3Com Switch 7750 Configuration Guide Guide

578 CHAPTER 55: DHCP RELAY CONFIGURATION
3 The DHCP server assigns IP addresses and transmits the configuration information
to the clients through the DHCP relay so that the clients can be configured
dynamically. The transmission mode depends on the flag field in the
DHCP-DISCOVER packet. For details, see section
“DHCP Packet Format”.
Option 82 Supporting Introduction to option 82 supporting
Option 82 is a relay agent information option in DHCP packets. When a request
packet from a DHCP client travels through a DHCP relay on its way to the DHCP
server, the DHCP relay adds option 82 into the request packet. Option 82 includes
many sub-options, but the DHCP server supports only sub-option 1 and
sub-option 2 at present. Sub-option 1 defines agent circuit ID (that is, Circuit ID)
and sub-option 2 defines remote agent ID (that is, Remote ID).
Option 82 enables a DHCP server to track the address information of DHCP clients
and DHCP relays, through which and other proper software, you can achieve the
DHCP assignment limitation and accounting functions.
Primary terminologies
■ Option: A length-variable field in DHCP packets, carrying information such as
part of the lease information and packet type. It includes at least one option
and at most 255 options.
■ Option 82: Also known as relay agent information option. This option is a part
of the Option field in DHCP packet. According to RFC3046, option 82 lies
before option 255 and after the other options. Option 82 includes at least one
sub-option and at most 255 sub-options. Currently, the commonly used
sub-options in option 82 are sub-option 1, sub-option 2, and sub-option 5.
■ Sub-option 1: A sub-option of option 82. Sub-option 1 represents the agent
circuit ID, namely Circuit ID. It holds the port number and VLAN-ID of the
switch port connected to the DHCP client, and is usually configured on the
DHCP relay. Generally, sub-option 1 and sub-option 2 must be used together to
identify information about a DHCP source.
■ Sub-option 2: A sub-option of option 82. Sub-option 2 represents the remote
agent ID, namely Remote ID. It holds the MAC address of the DHCP relay, and
is usually configured on the DHCP relay. Generally, sub-option 1 and
sub-option 2 must be used together to identify information about a DHCP
source.
Related specification
The specifications concerning option 82 supporting are as follows:
RFC2131 Dynamic Host Configuration Protocol
RFC3046 DHCP Relay Agent Information Option
Mechanism of option 82 supporting on DHCP relay
The procedure for a DHCP client to obtain an IP address from a DHCP server
through a DHCP relay is similar to that for the client to obtain an IP address from a