OSPF - Stuck in ATTEMPT neighbor state

When implementing OSPF, a router may enter the ATTEMPT neighbor state and may become stuck in such a state. In this case, the OSPF adjacency table will look something like this:

Neighbor ID     Pri   State           Dead Time   Address         Interface
N/A               0   ATTEMPT/DROTHER 00:01:13    172.16.123.1    Tunnel0

The ATTEMPT state is a state that is specific to NBMA networks. It means that the OSPF process is trying to establish a neighbor relationship with a router that it has been configured to statically attempt to contact such as in an OSPF non-broadcast network type. A prolonged such state means that something is not quite right.

This is an expected error if you have a broadcast network type (which is the default) along with statically defined neighbors. Such a config may result in duplicate efforts which interfere with each other. With a broadcast network type, OSPF will still attempt to dynamically discover neighbors using Multicast hellos, while also trying to establish a neighbor relationship through unicast hellos to the static neighbors. This can cause the above situation in the OSPF neighbor table.