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855 Views 5 Replies Latest reply: Apr 3, 2012 11:55 AM by mjones - CCNP RSS

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DR  or BDR which is elected  first

Apr 2, 2012 2:08 PM

mahesh18 105 posts since
Mar 10, 2009

Hi all,

 

In OSPf  Lan environment when election takes to elect DR  or  BDR  which is elected first DR  or BDR  ??

 

As per my understanding all updates first come to DR  then DR  talks to BDR  then other Non DR  or BDR  members are updated right?

 

Thanks

 

MAhesh

  • mjones - CCNP 196 posts since
    Aug 1, 2010
    Currently Being Moderated
    1. Apr 2, 2012 2:22 PM (in response to mahesh18)
    Re: DR or BDR which is elected first

    On a multiaccess network such as a LAN OSPF routers send updates to the multicast address 224.0.0.6 which is the "All OSPF designated routers" address, this will be the DR and BDR, as they both listen for packets sent to this multicast address.

     

    The DR in turn will send out updates to the 224.0.0.5 multicast address which is the "All OSPF router" multicast address, so all non DR/BDR routers get the updates. These routers are the DROthers.

     

    So in actual fact the DR doesnt talk to the BDR as you suggest, the DR and BDR both listen for, and process the updates to 224.0.0.6 simultaneously. The BDR just listens and processes the updates in its own topology tables, whereas the DR will process the updates and send the updates on to the DROthers using 224.0.0.5.

     

    The first two routers in a multiaccess network to come up and form an adjacency will elect the DR/BDR between themselves, with the highest router-id winning (bar IP OSPF priority set on the interface). Each other router that comes up and joins, forming its adjacenies, will notice the DR/BDR and become a DROther, even if it has a higher router-id.

     

    I think that's right - I'll wait and see if anyone corrects me, and anyone is more than welcome to do so!

     

    Hope that helps,

    MJ

  • Keith Barker - CCIE RS/Security, CISSP 5,351 posts since
    Jul 3, 2009
    Currently Being Moderated
    2. Apr 3, 2012 9:38 AM (in response to mahesh18)
    Re: DR or BDR which is elected first

    I like what Mike MJ had to offer.

     

    I labbed this up, with a single router with 2 interfaces, (each one in a different VRF), and had him talk to himself. 

     

    With a debug running, here is what is showed:


    OSPF: Neighbor change Event on interface GigabitEthernet2/0

    OSPF: DR/BDR election on GigabitEthernet2/0

    OSPF: Elect BDR 1.1.1.1

    OSPF: Elect DR 10.0.0.2

           DR: 10.0.0.2 (Id)   BDR: 1.1.1.1 (Id)

    OSPF: Neighbor change Event on interface GigabitEthernet2/0

    OSPF: DR/BDR election on GigabitEthernet2/0

    OSPF: Elect BDR 1.1.1.1

    OSPF: Elect DR 10.0.0.2

           DR: 10.0.0.2 (Id)   BDR: 1.1.1.1 (Id)

    OSPF: End of hello processing

    R1_with_VRFs#

    OSPF: Send hello to 224.0.0.5 area 0 on GigabitEthernet2/0 from 10.0.0.2

    OSPF: Rcv hello from 10.0.0.2 area 0 from GigabitEthernet1/0 10.0.0.2

    OSPF: Neighbor change Event on interface GigabitEthernet1/0

    OSPF: DR/BDR election on GigabitEthernet1/0

    OSPF: Elect BDR 1.1.1.1

    OSPF: Elect DR 10.0.0.2

    OSPF: Elect BDR 1.1.1.1

    OSPF: Elect DR 10.0.0.2

           DR: 10.0.0.2 (Id)   BDR: 1.1.1.1 (Id)

    OSPF: Neighbor change Event on interface GigabitEthernet1/0

    OSPF: DR/BDR election on GigabitEthernet1/0

    OSPF: Elect BDR 1.1.1.1

    OSPF: Elect DR 10.0.0.2

           DR: 10.0.0.2 (Id)   BDR: 1.1.1.1 (Id)

     

    It seems like, based on the debug, the router set up the DR and BDR at the same time, but the debug lists the BDR as being first, and DR as second (within milliseconds).

     

    Cheers,

     

    Keith

     

    Message was edited by: Keith Barker - CCIE RS/Security, CISSP, CCSI

  • mjones - CCNP 196 posts since
    Aug 1, 2010

    Kieth wrote "I like what Mike had to offer."

     

    Not sure if you are referring to me there Keith?  I'm not Mike , I'm Martyn, but since there is another well know Martin here I thought I'd go with MJ.

  • Keith Barker - CCIE RS/Security, CISSP 5,351 posts since
    Jul 3, 2009
    Currently Being Moderated
    4. Apr 3, 2012 9:39 AM (in response to mjones - CCNP)
    Re: DR or BDR which is elected first

    Hi MJ-

     

    I was referring to you.    Sorry about the name mistake.

     

    Thanks for the clarification.

     

    Cheers,

     

    Keith.

  • mjones - CCNP 196 posts since
    Aug 1, 2010

    No worries Keith, I'm just really glad I got the DR / BDR description right more than anything!

     

    Cheers

    MJ

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