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70
C
4: V
LAN
(VLAN
)
HAPTER
IRTUAL
S
S
Uses of Tagged VLANs
Tagging is most commonly used to allow VLANs to 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
9. 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 and only
one protocol-based VLAN. It can be a member of any number of tagged
VLANs, and 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
When a VLAN is configured to support tagging, it is assigned a tag. As
individual ports are added to a tagged VLAN, you decide whether the
port will use a tag.
Not all ports in a tagged VLAN must be tagged. As traffic from a port is
forwarded out of the switch, the switch adds and strips tags, as required,
by the port configuration 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.
Packets arriving tagged with a VLANid that is not configured on the
ingress port will be discarded.
Figure 10
illustrates the physical view of a network that uses tagged and
untagged traffic.
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