You can configure a native VLAN for each port. The native VLAN is like a default VLAN for untagged incoming packets. Outgoing packets for the native VLAN are sent as untagged frames.
The native VLAN is assigned to any untagged packet arriving at an ingress port.
At an egress port, if the packet tag matches the native VLAN, the packet is sent out without the VLAN header.