Allied Telesis AT-S25 - Manual (Page 119)

AT-S25 Management Software User’s Guide
119
VLAN Identifier Each VLAN in a network must have a unique number
assigned to it. This number is called the VLAN identifier
(VID). This number uniquely identifies a VLAN in the
switch and the network.
If a VLAN consists only of ports located on one physical
switch in your network, you would assign it a VID
unique from all other VLANs in your network.
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-8324 Switches, you
would assign the Marketing VLAN on each switch the
same VID.
You could 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 switches, 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 124.)
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.
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