Dell PowerEdge FN I/O Module Configuration Guide 9.10(0.0)

To add an interface to a VLAN, it must be in Layer 2 mode. After you place an interface in Layer 2 mode, it is automatically placed in the default VLAN. Dell Networking OS supports IEEE 802.1Q tagging at the interface level to filter traffic. When you enable tagging, a tag header is added to the frame after the destination and source MAC addresses. The information that is preserved as the frame moves through the network. The below figure shows the structure of a frame with a tag header. The VLAN ID is inserted in the tag header.

Figure 1. Tagged Frame Format

The tag header contains some key information used by Dell Networking OS:

• The VLAN protocol identifier identifies the frame as tagged according to the IEEE 802.1Q specifications (2 bytes).
• Tag control information (TCI) includes the VLAN ID (2 bytes total). The VLAN ID can have 4,096 values, but two are reserved.

Information contained in the tag header allows the system to prioritize traffic and to forward information to ports associated with a specific VLAN ID. Tagged interfaces can belong to multiple VLANs, while untagged interfaces can belong only to one VLAN.