Understanding the Difference Between an ESXi Host Not Responding and an ESXi Host Disconnected in VMware vCenter Server

Goal:
Understand the difference between a host that is showing as Not Responding and a host that is showing as Disconnected in VMware vCenter Server.

Facts:
VMware vCenter Server
ESXi

Information:

Not Responding
A host becomes unavailable (greyed out) and shows as Not Responding because of an external factor that vCenter Server is unaware of. If a host is showing as Not Responding, vCenter Server no longer receives a heartbeat from it.

This can happen because of several reasons, all of which prevent heartbeats being received from the host to the vCenter.

Some common reasons include:

• A network connectivity issue between the host and vCenter Server, such as UDP port 902 not open, routing issue, bad cable, firewall rule, and so forth
• Hostd is not running successfully on the host.
• Vpxa is not running successfully on the host.
• The host has failed.

A host can go from Not Responding back to a normal state if the underlying issue which brought the host to the Not Responding is resolved. However, a host that is in the Disconnected state is not monitored by vCenter Server. It stays in that state regardless of the status of the underlying issue. The user must right-click the host and select Connect to bring the host back to a normal state in vCenter Server after resolving the issue.

Disconnected
Disconnected is a state that is initiated from the vCenter Server side, and suspends vCenter Server host management. All vCenter Server services ignore the host.

A disconnected host is explicitly disconnected by the user, or the license on the host has expired. Disconnected hosts require the user to manually reconnect the host. 

The cause of a host being Disconnected is due to one of these three reasons (two of which require manual intervention):

• A user right-clicks the host and selects Disconnect.
• A user right-clicks a host that is listed as Not Responding and clicks Connect and that task fails.
• The host license expires.

When a host becomes disconnected, it remains in the vCenter Server inventory, but vCenter Server does not get any updates from the disconnected host. vCenter Server does not monitor the host, and has no knowledge of the health of that disconnected host. 

vCenter Server takes a conservative approach when considering disconnected hosts. Virtual machines on a host that is not responding affect the admission control check for vSphere HA. vCenter Server does not include those virtual machines when computing the current failover level for HA. vCenter Server assumes that any virtual machines running on a disconnected host is failed over if the host fails. Because the status of the host is not known, and because vCenter Server is not communicating with that host, HA cannot use it as a guaranteed failover target. As part of disconnecting a host, vCenter Server disables HA on that host. The virtual machines on that host are not failed over in the event of a host isolation. When the host becomes reconnected, the host becomes available for failover again.

Related Article:
For diagnosing a disconnected ESXi host, see VMware article: Troubleshooting an ESXi/ESX host in nonresponding state (1003409).