Found out an interesting thing while doing a Vol3 lab. We all know that EIGRP will not form an adjacency over a non common subnet unless special circumstances. What about OSPF? I had a lab task where I need to form an adjacency over an ip unnumbered link. To my surprise this works just fine. If you debug you will get a message like this:

*Mar 1 00:15:14.155: OSPF: Rcv pkt from 155.1.15.2, FastEthernet0/0, area 0.0.0.0 : src not on the same network

However the database is correct and routes are installed. If you want to try this yourself use two routers with a serial link between them and then do ip unnumbered from an Ethernet interface and enable OSPF as usual. Watch what happens. Does anyone have a good description of this behavior? I tried looking in the RFC but all I could find is this:

“All routers connected to a common network must agree on certain
parameters (Network mask, HelloInterval and RouterDeadInterval).
These parameters are included in Hello packets, so that differences
can inhibit the forming of neighbor relationships.”

“An adjacency is bound to the network that the two routers have
in common. If two routers have multiple networks in common,
they may have multiple adjacencies between them.”

OSPF adjacency over non common subnet
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7 thoughts on “OSPF adjacency over non common subnet

  • August 4, 2011 at 9:13 pm
    Permalink

    There’s certain misunderstandings about the common subnet issue, which I found out myself. I have a blog post about EIGRP, but the same seems to be true for OSPF.. unless a DR/BDR election takes..

    OSPF on point-to-point, point-to-multipoint networks, subnet mask does not matter, it’s only network boundaries that matter.. on Broadcast, non-broadcast networks (DR/BDR) are required, subnet mask matters

    I don’t want to spam with links to my blog on your own, but let me know if you want the link.. 🙂

    Reply
    • August 4, 2011 at 9:24 pm
      Permalink

      Hi Yandy,

      Glad you found your way in here. It’s perfectly fine to leave a link here. I read your last post on OSPF, very interesting. Please hit me with link.

      Reply
  • August 5, 2011 at 7:26 pm
    Permalink

    @Daniel

    Yea, it’s a bit over a month away, working hard on some things here and there.. balancing work, and trying to get ready. Changing approach a bit also, concentrating more on Vol1 type labs.. just tech focused and not so much full scale lab focused .. lets see how that works out.

    Reply
  • February 15, 2012 at 1:48 pm
    Permalink

    Hello man, did you found a solution for that?

    I’m with the same issue… but I can see everything works fine… in my design i don’t have direct connection to BDR from one router in area 0.. and then i receive this message.

    Is it fine?

    Thanks

    Reply
    • February 15, 2012 at 2:16 pm
      Permalink

      If you have connectivity I wouldn’t worry about it. Are you running an unnumbered link?

      Reply

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