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Cisco Employee

DR/BDR election in a multiaccess broadcast segment

In OSPF , who will be elected first DR or BDR ?what will router upon initialization will claim to - Dr or BDR ?

Thanks for your time

13 REPLIES

Re: DR/BDR election in a multiaccess broadcast segment

BDR is elected first among all eligible routers who didn't claim themselves DR, then DR is elected. Have a look at section 9.4 of RFC2329 for exact algorithm.

Re: DR/BDR election in a multiaccess broadcast segment

It is done via hello packets sent thru multicast packets.

You can get to know more about DR/BDR Election at :

http://www.cisco.com/warp/public/104/2.html#10.1

Hall of Fame Super Blue

Re: DR/BDR election in a multiaccess broadcast segment

Hi

Each interface on a multi-access network of a router has a router priority between 0 & 255.

When an OSPF router initialises and discovers it's neighbours it checks for a DR & BDR. If they are already there then they are accepted.

If there isn't a BDR an election is held. Router with highest priority for the interface on the network segment becomes the BDR. If interface priorities are the same router with highest router id wins.

If there is no DR then BDR is promoted and an election for another BDR is done.

HTH

Jon

Cisco Employee

Re: DR/BDR election in a multiaccess broadcast segment

thanks for your replies..

Please verify I am right in describing election process.

1. Routers will advertise themselves to be elected as BDR by placing their RID in the BDR field of hello packet.

2.then DR election takes place

**Also when only one router was enabled with OSPF process on the ethernet segment, It become a DR .

In this step has the router first elected as BDR and since no DR is present on that segment ,is that router promoted as DR?

Hall of Fame Super Silver

Re: DR/BDR election in a multiaccess broadcast segment

Padmanabhan

Yes you are correct that if there is one router with OSPF enabled that this router will first become the BDR and then will become the DR.

HTH

Rick

Re: DR/BDR election in a multiaccess broadcast segment

Normally all eligible routers will try to claim being DR. There are situations when due to partially broken connectivity some routers may lose DR and start election process, while other routers keep connection to DR and therefore will place themselves only to BDR field. But generally all routers want to be DR.

On thing to keep in mind is that election process performed independently by each router and in broken or misconfigured network (esp. on Frame-Relay or in private VLAN environments) you may see different DR elected on different routers.

Cisco Employee

Re: DR/BDR election in a multiaccess broadcast segment

Hi

The above confuses me.If the routers claim to be DR, and BDR election is taken first,

then how will be BDR is elected first ?

The statement 2 in RFC 2328 on DR election process and mentions BDR election first.

"Only those routers on the list that have not

declared themselves to be Designated Router are eligible to become Backup Designated Router."

Please clarify what the routers claim to themselves in hello packets?

Re: DR/BDR election in a multiaccess broadcast segment

Router first claims to be DR. The key part here is step (4) of the algorithm - if no router initially wanted to be BDR, then DR will be elected first (in step 3) and then step (2) will be executed again.

Cisco Employee

Re: DR/BDR election in a multiaccess broadcast segment

Hi Ilya

Keeping it simple .. Routers will claim themselves as DR when OSPF is initialized and DR will be elected first...

since all routers claim to be DR ..step 4 i.e DR election held first and BDR next.

So,is this way the DR/BDR elected?

Thanks

Hall of Fame Super Silver

Re: DR/BDR election in a multiaccess broadcast segment

Padmanabhan

Of the responses in this thread 4 have stated that BDR is first and no more than 1 (somewhat ambiguously) have suggested the DR is first. So why do you conclude that DR is first?

HTH

Rick

Re: DR/BDR election in a multiaccess broadcast segment

Padmanabhan,

As Rick and other stated BDR is first elected before DR. If you have access to a lab router then you can enable 'debug ip ospf adjacency' to see the sequence of DR/BDR election.

Here's a link that exactly shows the sequence. As you can see in the debug BDR is elected first.

http://www.cisco.com/en/US/tech/tk365/technologies_configuration_example09186a0080094058.shtml

Moreover, when DR becomes unavailable the BDR is automatically promoted to be the DR and only BDR election happens.

HTH

Sundar

Re: DR/BDR election in a multiaccess broadcast segment

Although it may look in many cases that DR is elected first, it isn't always true. There are situations where a router knows that it's not going to be DR, so general algorithm says that whenever there are some routers willing to be BDR, then first elect BDR among them and only after that proceed to electing DR. The fact that first BDR might be null-operation doesn't mean that it's not there.

Cisco Employee

Re: DR/BDR election in a multiaccess broadcast segment

Thanks to all

When I had discussion about this,we are not sure what router claim themselves as when OSPF is initialized on a broadcast multicast segment.

from the replies,the routers will claim themselves as BDR and so BDR will be elected first.

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