3-2
Figure 3-1 Typical network diagram for DHCP snooping application
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 DHCP-REQUEST packet
z DHCP-ACK packet
Introduction to DHCP-Snooping Option 82 Introduction to Option 82
For details about Option 82, refer to Option 82 Support on DHCP Relay Agent.
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 S4500 Series Ethernet Switches (enabled with DHCP
snooping) are padded as follows:
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.
z 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 S4500 Series Ethernet Switches serve as DHCP snooping devices, Option 82 adopts
the extended format. Refer to Figure 3-2 and Figure 3-3 for the extended format of the sub-options (with
the default padding contents). That is, the circuit ID or remote ID sub-option defines the type and length
of a circuit ID or remote ID.
The remote ID type field and circuit ID type field are determined by the option storage format. They are
both set to 0 in the case of HEX format and to 1 in the case of ASCII format.
Figure 3-2 Extended format of the circuit ID sub-option