3Com 1100 - Manual (Page 166)

166 C HAPTER 6: V IRTUAL LAN S (VLAN S )
Defining New VLANs If you want to move a port from the Default VLAN to another VLAN, you
must first define information about the new VLAN on your Switch. To do
this, you use the VLAN Setup page of the web interface; see
“Defining
VLAN Information” on page 84 .
Untagged and
Tagged VLANs When setting up VLANs you need to understand when to use untagged
and tagged VLANs. Quite simply, if a port is in a single VLAN it can be
untagged but if the port needs to be a member of multiple VLANs it must
be tagged.
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 data, 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. Routers interconnect
VLANs, so they must also understand 802.1Q tagging, so that they do
not become bottlenecks for inter-VLAN traffic.
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. To do this, use the Untagged VLAN listbox on the Port
Setup page of the web interface; see
“Configuring a Port” on page 59 .
Placing a Port in
Multiple VLANs
Your Switch supports VLAN tagging, a system that allows traffic for
multiple VLANs to be carried on a single link. Two methods of VLAN
tagging are supported: 802.1Q tagging and VLT (Virtual LAN Trunk)
tagging.
802.1Q Tagging
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.