HP Network Direr Software Products manual Tagged and untagged Vlan membership

Key Concepts 503

Tagged and untagged VLAN membership

Switches typically allow ports to be either untagged or tagged members of a VLAN. If a port is an untagged member of a VLAN, packets transmitted on the port contain no VLAN information. If a port is a tagged member of a VLAN, packets transmitted on the port contain the VLAN ID as specified in the 802.1Q standard. Tagged and untagged ports can coexist in the same VLAN group. When setting up VLANs, you need to understand when to use untagged and tagged membership of VLANs.

Ports should be untagged if they are connected to devices that do not support 802.1Q VLAN tagged packets. Typically, end-stations will drop VLAN tagged packets and so should be connected to untagged ports.

A port can also be an untagged member of a VLAN if the port is a member of one VLAN only. However, if a port needs to be a member of multiple VLANs, tagged membership must be defined. For example, if multiple VLANs need to be carried over an inter-switch link, the ports at each end of the link should be defined to be tagged members of the VLANs. When a port needs to be a member of multiple VLANs, it can still be an untagged member of one of these VLANs but it has to be defined to be a tagged member of all other VLANs.

In a network where the VLANs are distributed amongst more than one switch, you must use 802.1Q tagged connections so that all VLAN traffic can be passed along the links between the switches. 802.1Q tagging can only be used if the devices at both ends of a link support IEEE 802.1Q.

In Figure 260, the simple example below, each switch has end-stations in VLAN 1 and VLAN 2. All end-stations in VLAN 1 need to be able to connect to the server in VLAN 1 that is attached to Switch 1 and all end-stations in VLAN 2 need to connect to the server in VLAN 2 that is attached to Switch 2. The two VLANs are distributed between the two switches by making the ports at each end of the link that connects Switch 1 to Switch 2 tagged members of VLANs 1 and 2.