Discovery error messages occur most frequently because of the following conditions:
The element is unreachable.
The running configuration for the element does not match the element defined in topology. Two examples of this are:
The running configuration is for a gateway, but the topology lists this element as a gatekeeper.
There is an IP address mismatch where the running configuration does not contain the voice-enabled IP address that the element should have.
The gateway has no voice ports defined.
When you encounter Discovery error messages, use the console to set the debug of the Discovery subsystem to on.