H3C S5120-EI Series, Operation Manual

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