3Com WX3000 Overview of Dhcp Snooping Option, Padding content and frame format of Option

Figure 3-1Typical network diagram for DHCP snooping application

DHCP Server

DHCP Client DHCP Client
			Internet
	GE1/0/1	GE1/0/2
		Switch A	Switch B
	(DHCP Snooping)		(DHCP Relay)

DHCP Client DHCP Client

DHCP snooping listens the following two types of packets to retrieve the IP addresses the DHCP clients obtain from DHCP servers and the MAC addresses of the DHCP clients:

z

z

DHCP-REQUEST packet DHCP-ACK packet

Overview of DHCP Snooping Option 82

Introduction to Option 82

Option 82 is the relay agent information option in the DHCP message. It records the location information of the DHCP client.

When a DHCP relay agent (or a device enabled with DHCP snooping) receives a client’s request, it adds the Option 82 to the request message and sends it to the server.

The administrator can locate the DHCP client to further implement security control and accounting. The Option 82 supporting server can also use such information to define individual assignment policies of IP address and other parameters for the clients.

Option 82 involves at most 255 sub-options. If Option 82 is defined, at least one sub-option must be defined. Currently the DHCP relay agent supports two sub-options: sub-option 1 (circuit ID sub-option) and sub-option 2 (remote ID sub-option).

Padding content and frame format of Option 82

There is no specification for what should be padded in Option 82. Manufacturers can pad it as required. By default, the sub-options of Option 82 for the device (enabled with DHCP snooping) are padded as follows:

z

z

Sub-option 1 (circuit ID sub-option): Padded with the port index (smaller than the physical port number by 1) and VLAN ID of the port that received the client’s request.

Sub-option 2 (remote ID sub-option): Padded with the bridge MAC address of the DHCP snooping device that received the client’s request.

By default, when the device serves as a DHCP snooping device, Option 82 adopts the extended format. Refer to Table 3-2 and Figure 3-3 for the extended format of the sub-options (with the default padding

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