Extreme summit1 Mode D'Emploi

5-6

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 LAN

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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 

. 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.

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.

Packets arriving tagged with a VLANid that is not configured in the switch will be 
discarded.

Summit.bk : 5VLAN.FM  Page 6  Thursday, June 18, 1998  9:27 AM