H3C Technologies H3C S12500-X Series Switches - Manual (Page 140)

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Figure 36 Single-tagged Ethernet frame header and double-tagged Ethernet frame header
For correct transmission of tagged frames, H3C recommends that you set the MTU of each interface on
the service provider network to at least 1504 bytes, which is the sum of the default interface MTU (1500
bytes) and the size of a VLAN tag (4 bytes).
The devices in the service provider network forward a tagged frame according to its SVLAN tag only,
and they transmit the CVLAN tag as part of the frame's payload.
Figure 37 Typical QinQ application scenario
As shown in Figure 37 , customer network A has CVLANs 1 through 10, and customer network B has
CVLANs 1 through 20. The service provider assigns SVLAN 3 to customer network A, and assigns
SVLAN 4 to customer network B.
1. When a tagged Ethernet frame from customer network A arrives at PE1, the PE tags the frame with
SVLAN 3. When a tagged Ethernet frame from customer network B arrives at PE2, the PE tags the
frame with SVLAN 4.
2. The double-tagged Ethernet frame is then transmitted over the service provider network and arrives
at the other PE. The PE removes the SVLAN tag of the frame before sending it to the target CE.
PE 1
Public network
PE 2
VLANs 1 to 20
Customer
network B
Customer
network A
VLANs 1 to 10
VLANs 1 to 10
Customer
network A
VLANs 1 to 20
Customer
network B
Data VLAN 4 CVLAN B
Data VLAN 3 CVLAN A Data VLAN 4 CVLAN B
Data VLAN 3 CVLAN A
Data CVLAN B
Data CVLAN B Data CVLAN A
Data CVLAN A
CE 3 CE 4
CE 1 CE 2
Data flow
Network