Allied Telesis AT-8550 - Manual (Page 72)

Types of VLANs
4-6
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 4-3 . 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.
Note
Packets arriving tagged with a VLANid that is not configured in the
switch will be discarded.
Figure 4-4 illustrates the physical view of a network that uses tagged
and untagged traffic.