Introduction to VLAN Termination

Definition

VLAN termination is a VLAN tag processing mechanism. After VLAN termination is enabled on a device, the device identifies VLAN tags in received packets, removes single or double tags from the packets, and then forwards packets at Layer 3 or takes other actions. These VLAN tags are only useful before termination, and are not used in Layer 3 forwarding or other processing.

Classification

Depending on the modes in which VLAN tagged packets are processed, VLAN termination has the following sub-functions:

• Dot1q termination: removes the outer VLAN tag from the received single-tagged or double-tagged packets, and adds a VLAN tag to the packets to be sent by an interface.
• QinQ termination: removes double VLAN tags from the received double-tagged packets, and adds double VLAN tags to the packets to be sent by an interface.

Generally, VLAN termination is configured on sub-interfaces. A sub-interface that terminates single tags in packets is called a Dot1q termination sub-interface, and a sub-interface that terminates double tags in packets is called a QinQ termination sub-interface.

Dot1q and QinQ VLAN tag termination sub-interfaces do not support transparent transmission of packets that do not contain a VLAN tag, and discard received packets that do not contain a VLAN tag.

Purpose

After VLANs are assigned on a network, hosts in the same VLAN can communicate with each other at Layer 2, whereas hosts in different VLANs cannot. You can use VLANIF interfaces on a Layer 3 switch to implement inter-VLAN Layer 3 connectivity. As shown in Figure 8-1, when a Layer 3 switch uses only one Layer 3 Ethernet interface to connect to users or a network, this interface needs to transmit packets from multiple VLANs. A VLANIF interface cannot provide this function. You can virtualize a Layer 3 Ethernet interface into multiple logical sub-interfaces. The Layer 3 Ethernet interface is the main interface for the logical sub-interfaces.

Figure 8-1  Networking of configuring sub-interfaces to implement interworking

By default, a Layer 3 Ethernet sub-interface treats received VLAN packets as invalid packets and discards them; therefore, VLAN termination needs to be configured on the Layer 3 Ethernet sub-interface so that the sub-interface can remove VLAN tags from packets.