we were updating an ESXi host with HPP offline bundle, HPP SPP (firmware) and VMware patches manuall. All successful, no error.
ESXi host = HPE Gen10 running ESXi 6.7.0 Update 3 Build 19195723
vCenter = vCSA with embedded Platform Services Controller 126.96.36.199000 Build number 19300125
During the updates the host and the VM's on it changed status to 'disconnected' in vCenter. After all updating finished, trying to reconnect that host failed. Removing from vCenter inventory and adding failed. We tried adding to cluster and adding to datacenter (which makes a difference when adding) but both failed.
This host has been part of vCenter and our cluster for years, no problems. vSphere license was also available and host is in maintenance mode when trying to add again.
Error message pop-up in vCenter:
Add standalone host
Cannot contact the specified host (<IP-of-standalone-host>). The host may not be available on the network, a network configuration problem may exist, or the management services on this host may not be responding.
The host and the VM's on it are working when using the host stand alone. We restarted the stand alone host several times, but no success to add it to vCenter, always same error messages.
From the CLI vCSA can reach that standalone host by ping and vice versa. SSH is working, but from ESXi to vCSA, not from vCSA to ESXi.
Found the issue. It was the firewall. Disabling it on the ESXi stand alone host did it.
But how can that ESXi have been part of the inventory of the vCenter with firewall set to 'enabled = true'?