OSPF DR election question

steele84steele84 Member Posts: 62 ■■□□□□□□□□
in CCNA & CCENT
Hey everyone, I was just doing a couple questions on troubleshooting OSPF and I came across a concept that I thought I knew the correct answer but turns out I was wrong so I wanted to post it so you guys can get me righted.

So the question scenario is: No loopbacks or specific router IDs have been configured, and all routers have been brought up at the same time. In a single area I have 3 routers R1, R2 and R3. The configured interface address are (crap I think I just figured it out, but i'll explain the logic just to make sure) R1 Fa0/0 10.11.0.1, R2 Fa0/0 10.11.0.2,R2 S0/0 10.22.0.2, R3 Fa0/0 10.11.0.3.

When I first looked at this I thought "Ok no loopbacks, priority, or RIDs have been manually configured so the DR would be granted to the router with the highest configured interface." (is this the right thinking?) At first glance I was like R3 with the ip 10.11.0.3 is the highest. But what I think I did was I didn't take into account the S0/0 on router 2 which is connected to a different subnet. The subnet being 22 would give it the higher IP address configured as 10.22.0.2 (correct?).

Am I on the right track or am I not quite correct on understanding the DR election process? Thanks!

“What lies behind us and what lies before us are tiny matters compared to what lies within us.”

― Ralph Waldo Emerson
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  • mikeybinecmikeybinec Member Posts: 484 ■■■□□□□□□□
    No DRs or BDRs on wan links. From top to bottom the rules are: router id, highest loopback, highest ip address. Use the command show ip ospf neighbor to determine your DR on a broadcast link
    Cisco NetAcad Cuyamaca College
    A.S. LAN Management 2010 Grossmont College
    B.S. I.T. Management 2013 National University
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  • steele84steele84 Member Posts: 62 ■■□□□□□□□□
    mikeybinec wrote: »
    No DRs or BDRs on wan links. From top to bottom the rules are: router id, highest loopback, highest ip address. Use the command show ip ospf neighbor to determine your DR on a broadcast link

    Right, but will ospf look at a wan links IP address as a configured interface value when considering DR elections?
    “What lies behind us and what lies before us are tiny matters compared to what lies within us.”

    ― Ralph Waldo Emerson
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  • networker050184networker050184 Mod Posts: 11,962 Mod
    mikeybinec wrote: »
    No DRs or BDRs on wan links.

    It's not that straight forward. The router doesn't care if you consider something a WAN or not. Your WAN link could certainly be Ethernet.

    steele84 wrote: »
    Right, but will ospf look at a wan links IP address as a configured interface value when considering DR elections?

    It could come into play if it was used as the router ID, but it doesn't just look at other interface IPs in the process. Remember, the DR is a per segment deal.
    An expert is a man who has made all the mistakes which can be made.
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  • steele84steele84 Member Posts: 62 ■■□□□□□□□□
    So in my example two interfaces on R2 fa0/0 10.11.0.2 and S0/0 10.22.0.2 with know no loopback or manual RID set, would the RID of R2 be 10.22.0.2 ? If so that should make R2 my DR for the 10.11.0.0/24 subnet correct ?
    “What lies behind us and what lies before us are tiny matters compared to what lies within us.”

    ― Ralph Waldo Emerson
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  • networker050184networker050184 Mod Posts: 11,962 Mod
    Yeah sounds about right. Assuming everything boots simultaneously and that WAN IP was there when OSPF selected it's RID.
    An expert is a man who has made all the mistakes which can be made.
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  • steele84steele84 Member Posts: 62 ■■□□□□□□□□
    Cool, thanks for the clarification!
    “What lies behind us and what lies before us are tiny matters compared to what lies within us.”

    ― Ralph Waldo Emerson
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  • volfkhatvolfkhat Member Posts: 1,072 ■■■■■■■■□□
    steele84 wrote: »
    Right, but will ospf look at a wan links IP address as a configured interface value when considering DR elections?

    According to the self-study that i use.... YES, the 'WAN interface' definitely impacts the election.
    https://www.youtube.com/watch?v=L_OxFxk6fJM
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  • james43026james43026 Member Posts: 303 ■■□□□□□□□□
    steele84 wrote: »
    Hey everyone, I was just doing a couple questions on troubleshooting OSPF and I came across a concept that I thought I knew the correct answer but turns out I was wrong so I wanted to post it so you guys can get me righted.

    So the question scenario is: No loopbacks or specific router IDs have been configured, and all routers have been brought up at the same time. In a single area I have 3 routers R1, R2 and R3. The configured interface address are (crap I think I just figured it out, but i'll explain the logic just to make sure) R1 Fa0/0 10.11.0.1, R2 Fa0/0 10.11.0.2,R2 S0/0 10.22.0.2, R3 Fa0/0 10.11.0.3.

    When I first looked at this I thought "Ok no loopbacks, priority, or RIDs have been manually configured so the DR would be granted to the router with the highest configured interface." (is this the right thinking?) At first glance I was like R3 with the ip 10.11.0.3 is the highest. But what I think I did was I didn't take into account the S0/0 on router 2 which is connected to a different subnet. The subnet being 22 would give it the higher IP address configured as 10.22.0.2 (correct?).

    Am I on the right track or am I not quite correct on understanding the DR election process? Thanks!

    Yes R2 would be the DR for the segment because of the S0/0 interface, the IP address of this interface would be the RID for R2.

    The interface OSPF priority value is the first thing that determines the DR and BDR, a priority can range from 0 - 255 with 0 saying to not even participate in the DR / BDR election, and 1 being the default, then comes the configured RID, then comes highest loopback address, then comes highest interface IP address on the router. This is just to clear up any confusion that may have been presented by others.
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  • mikeybinecmikeybinec Member Posts: 484 ■■■□□□□□□□
    CONFUSING~!!! For a minute there I thought the Mod was saying that 'Wan links can be DRs and/or BDRs. My comment was in a straight
    serial link network (P to P) there are NO DRs/BDRs... That red herring about an etherlink being a wan link I agree with but when in straight theory mode, P to Ps do not have DRs or BDRs

    Uggh, dont screw me up, I have my ICND2 test in four days (Thanks to Volf for that great youtube packet tracer)
    Cisco NetAcad Cuyamaca College
    A.S. LAN Management 2010 Grossmont College
    B.S. I.T. Management 2013 National University
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