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TOPIC: Can i have a question on OSPF DR and DBR election

Can i have a question on OSPF DR and DBR election 11 years 9 months ago #17108

  • ccnx
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Hi

have been very puzzling with the DR and BDR election in a OSPF area network. as predetermined, the first condition of being a DR is the parameter "priority". however, if all the routers were having the same priority number such as 1, the router with the higher router ID would be the DR. also, the router ID is based on the highest numerical active ip address. Therefore, based on the all those understanding, here comes a example....

router A f0: 10.4.0.2 /16

f0:10.4.0.1/16 Router B f0:10.5.0.1 /16

Router C f1: 10.5.0.2 /16

(Assume that all the routers are having same priority)

as shown above, two network are known, they are the 10.4.0.0 and 10.5.0.0 /16. the election of DR and DBR would be as following:

network 10.4.0.0: router A is the DR(its address is numerically higher than the other), Router B is the BDR

network 10.5.0.0: Router C is the DR(same as above), Router B is the BDR

However, as the answer given, my thought was proved to be wrong. the answer is router A is the BDR, router B is DR, Router C is DR.

Finally, my question is: any 1 can tell me where i m wrong???

thnx so much[/img]

Re: Can i have a question on OSPF DR and DBR election 11 years 9 months ago #17184

election of the dr and bdr is only for the subnetwork that the link is on. Therefore, for the subnetwork that links both router A and Router B, router B would be the DR because the highest IP address on the router is used as the router ID and therefore for the link between A and B, B has the highest router ID. Now, for the link between router B and C, C has a higher iP addressed, in turn, that became the router ID and so for that link C is the DR.

Hi

have been very puzzling with the DR and BDR election in a OSPF area network. as predetermined, the first condition of being a DR is the parameter "priority". however, if all the routers were having the same priority number such as 1, the router with the higher router ID would be the DR. also, the router ID is based on the highest numerical active ip address. Therefore, based on the all those understanding, here comes a example....

router A f0: 10.4.0.2 /16

f0:10.4.0.1/16 Router B f0:10.5.0.1 /16

Router C f1: 10.5.0.2 /16

(Assume that all the routers are having same priority)

as shown above, two network are known, they are the 10.4.0.0 and 10.5.0.0 /16. the election of DR and DBR would be as following:

network 10.4.0.0: router A is the DR(its address is numerically higher than the other), Router B is the BDR

network 10.5.0.0: Router C is the DR(same as above), Router B is the BDR

However, as the answer given, my thought was proved to be wrong. the answer is router A is the BDR, router B is DR, Router C is DR.

Finally, my question is: any 1 can tell me where i m wrong???

thnx so much[/img]

Re: Can i have a question on OSPF DR and DBR election 11 years 9 months ago #17189

Highest active interface IP = Router ID, no matter which network it connects to.

That's why your Router B is elected DR over Router A.

Re: Can i have a question on OSPF DR and DBR election 11 years 9 months ago #17197

hello!
-i have question regarding ospf priority-
My understanding about ospf priority is that the lowest priority number will have the highest priority value such that if I set the priority number of Router A to 1 and Router B to 5, then Router A will have the highest priority.
Am i correct?

Re: Can i have a question on OSPF DR and DBR election 11 years 9 months ago #17200

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Yes, Correct, while select if any of the router having the lowest priority then that will be elected as DR. If more than one router priority is same then the IP address will be considered.

Re: Can i have a question on OSPF DR and DBR election 11 years 9 months ago #17215

Ardeef,


OSPF priority ranges from 0 to 255, where

0 = Router will never be elected DR
1= Default priority

The HIGHER the priority, the higher the elecitive value.

Your Router B will be DR.

http://www.cisco.com/warp/public/104/17.html#priority
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