What happens in the ospf process if I have 2 routers with same loopback address

hitmenhitmen Banned Posts: 133
R1 - lo1
R2- lo1
R3- lo1 (deliberately configured wrongly)

In that case who will be BR, BDR and BROther.

What difference does it make if the loopback routes are advertised by ospf and if they are not?


  • d4nz1gd4nz1g Member Posts: 464
    OSPF router will not form a neighbor relationship with another router with the same Router-id.
    In your case, R2 and R3 will never become neighbors. R1 will for a neighborship with both routers, however, R1 will only learn topology information from only one of them.
    Loopback addressess only need to be advertised if you need reachability for that specific address (for example, when you use redundant links on BGP)
    For ospf, loopback addressess are used only when determining router-id.
  • atorvenatorven Member Posts: 319
    Ignoring Router IDs in the election process, R1 will be the DR and then it would be between R2 and R3 depending on which came online first (sent the first hello) as to who would be the BDR, R3 would then be the DROther. With regards to your 2nd question, R1 would load balance the routes which would result in suboptimal routing because it sees 2 routes to get to the same network but they are actually for different destinations - someone correct me if I'm wrong.
  • hitmenhitmen Banned Posts: 133
    I tried it and error messages came out if log adjacency was input
  • SteveO86SteveO86 Member Posts: 1,423
    You will receive an error message along the lines of "flood war".

    To get the most of this question, I'd lab it up in GNS3 and see what happens first hand.
    My Networking blog
    Latest blog post: Let's review EIGRP Named Mode
    Currently Studying: CCNP: Wireless - IUWMS
  • AhmadrezaMohseniAhmadrezaMohseni Registered Users Posts: 3 ■□□□□□□□□□
    OSPF-X-DUP-PTRID-NBR: OSPF detected duplicate router-id x.x.x.x from x.x.x.x on interface fastethernet0/0
  • Danielh22185Danielh22185 Member Posts: 1,195 ■■■■□□□□□□
    Yes you will get a Syslog message indicating a duplicate IP detected. This can be a VERY bad thing in production network. I've seen where a duplicate IP was introduced into a live production network and immediately creates and outage because the DR loses routing functions.
    Currently Studying: IE Stuff...kinda...for now...
    My ultimate career goal: To climb to the top of the computer network industry food chain.
    "Winning means you're willing to go longer, work harder, and give more than anyone else." - Vince Lombardi
Sign In or Register to comment.