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Types of VLANs
Summit 300-48 Switch Software User Guide 67
NOTE
The use of 802.1Q tagged packets may lead to the appearance of packets slightly bigger than the
current IEEE 802.3/Ethernet maximum of 1,518 bytes. This may affect packet error counters in other
devices, and may also lead to connectivity problems if non-802.1Q bridges or routers are placed in the
path. The tag also carries the 802.1 (802.1p) priority bits. This is the only way priority information can
be shared between seperate devices (hosts, switches/routers and so on).
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 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 on a port will be discarded.
Figure 4 illustrates the physical view of a network that uses tagged and untagged traffic.