DR/BDR election - priority

[nuggetinu]

Hi,

I am currently reading how the election process takes place, and am confused a bit. This is because, on one slide there is that Router priority is taken first preference for DR/BDR election and on the other there is that the preference is taken for the interface priority.

I am attaching both statements. Can some clarify this please?

Also,

1) If it is Router priority, where is this set?

2) If it is interface priority, which interface priority matters in the election?

[Deepak Kumar]

Hi,

1) If it is Router priority, where is this set?

Ans: You have to configure priority under the interface.

2) If it is interface priority, which interface priority matters in the election?

Ans: OSPF uses a DR (Designated Router) and BDR (Backup Designated Router) on each multi-access network.  Where you want to change the DR and BDR election? I think better to change the priority on the interface which you want to make DR and clear OSPF Process. 

http://packetlife.net/blog/2011/jun/2/ospf-designated-router-election/

https://networklessons.com/ospf/ospf-drbdr-election-explained/

Regards,

Deepak Kumar

[Julio E. Moisa]

Hi

As it was mentioned previously the priority is configured under the interface using the command:

ip ospf priority <0 - 255>.

By default the priority 1 on all the interface on a router, if you set up a priority with 255 you are saying this router will be the DR other router can be the BDR using a priority of 254. So you could have something like:

      ROUTER1  (f0/0) -----------  (f0/0) ROUTER 2

          (fa0/1)                                               (f0/1)

              |                                                        |

              |                                                        |

           (fa0/0)                                              (f0/0)

      ROUTER 3 (fa0/1) ----------- (fa0/1) ROUTER 4

Under router 1 interfaces you can configure: ip ospf priority 255

Under router 2 interfaces you can configure: ip ospf priority 254

Under router 3 and 4 interfaces configure: ip ospf priority 0

So the result: Router 1 will be the DR, Router 2 will be the BDR and Router 3 and 4 will be DROTHER. 

It is a method to select DR/BDR manually. To take effect you must execute: clear ip ospf neighbor command line. 

Hope it is useful

:-)