3Com 9100 manual Uses of Tagged VLANs, Assigning a Vlan Tag

70CHAPTER 4: VIRTUAL LANS (VLANS)

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.