Allied Telesis AT-S41 - Manual (Page 95)

AT-S41 User’s Guide
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Tagged VLANs are also useful where multiple VLANs span across
switches. You can use one port per switch for connecting all VLANs on
the switch to another switch.
The IEEE 802.1Q standard deals with how this tagging information is
used to forward the traffic throughout the switch. The handling of
frames tagged with VIDs coming into a port is straightforward. If the
incoming frame’s VID tag matches one of the VIDs of a VLAN that the
port is a tagged member of, the frame will be accepted and forwarded to
the appropriate ports. If the frame’s VID does not match any of the
VLANs that the port is a member of, the frame will be discarded.
So how do you indicate which ports are to be tagged and which are to
be untagged when you create a VLAN? The rule is straightforward. If you
assign a port to only one VLAN, the switch assumes it is to be an
untagged port. If you assign a port to more than one VLAN, the switch
assumes that the port is to be both a tagged and untagged port.
A VLAN that contains only tagged frames or that contains a combination
of tagged and untagged ports is referred to as a tagged VLAN. And, as
explained previously, any device that you connect to a tagged port of a
tagged VLAN must be IEEE 802.1Q-compliant.
General Rules
to Creating an
Untagged or
Tagged VLAN
Below are general rules to observe when creating a VLAN.
❑ An AT-8326GB switch can support up to256 tagged and untagged
VLANS.
❑ Each VLAN must be assigned a unique VID. However, if a particular
VLAN spans multiples AT-8326GB stacks, each part of the VLAN on
the different stacks must be assigned the same VID.
❑ A port can be an untagged member of only one VLAN at a time.
❑ A port can be a tagged member of multiple VLANs.
❑ You must assign each untagged port a PVID. The PVID of an
untagged port must match the VLAN’s VID. You must assign this
value manually when you create the VLAN.
❑ An untagged VLAN that spans multiple stacks requires a port on
each stack where the VLAN is located to function as a connection
between the various parts of the VLAN reside.
❑ If there are end nodes in different VLANs that need to
communicate with each other, a router or Layer 3 switch is
required to connect the VLANs.