A great feature released as part of NSX 6.2 is the Communication Channel Health which can be used to troubleshoot communications between NSX Managers and Controllers to the individual hosts or clusters. The following diagram taken from a VMware presentation shows the communication channels involved:
The VSFWD is the Firewall Agent and NETCPA is the Network Control Plane agent that both run on all hosts / clusters that have been prepared for NSX. To check the Communication Channel Health log into the vSphere Web Client and navigate to the Networking & Security plug in -> Installation and then the Host Preparation. Click on a cluster or individual host and selection Actions and then Communications Channel Health:
The following shows the health of the PA-MGMT cluster:
The following shows the health of an individual host:
When your hosts looses the ability to communicate with an NSX Controller then you will see the Control Plane Agent to Controller being down (To simulate this I just powered off the only controller in my lab):
If there are issues with the NETCPA agent then you will see the following (To simulate this I just stopped the NETCAP service on a single host):
Because the host uses the NETCPA agent to communicate with the controllers you will see an unknown error as well. Finally if there are issues with the firewall agent you will see the following (To simulate this I just stopped the vShield-Stateful-Firewall service on a single host):
You will also notice that NSX Manager to Control Plane Agent is down and Control Plane Agent to Controller is unknown and this is because the connection status from each host to controller is locally monitored and the health status is then reported by each host to the NSX Manager over the messaging bus which is dependent on the VSFWD service.