H3C Technologies H3C S3100 Series Switches - Manual (Page 860)

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Figure 1-3 After many-to-one VLAN mapping
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Configuring the DHCP Option 82 for Many-to-One VLAN Mapping
Option 82 is the relay agent option in the option field of the DHCP message. A DHCP snooping-enabled
device that supports option 82 can insert the location information (including the port number and VLAN)
of the DHCP client to the DHCP request. Upon receiving the DHCP request, the DHCP server can use
the option to locate the DHCP client, thus securing and accounting the client.
In a many-to-one VLAN mapping network, the DHCP server is usually deployed above the distribution
layer. To record the client information correctly, the DHCP server must know the port through which a
user is connected to the corridor switch and the original VLAN of the traffic. The port number locates the
user, and the original VLAN identifies the device type of the client. For this purpose, you must enable
DHCP snooping to support option 82 on the corridor switch, and configure option 82 to carry the original
VLAN of traffic. After the configuration above is complete, the DHCP snooping option 82 works as
shown in Figure 1-4 :
Figure 1-4 How DHCP snooping option 82 works for many-to-one VLAN mapping