3Com 7757 Configuration Manual page 163

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The following describes how a packet is forwarded from Switch A to Switch C.
As QinQ is enabled on Ethernet2/0/1 port of Switch A, when a packet from the
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user's private network reaches Ethernet2/0/1 port of Switch A, it is tagged with
the default VLAN tag of the port (VLAN 10 tag) and is then forwarded to
Ethernet2/0/2 port.
When the packet reaches Ethernet2/0/2 port of Switch B, it is forwarded in
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VLAN 10 and is passed to Ethernet2/0/1 port.
The packet is forwarded from Ethernet2/0/1 port of Switch B to the network
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on the other side and reaches Ethernet2/0/2 port of Switch C. Switch C
forwards the packet in VLAN 10 to its Ethernet2/0/1 port. As Ethernet2/0/1
port is an access port, the outer VLAN tag of the packet is stripped off and the
packet restores the original one.
It is the same case when a packet travels from Switch C to Switch A.
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After the configuration, the networks connecting Switch A and Switch C can
receive packets from each other.
QinQ Configuration Example 163