Typical Application of the Option 82 Field

Figure 1 Application of the Option 82 field

In Figure 1, clients use DHCP to obtain IP addresses. To improve network security, the administrator configures the device to control network access of clients connected to Interface1.

The DHCP server cannot detect the DHCP client location based only on the DHCP Request message. As a result, users in the same VLAN have the same rights to access network resources.

To address this issue, the administrator can enable the Option 82 insertion function after DHCP snooping is enabled on SwitchA. Upon receiving a DHCP Request message, SwitchA inserts the Option 82 field in the message to notify the DHCP server of the DHCP client location. The Option 82 field contains the MAC address, VLAN ID, and interface number of the client. The DHCP server can properly assign an IP address and other configurations to the client based on the IP address assignment or security policies on the server.

The Option 82 field records only the location of a DHCP client and is encapsulated in a DHCP Request message sent to the DHCP server. To deploy different IP address assignment or security policies for different clients, the DHCP server must support the Option 82 field and be configured with IP address assignment or security policies.