Allied Telesis AT-S39 - Manual (Page 94)

Section II: Local and Telnet Management
94
If a VLAN spans multiple switches, then the VID for the VLAN on the
different switches must be the same. In this manner, the switches are
able to recognize and forward frames belonging to the same VLAN even
though the VLAN spans multiple switches.
For example, if you had a port-based VLAN titled Marketing that
spanned three AT-8024 switches, you would assign the Marketing VLAN
on each switch the same VID.
You can assign this number manually or allow the management
software to do it automatically. If you allow the management software to
do it automatically, it will simply select the next available VID. This is
acceptable when you are creating a new, unique VLAN.
If you are creating a VLAN on a switch that will be part of a larger VLAN
that spans several switch, then you will need to assign the number
yourself so that the VLAN has the same VID on all switches.
Untagged Ports
Naturally, you need to specify which ports on the switch 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 ports as untagged
frames. The names derive from the fact that the frames received on a
port will not contain any information that indicates VLAN membership,
and that VLAN membership will be determined solely by the port’s PVID.
(There is another type of VLAN where VLAN membership is determined
by information within the frames themselves, rather than by a port’s
PVID. This type of VLAN is explained in Tagged VLAN Overview on page
100.)
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.
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.