One common issue with networking in virtualized environments is the outgoing ports from the virtual switches to the physical network switches. Configuring with VLANs and similar must be done very carefully to ensure that all ports are correctly setup. It is often quite hard to track patch cables going from the server chassis to the switch port and it is often also difficult to determine the relation between the ESXi server NIC ports to the VMNICs visible inside ESXi.
A very helpful tool is a network discovery protocol, for example CDP or LLDP.
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