Implementations Of Qinq; Modifying The Tpid In A Vlan Tag - 3Com 4210G Series Configuration Manual

Figure 10-2 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 frames 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.
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