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

hitmen

R1 - lo1 10.1.1.1
R2- lo1 10.1.1.2
R3- lo1 10.1.1.2 (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?

d4nz1g

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.

atorven

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.

hitmen

I tried it and error messages came out if log adjacency was input

SteveO86

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.

AhmadrezaMohseni

OSPF-X-DUP-PTRID-NBR: OSPF detected duplicate router-id x.x.x.x from x.x.x.x on interface fastethernet0/0

Danielh22185

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.