OSPF Router id

Unanswered Question
Mar 26th, 2007


Iam doing my CCNP BSCI and i had a small confusion on the DR and BDR selection and i want to know how they are selected.

Can u help me in short points How ospf process election is done in NBMA,PPP,Point to multipoint

Thanks in Advance


  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 3 (1 ratings)
achalante Tue, 03/27/2007 - 04:27

It's quite simple. You must understand that there is no DR/BDR election in ppp and ppmp modes. It only occurs in Broadcast and NBMA modes. Apparently, it bases it's selection a couple of things, but the most important is the priority. The router with the highest priority would be elected as the DR, while the router with the higher priority remains the BDR. Should there be a DR failure, the BDR takes the place of a DR, even after the former DR comes back online, the new DR still remains the DR. In a sense, the DR election works in a round-robin fashion, in that you'd need to clear the ospf processes until the original DR finally becomes the DR again. I hope this helps.

vinoth.kumar Tue, 03/27/2007 - 05:20


Thanks to you for replying me, my comment is what are all the factor that involves in DR and BDR election

Like Higher loopback address



Jon Marshall Tue, 03/27/2007 - 05:34

Hi Vinoth

Each interface on a router that is part of a multiaccess network has a router priority in OSPF. If the priority is 0 that router cannot become a DR or BDR for that network segment.

When a router becomes active, once it has discovered it's neighbours it then checks to see if there is a DR and BDR. If there isn;t a BDR an election is held. if there isn't an active DR the BDR gets promoted and another election is held for the BDR.

In the event of a tie in terms of router priority the router with the highest router ID wins.



cpembleton Tue, 03/27/2007 - 05:40

Just to add:

Router ID is the hightes loopback address. If no loopback address is set the the highest active interface is used.

Set a loopback! The highest active interface can change if an interface goes down.



vinoth.kumar Mon, 04/02/2007 - 23:53

Thanks for reply

I had got a router 3845and 3725 and i want to run a BGP protocol on the 3845 router but when i try to enter #router bgp AS number it is showing protocol not matched for this image (my IOS version is 12.3 (11))

I need your help in this to configure my BGP protocol and which version will support for this

Thanx in Advance


vinoth.kumar Tue, 04/17/2007 - 01:32


I want to know that which routing protocol will support for multi brand routers

Thanks in Advance


c.lemaire Tue, 04/17/2007 - 03:23

All standardized protocols should work. OSFP, BGP, RIP, IS-IS

vinoth.kumar Tue, 05/08/2007 - 02:56


I need information that when i configuring the ospf authentication as example

#area 2 authentication

#interface serial 0

#ip ospf message digest-key ? when i use IOS help

it is showing 0 7

what it means of range choosing the option

And the second information is while enabling the debug to see the OSPF adjcency after configuring the OSPF authentication (md5)it is showing as

#debug ip ospf adj

it is showing some line as

send me the youngest key- what it means

can u help me in getting the both information

Thanks in advance


mohammedmahmoud Tue, 05/08/2007 - 03:11


0 for not yet encrypted, 7 for proprietary encrypted (using the weak reversible algorithm - needs service password-encryption enabled).

And the reason for this error is most probably that you didn't add the " ip ospf message-digest-key" command under the interface properly.

HTH, please rate all helpful posts,

Mohammed Mahmoud.

vinoth.kumar Tue, 05/08/2007 - 03:17


Thanks fo your prompt reply

from that 0 - for not yet encrypted (what it defines )if enabled with 0 what will be the result

7 - what will be the result

please do the needfull



mohammedmahmoud Tue, 05/08/2007 - 03:27


If you are going to add the OSPF password (key) in plain text use 0, and if you are going to enter it in the encrypted form use 7.

HTH, please do rate helpful posts,

Mohammed Mahmoud.


This Discussion