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HAPTER
8: S
ETTING
U
P
V
IRTUAL
LAN
S
VLANs: Tagged and
Untagged
Membership
Your Switch supports 802.1Q VLAN tagging, a system that allows traffic
for multiple VLANs to be carried on a single physical (backbone) link.
When setting up VLANs you need to understand when to use untagged
and tagged membership of VLANs. Quite simply, if a port is in a single
VLAN it can be an untagged member but if the port needs to be a
member of multiple VLANs tagged membership must be defined.
Typically endstations (for example, clients or servers) will be untagged
members of one VLAN, while inter-Switch connections will be tagged
members of all VLANs.
The IEEE 802.1Q standard defines how VLANs operate within an open
packet-switched network. An 802.1Q compliant packet carries additional
information that allows a switch to determine to which VLAN the port
belongs. If a frame is carrying the additional information, it is known as
tagged
.
To carry multiple VLANs across a single physical (backbone) link, each
packet must be tagged with a VLAN identifier so that the Switches can
identify which packets belong in which VLANs. To communicate between
VLANs a router must be used.
Placing a Port in a
Single VLAN
Once the information for a new VLAN has been defined, you can place a
port in that VLAN.
Creating an IEEE 802.1Q Tagged Link
This method of tagging is defined in the IEEE 802.1Q standard, and
allows a link to carry traffic for any of the VLANs defined on your Switch.
802.1Q tagging can only be used if the devices at both ends of a link
support IEEE 802.1Q.
To create an 802.1Q tagged link:
1
Ensure that the device at the other end of the link uses the same 802.1Q
tags as your Switch, that is, the same VLAN IDs are configured (note that
VLAN IDs are global across the network).
2
Place the Switch ports in the required VLANs as tagged members.
3
Place the port at the other end of the link as a tagged member of the
same VLANs as the port on your Switch.