Overview of VLAN Termination

Definition

VLAN termination is a VLAN tag processing mechanism. VLAN termination enables a device to identify VLAN tags and remove single or double VLAN tags from received packets. It then forwards the packets over Layer 3 or takes other actions as required. These VLAN tags are only useful before termination, and are not used in Layer 3 forwarding or other processing.

A device with VLAN termination enabled processes incoming and outgoing packets as follows:

Removes single or double VLAN tags from the packets received on interfaces, and then selects an appropriate action such as forwarding the packets over Layer 3.
Adds VLAN tags to the packets that will be sent out through interfaces.

Classification

Depending on the selected method for VLAN tagged packets processing, VLAN termination has the following sub-functions:

Dot1q termination: The device removes the outer VLAN tag from any received single-tagged or double-tagged packets, and adds a VLAN tag to the packets to be sent by an interface.
QinQ termination: The device removes double VLAN tags from any 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 sub-interfaces do not support transparent transmission of packets that do not contain a VLAN tag, and discard these packets instead.

Purpose

After VLANs are assigned on a network, hosts in the same VLAN can communicate with each other over Layer 2 but in different VLANs cannot communicate with each other. You can use VLANIF interfaces on a Layer 3 switch to implement inter-VLAN Layer 3 connectivity, but this encounters the following problem. As shown in Figure 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. To solve this problem, you can virtualize a Layer 3 Ethernet interface into multiple logical sub-interfaces with the Layer 3 Ethernet interface as the main interface.

Figure 1 Sub-interface configuration to implement interworking

However, 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.