OSPF DR/BDR election

Hi friends ,I have a topology from INE's video.Please help me to understand what is going on in that toplogy.When I use sh ip ospf neighbor command in R2 ,I see following output

 

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R2#sh ip ospf neighbor


Neighbor ID     Pri   State           Dead Time   Address         Interface

4.4.4.4           1   FULL/DR         00:00:31    172.16.30.4     FastEthernet0/0

3.3.3.3           1   FULL/DROTHER    00:00:33    192.168.1.3     FastEthernet0/1

5.5.5.5           1   FULL/DR         00:00:36    192.168.1.5     FastEthernet0/1

 

R2#sh ip ospf neighbor


Neighbor ID     Pri   State           Dead Time   Address         Interface

4.4.4.4           1   FULL/DR         00:00:31    172.16.30.4     FastEthernet0/0

3.3.3.3           1   FULL/DROTHER    00:00:33    192.168.1.3     FastEthernet0/1

 

5.5.5.5           1   FULL/DR         00:00:36    192.168.1.5     FastEthernet0/1image



As you see that there is 2 DRs .


And followings are like that .



R3#sh ip ospf neighbor



Neighbor ID     Pri   State           Dead Time   Address         Interface

4.4.4.4           1   FULL/BDR        00:00:35    10.1.1.4        FastEthernet0/0

2.2.2.2           1   FULL/BDR        00:00:37    192.168.1.2     FastEthernet0/1

5.5.5.5           1   FULL/DR         00:00:30    192.168.1.5     FastEthernet0/1




2 BDRs are in same output.Can anyone make clear ?

Comments

  • DRs and BDRs are elected per segment.  So you have three segments in your diagram:

    The link between R2 and R4 (R4 is the DR presumably R2 is the BDR)

    The link between R3 and R4 (R4 is the DR presumably R3 is the BDR)

    The link (switched) between R2,R3 and R5 (R5 is the DR, R2 is the BDR and R3 is DROTHER)

    So yes, you have two DRs, but only one per broadcast segment.

  • peetypeety ✭✭✭

    Even where there are only 2 routers on ethernet, there will be election. short but still one.  of course in case of only 2 routers on segment, DR/ BDR does not matter. you can change that by changing network type to p2p on ethernet. someone mentioned that is recommanded action by cisco.

    There's an election as soon as the first router is up and the dead interval has passed (it needs to confirm that nobody else is hanging out).

    When the second router comes up, another election happens after the dead interval has passed on the second router, as there needs to be agreement on which router is the BDR (we know which one it'll be, but the routers don't).

    My underlying point here is that the election itself is quick: who's got highest priority? if tied, who's got the winning router ID?  It's the "we have to wait for the dead interval" that makes the election seem long.

  • so you saying benefit of not having elections is 40 sec saved on those p2p ethernet links, right?

     

    cleans up the ospf database too , no type 2 LSAs

     

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