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3com WX3000 Manuel D’Utilisation

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Figure 1-2 Structure of packets with double-layer VLAN tags 
Destination MAC address
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31
Data
Source MAC address
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Inner VLAN Tag
Outer VLAN Tag
 
 
Compared with MPLS-based Layer 2 VPN, VLAN-VPN has the following features: 
z 
It provides Layer 2 VPN tunnels that are simpler. 
z 
VLAN-VPN can be implemented through manual configuration. That is, signaling protocol-related 
configuration is not needed. 
The VLAN-VPN feature provides you with the following benefits: 
z 
Saves public network VLAN ID resource. 
z 
You can have VLAN IDs of your own, which is independent of public network VLAN IDs. 
z 
Provides simple Layer 2 VPN solutions for small-sized MANs or intranets. 
Implementation of VLAN-VPN 
With the VLAN-VPN feature enabled, no matter whether or not a received packet already carries a 
VLAN tag, the device will tag the received packet with the default VLAN tag of the receiving port and add 
the source MAC address to the MAC address table of the default VLAN. When a packet reaches a 
VLAN-VPN-enabled port: 
z 
If the packet already carries a VLAN tag, the packet becomes a dual-tagged packet.  
z 
Otherwise, the packet becomes a packet carrying the default VLAN tag of the port. 
Adjusting the TPID Values of VLAN-VPN Packets 
Tag protocol identifier (TPID) is a field of the VLAN tag. IEEE 802.1Q specifies the value of TPID to be 
0x8100. 
 illustrates the structure of the Tag packet of an Ethernet frame defined by IEEE 802.1Q. 
Figure 1-3 The structure of the Tag packet of an Ethernet frame 
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15
TPID
Priority
VLAN ID
CFI
 
 
By default, the device adopts the TPID value (0x8100) specified in the protocol. Some vendors use 
other TPID values (such as 0x9100). 
To be compatible with devices of other vendors, the device can adjust the TPID values of VLAN-VPN 
packets globally. You can configure TPID values by yourself. When forwarding packets, the VLAN-VPN 
uplink port sets the TPID value for outer VLAN tags of these packets to the user-defined value, so that 
these packets forwarded to the public network can be recognized by devices of other vendors.