Troubleshooting OSPF Neighbor Adjacency

[lagapidis (Lazarus Agapidis)]

Hello Shahlar

I just went in and labbed this up and found that OSPF neighbors will not form if the subnet mask is different. For example, I had an L3 EtherChannel created between two switches, and the IP address on one port-channel interface was 192.168.12.1/24 and the IP address on the other was 192.168.12.2/25. The interfaces can ping each other successfully, but because of the different subnet masks (/24 and /25), they failed to become neighbors. Notice that we’re talking about the subnet masks of the interfaces used to create the neighborship.

Can you recheck your configuration and let us know if you get different results? If so, you may want to share your config with us so we can further discuss it.

Concerning the BROADCAST network type, the PortChannel is not considered a point-to-point link, because of the fact that Ethernet, which is a multi-access technology, is being used. Take a look at this post, which should clarify further.

OSPF Multi-Area Configuration

Hello Shahlar It is true that a DR/BDR election won’t take place in a point-to-point network between OSPF routers. However, we must define what “point-to-point” means. OSPF will automatically detect a point-to-point network based on the underlying technology. For example, a serial connection is considered point-to-point, because, by definition, serial connections can have only one device connected on the opposite end of the circuit. OSPF will interpret an OSPF peering over a serial circuit …

I hope this has been helpful!

Laz