When does a switch add a VLAN tag to a frame?

A switch adds a VLAN tag to a frame primarily when it forwards the frame to another switch. This process is a part of VLAN (Virtual Local Area Network) management and is integral to how switches handle traffic within a network that employs VLANs.

When a switch receives a frame that is destined for a different VLAN or when it is forwarding a frame across switch boundaries, it tags the frame to inform the receiving switch about the VLAN to which the frame belongs. This tagging enables the receiving switch to appropriately handle and route the frame based on its VLAN membership. The VLAN tags are typically added according to the IEEE 802.1Q standard, which is a method used for VLAN tagging.

In scenarios where frames are forwarded to servers or originate from clients, the need to tag a frame with a VLAN ID may not arise immediately. For example, if the server and client are on the same VLAN and directly connected to the same switch, the switch does not need to add a tag when it processes traffic in this context until it is required for routing between VLANs. Similarly, the act of processing inbound traffic does not inherently involve tagging unless the traffic is being prepared for inter-VLAN routing or switch-to-switch communication.