OSPF: IP MTU on neighbor relationships

aocferreira

Hello,

Question: Does the IP mtu mismatch on interfaces actually prevent neighbor relationships? Or influence them?

According to the book I'm reading (Cisco Press CCNP Route by Wendell Odom), when we have IP mtu mismatch, the neighbor relationship will be established but the routers will not be able to exchange topology information. Please take a look on attached picture.
Anyway, I have designed this topology in GNS3, and after changing IP MTU of Fa0/0 on R4 to something different than 1500 bytes, all the other routers keep their neighbor relationship with R4 (this is OK & according to what they say) but they keep their routes to 10.4.4.0.

Is this supposed to happen or not? According to the book it's not... But I have my doubts because I already found some errors like this.

Thanls!

aocferreira

Hi,

Problem fixed. The problem was that I needed to clear ospf process on router 4... after that, everything happened like in the book.

Neighbors state is always changing from EXCHANGE to DOWN to INIT and so on....

Danielh22185

The beauty of using labs to prove a theory. I was going to replay with it will never become a neighbor due to being stuck in an INT state trying to become a full neighbor.

Zartanasaurus

The MTU is only sent in a DBD packet which is why everything was okay when you changed the MTU after the neighborship was established.