Allied Telesis AT-S84 - Manual (Page 107)

AT-S84 Management Software User’s Guide
Section I: Using the Menus Interface 107
Note
The switch is preconfigured with the Default_VLAN only. All ports on
the switch are initially members of the Default_VLAN.
If a port is assigned to be a new member of a VLAN, its membership can
be defined as either tagged or untagged.
Tagged Port Members
A port is a tagged member of a VLAN when the PVID does not equal the
VID. In this case, the port must be a member of two or more VLANs. If a
port is a tagged member of one VLAN, then the same port is also an
untagged member of another VLAN where the PVID matches its VID.
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 be
an untagged member of one VLAN and be a tagged member of one or
more VLANS at the same time. (See Figure 27 on page 111.)
Incoming and
Outgoing Tagged
and Untagged
Frames
The VLAN information within an Ethernet frame is referred to as a tag or
tagged header. An Ethernet frame can contain VLAN information within its
header. Likewise, a frame that does not contain this VLAN tag information
is referred to as an untagged or standard frame. A tag contains the VID
information of the VLAN to which the frame belongs, according to the
IEEE802.1Q 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 or
discarded) 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. As described in “Port VLAN Identifier” on page 106, each
untagged frame received by the switch is assigned a VLAN number equal
to the PVID. The switch then forwards this frame to one of the other
member ports of that VLAN.