Incoming and
Outgoing Tagged
and Untagged
Frames
AT-S82 Management Software User’s Guide
Tagged Port Members
A port is a tagged member of a specific VLAN when it is a member of more than one VLAN. If a port is a tagged member of one VLAN, then the same port is also an untagged member of another VLAN.
Untagged Port Members
A port is an untagged member of a VLAN if the PVID is equal to the VID of that VLAN. A port can be an untagged member of only one VLAN. An example of this is the Default VLAN configuration where all ports are initially configured to be untagged members of VLAN 1 only. A port can also be an untagged member of one VLAN and be a tagged member of one or more VLANS.
The VLAN information within an Ethernet frame is referred to as a tag or tagged header. The frame containing this VLAN tag information is referred to as a tagged frame. Likewise, a frame that does not contain this VLAN tag information is referred to as an untagged or standard frame. A tag, which follows the source and destination addresses in the frame’s header, contains the VID information of the VLAN to which the frame belongs, according to the IEEE802.3ac VLAN tagging standard.
When a switch receives a frame, it examines the frame header to see if it contains a VLAN tag (tagged frame) or no tag (untagged frame). After switching the frame to an outgoing port and before transmitting it, the switch determines if the tag information should be kept in the header or should be stripped out and made into an untagged frame.
Incoming Frames
Tagged frames received by the switch are only accepted (not dropped) if the tag information contained in the frame is equal to one of the VIDs of which the port is a member. If the tag information contained in the frame does not match one of these VIDs, the frames are dropped or discarded.
Untagged frames received by the switch are always accepted by all ports on the switch. Each untagged frame is assigned to the VLAN number of which the port is an untagged member. The switch then forwards this frame to one of the other member ports of that VLAN.
Outgoing Frames
Frames being transmitted from the switch retain their VLAN tag information in the frame header if the frame’s tag does not match the PVID of the port (a tagged member of that VLAN). These frames are untagged after transmission from the switch.
The VLAN tag information in the header of the frame is stripped from the frame’s header if the tag matches the PVID of the port (an untagged
45
