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DR

if we have already dr and BDR elected and for design perpuse we want to make another router in that same BMA as a DR.....what is the procedure to that??? cheers and thanks in advance.....

3 REPLIES
Cisco Employee

Re: DR

Hi Guroo,

There is no preempt in case of DR & BDR in ospf so any DR once elected will remain as DR till the time it goes down and once it goes down BDR will be promoted as DR.

Regards,

Ankur

New Member

Re: DR

Hi,

You must take into consideration the following about DR and BDR in OSPF:

- The router with the highest OSPF priority P will be elected as the DR and the one with P-1 will be the BDR

- If the 2 routers has 2 equal priorities, OSPF choose the router with the highest router-ID.

- On each router, OSPF will choose the highest ip address from loopback interfaces as the router id, only if you assign it explicitly by command.

- The default priority is 1, and you can exclude a router from the election process if you assign it a priority =0.

So you can make as many potential DR/BDR as you like by controlling priority/router-id but OSPF will take into consideration only DR and BDR

! Setup ospf priority

Router(config)#inteface X/X

Router(config-if)#ip ospf priority

! Setup ospf router-id

Router(config)#router ospf

Router(config-router)#router-id

Hall of Fame Super Silver

Re: DR

As others have observed OSPF treats the introduction of a new router with higher priority as non-preemptive and the new router will not displace the existing DR or BDR without some manual intervention.

It may help to have an example to discuss. If we assume that routerA has priority = 1 and routerB has priority = 2 then routerB will be the DR and routerA will be the BDR. Now assume that routerC has priority = 3 and is booted up. Since the election for DR and BDR has already taken place routerC will not automatically become the DR. But if you want it to become DR you can manually intervene and cause it to happen. If you configure routerA to have priority = 0 then it stops being BDR and routerC will become BDR. You can then configure routerA to have priority = 1 to return to its original priority and routerC remains as BDR. Then you can configure routerB to have priority = 0. It will cease being DR and routerC will become DR. You can then configure routerB to have priority = 2 to return to its original priority.

So the routers will not change DR automatically when a new router joins the network with a higher priority. But you can manually intervene and cause it to happen.

HTH

Rick

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