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C H A P T E R 5
92
Virtual LANs (VLANs)
Uses of Tagged VLANs
Tagging is most commonly used to create VLANs that span
switches. The switch-to-switch connections are typically called
trunks. Using tags, multiple VLANs can span multiple switches
using one or more trunks. In a port-based VLAN, each VLAN
requires its own pair of trunk ports, as shown in Figure 7. Using
tags, multiple VLANs can span two switches with a single trunk.
Another benefit of tagged VLANs is the ability to have a port be a
member of multiple VLANs. This is particularly useful if you have
a device (such as a server) that must belong to multiple VLANs. The
device must have a NIC that supports 802.1Q tagging.
A single port can be a member of only one port-based VLAN. All
additional VLAN membership for the port must be accompanied by
tags. In addition to configuring the VLAN tag for the port, the
server must have a Network Interface Card (NIC) that supports
802.1Q tagging.
Assigning a VLAN Tag
Each VLAN may be assigned an 802.1Q VLAN tag. As ports are
added to a VLAN with an 802.1Q tag defined, you decide whether
each port will use tagging for that VLAN. The default mode of the
switch is to have all ports assigned to the VLAN named default with
an 802.1Q VLAN tag (VLANid) of 1 assigned.
Not all ports in the VLAN must be tagged. As traffic from a port is
forwarded out of the switch, the switch determines (in real time) if
each destination port should use tagged or untagged packet formats
for that VLAN. The switch adds and strips tags, as required, by the
port configuration for that VLAN. Any packets arriving tagged with
a VLANid that is not configured on a port will be discarded.
Figure 8 illustrates the physical view of a network that uses tagged
and untagged traffic.
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