Assuming VLANs are not configured, which of the following is true about traffic from a virtual machine connected to a port group on a vNetwork Standard Switch with no uplinks?
Virtual machines in any port group on the virtual switch can receive the traffic
The virtual switch will drop the packets if no uplink is present
Virtual machines on any virtual switch on the same ESX/ESXi host can receive the traffic
Only virtual machines in the same port group on the virtual switch can receive the traffic
VMware Virtual Networking Concepts, page 6.
It is possible, and even reasonable, to assign the same VLAN ID to multiple port groups. This would be useful if, for example, you wanted to give different groups of virtual machines different physical Ethernet adapters in a NIC team for active use and for standby use, while all the adapters are on the same VLAN.
Since a port group without an assigned VLAN has an implicit VLAN ID of 0, all port groups without a VLAN ID will be in VLAN0. Multiple port groups with the same VLAN can communicate with each other.
Physical Ethernet adapters serve as bridges between virtual and physical networks. In VMware Infrastructure, they are called uplinks, and the virtual ports connected to them are called uplink ports.
If no uplink exists there will be no communication with physical networks, but traffic will propagate across virtual interfaces connected to the same switch.