MNS-BB Software User Guide
-87-
is based on the TAG information contained in the packet. Hence, we can drop or allow through
packets arriving on a set of ports based on the source TAG information contained in the packets.
16.2 VLAN Tagging Information
VLAN tagging enables traffic from more than one VLAN to use the same port. (Even when two or
more VLANs use the same port they remain as separate domains and cannot receive traffic from each
other without going through an external router.) As mentioned earlier, a “tag” is simply a unique
VLAN identification number (VLAN ID, or VID) assigned to a VLAN at the time that you configure
the VLAN name in the switch. In the LE2425A and LEV2525A switches, the tag can be any number
from 1 to 4095 that is not already assigned to a VLAN. When you subsequently assign a port to a
given VLAN, you must implement the VLAN tag (VID) if the port will carry traffic for more than
one VLAN. Otherwise, the port VLAN assignment can remain “untagged” because the tag is not
needed. On a given switch, this means you should use the “Untagged” designation for a port VLAN
assignment where the port is connected to a non 802.1Q-compliant device or is assigned to only one
VLAN. Use the “Tagged” designation on at least one of the VLAN’s when the port is assigned to
more than one VLAN or the port is connected to a device that does comply with the 802.1Q standard.
Ports 1-6: Untagged
Port 7: Red VLAN Untagged Green VLAN Tagged
Ports 1-4: Untagged Port 5: Red VLAN Untagged
Example of Tagged and Untagged VLAN Port Assignments
For example, if port 7 on an 802.1Q-compliant switch is assigned to only the Red VLAN, the assignment
can remain “untagged” because the port will forward traffic only for the Red VLAN. However, if both the
Red and Green VLANs are assigned to port 7, then at least one of those VLAN assignments must be
“tagged” so that Red VLAN traffic can be distinguished from Green VLAN traffic. The above illustration
shows this concept.
¾ In switch X:
• Suppose the ports X1 - X6 each only have one VLAN per port. The VLANs assigned can all be
untagged. Red VLAN traffic will go out only the Red ports; Green VLAN traffic will go out only the
Green ports, and so on. Devices connected to these ports do not have to be 802.1Qcompliant.
• However, if both the Red VLAN and the Green VLAN are assigned to port X7, at least one of the
VLANs must be tagged for this port.
To Next Page
Loading...
