3Com S7902E, Configuration 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. For how to configure the MTU of an
interface, refer to Ethernet Port Configuration in the Access Volume .
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 an implementation more flexible than basic QinQ. In addition to all the functions of
basic QinQ, selective QinQ can tag frames with different outer VLAN tags based on their inner VLAN
IDs.
The S7900E series implements selective QinQ by using customer VLAN IDs as match criteria to
classify frames and then tagging the frames that match a certain VLAN ID with the outer VLAN tag
defined in the associated traffic behavior.
Modification of the TPID Value in VLAN Tags
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.
Figure 1-3 shows the 802.1Q-defined tag structure of an Ethernet frame.