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DR/BDR , Higher or Lower IP address Decision

I have been asked with the attachment scenario,,,,Why will R1 be the DR for the 10.1.2.0/24 network,,,,,and R2 be the DR for the 10.1.1.0/24 network,,,,,and R4 be the DR for the 10.1.3.0/24 network ? because as we know that the highest IP address (there is no loopback here,,,no priority) decide how is DR/BDR.

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Re: DR/BDR , Higher or Lower IP address Decision

Zillah,

Note that DR election process will happen per lan segment. So the same router could be DR on one segment while be a BDR or a regular router (neither DR or BDR) on another segment. So in your case,

a. you have not configured any priority, so default priority is 1.

b. There is no loopback, so the router ID used by OSPF could be the highest ip address of a physically up/up interface, which is

a. for R1 it is 10.1.3.2

b. for R2 it is 172.1.1.2

c. for R4 it is 192.1.2.1

d. for R3 it is 10.1.2.2

So when election happens between R1 and R2, R2 becomes DR. B/n R1 and R3, R3 becomes DR and b/n R1 and R4, R4 becomes DR.

Hope that answers your question.

Also, do a sh ip ospf neighbor and it should each neighbor's router id. In addition to all the above read this link for further clarification.

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

Sankar

PS: please remember to rate helpful posts!

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