Allied Telesis AT-S25 - Manual (Page 142)

Introduction to Virtual LANs
142
You can assign this number manually or allow the AT-S25 software to do
it automatically. The range of the VID varies depending on whether
IGMP snooping is activated on an AT-8300 Series stack. If IGMP snooping
is not activated, the VID range is from 2 to 4096. If IGMP snooping is
activated, the range is 2 to 2047. You should take this into account when
planning your VLANs and assigning VID values. If IGMP snooping is not
activated on a stack but that you might activate it at a later time, it is
recommended that you assign VLANs VID values in the range of 2
through 2047. This could save you from having to reconfigure your
VLAN VID assignments should you later activate IGMP snooping.
Port VLAN Identifier
Each port in a port-based VLAN must have a port VLAN identifier (PVID).
The switch associates a frame to a port-based VLAN by the PVID
assigned to the port on which the frame is received, and forwards the
frame only to those ports with the same PVID. Consequently, all ports of
a port-based VLAN must have the same PVID. Additionally, the PVID of
the ports in a VLAN must match the VLAN’s VID.
For example, assume that you were creating a port-based VLAN on a
switch and you had assigned the VLAN the VID 5. Consequently, the
PVID for each port in the VLAN would need to be assigned the value 5.
Some switches and switch management programs require that you
assign the PVID value for each port manually. However, the AT-S25
management software performs this task automatically. The software
automatically assigns a PVID to a port when the port is assigned to a
port-based VLAN. A port’s PVID assigned by the management software is
the same as the VID of the VLAN to which the port is made a member.
Untagged Ports
Naturally, you also need to specify which ports on a switch or stack are to
be members of a port-based VLAN. Ports in a port-based VLAN are
referred to as untagged ports and the frames received on the port as
untagged frames. The names originate from the fact that the frames
received on a port will not contain any tagged information that indicates
VLAN membership, and that VLAN membership is determined by the
port’s PVID. (As explained in Tagged VLAN on page 147, there is another
type of VLAN where VLAN membership is determined by information
within the frames themselves, rather than by a port’s PVID.)
A port on a switch can be an untagged member of only one port-based
VLAN at a time. An untagged port cannot be assigned to two port-based
VLANs simultaneously.