OSPF DR BDR Election

Posted by September 20, 2011 in OSPF

Scenario:

You end up working as one of the network engineers for a nuclear powerplant located in a cartoony looking town. OSPF is being used as the IGP routing protocol but there have been some problems with the network. It is uncertain which router is being chosen as the designated or backup designated router. Up to you to fix these issues!

Goal:

  • All IP addresses have been preconfigured for you as specified in the topology picture.
  • Configure OSPF on all routers, achieve full connectivity.
  • Ensure router Marge is the DR for network 192.168.1.0 /24.
  • Ensure router Bart is the BDR for network 192.168.1.0 /24.
  • Ensure router Home is the DR for network 192.168.2.0 /24. You are not allowed to change the priority.
  • Ensure router Maggie is the BDR for network 192.168.2.0 /24. You are not allowed to change the priority.

It took me 1000s of hours reading books and doing labs, making mistakes over and over again until I mastered all the protocols for CCNP.

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You will learn all the secrets about OSPF, DR and BDR election, priority and more.

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IOS:

c3640-jk9s-mz.124-16.bin

Topology:

OSPF DR BDR Election Network Topology

Video Solution:

Configuration Files

You need to register to download the GNS3 topology file. (Registration is free!)

Once you are logged in you will find the configuration files right here.

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About the Author: Rene Molenaar

René - CCIE #41726 is the creator of GNS3Vault.com where he shares CCNA, CCNP and CCIE R&S labs. He also blogs about networking on http://networklessons.com

13 Comments

    1. Yes that’s correct. The router with the highest priority will become the DR and if it’s a tie OSPF will compare the router ID.

  2. Hi Rene,

    I tried putting a loopback on one of the routers thinking that it would change the router ID for a particular router and therefore be promoted to the DR. It didnt work and Im wondering why? In addition, when I look at the show ip ospf interface it still doesnt show the router ID as the loopback I put on, it still shows it as the 192.x.x.x thats on the interface. I did do a clear ospf process on all the routers in the segment too. FYI, Im looking at page 93 in your book. Any thoughts-What am I missing.

    I used the router -id command but curious why the loopback thing isnt working?

    1. Hi Joe,

      Clearing the OSPF process should change the router ID. Once the DR has been elected it will stay the DR unless you reset OSPF on the DR.

      Did you reset the DR itself?

      Rene

  4. Hello. I have a few questions:
    1- priority is related to the interface. If one router has two interfaces with different priorities in one OSPF area, how are the DR/BDR elected ?
    Let’s say we have R1 with 2 interface (prio 10& 20), R2 with 1 interface (prio 15) and R3 with 1 interface (16). We suppose that all 3 get rebooted at the same time. The DR will become R1 because he has an interface with OSPF priority 20 ?
    What happens if we have the above case but only interface 1 from R1 is in OSPF and has priority of 10 ? Who is elected DR ?
    More: let’s assume that all R1,R2 and R3 have equal priorities and no loopback interfaces defined. The RID is the highest IP address of any interface (active) from the router participating in the OSPF or just any interface at all.

    thank you in advance for your answers.

  5. Thanks for giving me chance to be a part of your community. May ALLAH bless you with more knowledge!

  6. Rene… The DR/BR election varies depending on which router is OSPF-enabled first?

    1. Yes, it depends.
      If you have a segment with two routers and you enable OSPF in only one, it will become the DR automatically. When second router comes up, even if it has higher priority or higher RID, it will not become DR because there is no preemption.
      Anyway, if you clear ospf processes, second router will become DR.

  7. Hi,
    startup config and final configs are same, please kindly correct them

    thank you

  8. Hello,

    I have done some tests and I do think that the oldest router on the area will be elected as DR, then the second oldest router will be elected BDR.

    I think that values like Priority, Router ID, highest Loopback address and highest IP address are just theory, this is what I discover by doing this lab with out configuring the values I mentioned before.

  9. Miguel, you have to reset the OSPF process in order for the priority and router ID changes to take place. From privileged exec mode, type ‘clear ip ospf process’ and then ‘yes’

Comments are closed.