router.

BR,

Mohammed Mahmoud.

k.cherian Fri, 04/18/2008 - 19:38

Yes Mohammaed.

The property of being a DR or BDR is specific to the interface and not to the whole device as such. He could be the DR/BDR of a network on one of it's interface, but just another device to another network in another interface.

-/ Kiran Cherian

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