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  1. #1
    Join Date
    Oct 2008

    Default question about DR/BDR

    Page No. 461.

    R3# sh ip ospf nei Full/BDR (shouldn't it be Full/DR, since the neighbor 871w has highest RID)

    871W#sh ip ospf nei Full/DR (shouldn't it be Full/BDR, since the neighbor R3 has low RID)

    i have the same question.
    why nobody answer this question,i hope someone give me a hlpe.

  2. #2
    Join Date
    Dec 2006

    Default RID's are tricky

    There are two possible answers and the book is not wrong. The RID for the 871w is going to be the highest IP address that is active on the router. Which is the network, and that is higher than any other address that the R3 router has. As long as all the interfaces were up when the RID was chosen then that is what would happen.

    Even if the 871 didn't have the 12 subnet (which is higher), the two routers were comparing their 11 subnet addresses to each other. Sure the R3 should win because it has the .2 host, but I must have brought the 871 router up first and let it go through the entire election before I brought the R3 router online. The 871 would still stay the DR and R3 would have to just become the BDR. This is because OSPF does not preempt in the DR election.

    However, this brings up a good point. Don't let OSPF have a free election. Fix the election with the priority command and with loopback interfaces as discussed in the chapter.

    Todd Lammle
    Last edited by lammle; 10-18-2008 at 06:16 PM.

  3. #3
    Join Date
    Mar 2008


    Thanks Todd.

  4. #4
    Join Date
    Apr 2010
    Sunnyvale, CA


    I didn't want to start a new topic. So I ask a simple question here:
    Router1 with Ethernet interface is connected to Router2.
    Router2 with Ethernet interface is connected to Router3.
    So we have 2 networks here.
    The question is, do we have one DR and one BDR for EACH network or we have one DR and one BDR for both Ethernet networks?

  5. #5
    Join Date
    Oct 2009


    Put them in the same Area and you'll get one of each. Otherwise, each area will elect their own.
    CompTIA: Network+, A+
    Cisco: CCENT
    Cisco: CCNA

  6. #6
    Join Date
    Nov 2008
    Worcester, UK


    It's not quite that clear. It is possible for a single router to be the DR for all networks in an area, but the truth is each network segment will elect it's own DR and BDR. Give router3 the highest RID and there will still be an election for DR between routers 1 & 2, regardless of which area the segment is in.
    Currently studying: CCDP, CCIE R&S
    Follow my CCIE progress with study notes on my blog:

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