10-3Figure 10-2 Single-tagged frame structure vs. double-tagged Ethernet frame structureThe default maximum transmission unit (MTU) of an interface is 1500 bytes. The size of an outerVLAN tag is 4 bytes. Therefore, you are recommended to increase the MTU of each interface on theservice provider network. The recommended minimum MTU is 1504 bytes.Implementations of QinQThere are two types of QinQ implementations: basic QinQ and selective QinQ.1) Basic QinQBasic QinQ is a port-based feature. When a frame arrives at a basic QinQ-enabled port, the port tags itwith the port’s default VLAN tag, regardless of whether the frame is tagged or untagged. If the receivedframe is already tagged, it becomes a double-tagged frame; if it is untagged, it becomes a frametagged with the port’s default VLAN tag.2) Selective QinQSelective QinQ is a more flexible, VLAN-based implementation of QinQ. In addition to all the functionsof basic QinQ, selective QinQ can tag frames with different outer VLAN tags based on different innerVLAN IDs.Modifying the TPID in a VLAN TagA VLAN tag uses the tag protocol identifier (TPID) field to identify the protocol type of the tag. Thevalue of this field, as defined in IEEE 802.1Q, is 0x8100.0 shows the 802.1Q-defined tag structure of an Ethernet frame.
