Allied Telesis AT-S82 - Manual (Page 45)

AT-S82 Management Software User’s Guide
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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 as 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.
Incoming and
Outgoing Tagged
and Untagged
Frames
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 the frame,
the switch determines if the tag information should be kept in the header or
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 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.