Understanding the difference between Not Responding and Disconnected ESXi hosts in VMware vCenter Server

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Purpose
This article explains the difference between a host that is showing as Not Responding and Disconnected in VMware vCenter Server.

Resolution
Not Responding

A host can become greyed out and shown 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 heartbeats from it.
This happens because of several reasons, all of which prevent heartbeats being received from the host to vCenter.

Some common reasons include:
  • A network connectivity issue between the host and vCenter Server, such as UDP port 902 not open, a routing issue, bad cable, firewall rule, etc.
  • 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 state is resolved. However, a host that is in the Disconnected state ceases to be monitored by vCenter Server and stays in that state regardless of the status of the underlying issue. After resolving the issue, the user must right-click on the host and select Connect to bring the host back to a normal state in vCenter Server.

Disconnected

Disconnected is a state initiated from the vCenter Server side and suspends vCenter Server host management, and thus all vCenter Server services ignore the host.
A disconnected host is the one that has been explicitly disconnected by the user, or the license on the host has expired. Disconnected hosts also require the user to manually reconnect the host. 

Ultimately, a host that is Disconnected due to one of these three reasons (2 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 still remains in the vCenter Server inventory, but vCenter Server does not get any updates from the disconnected host, does not monitor it, and therefore 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, but assumes that any virtual machines running on a disconnected host will be 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 therefore not failed over in the event of a host isolation. When the host becomes reconnected, the host becomes available for failover again.

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