If you add or configure a serial port that is backed by a remote network connection, ESXi firewall settings can prevent transmissions.
Before you connect network-backed virtual serial ports, you must add one of the following firewall rule sets to prevent the firewall from blocking communication:
VM serial port connected to vSPC. Use to connect the serial port output through a network with the Use virtual serial port concentrator option enabled to allow only outgoing communication from the host.
VM serial port connected over network. Use to connect the serial port output through a network without the virtual serial port concentrator.
Do not change the allowed IP list for either rule set. Updates to the IP list can affect other network services that might be blocked by the firewall.
For details about allowing access to an ESXi service through the firewall, see the vSphere Security documentation.