3Com Switch 7750 Configuration Guide

Configuring DHCP Relay Agent 613
Mechanism of option 82 supporting on DHCP relay agent
The procedure for a DHCP client to obtain an IP address from a DHCP server
through a DHCP relay agent is similar to that for the client to obtain an IP address
from a DHCP server directly. The following are the mechanism of option 82
supporting on DHCP relay agent.
1 A DHCP client broadcasts a request packet when it initiates.
2 The DHCP relay agent on the local network receives the request packet, and then
checks whether the packet contains option 82 and processes the packet
accordingly.
3 If the packet contains option 82, the DHCP relay agent processes the packet
depending on the configured policy (that is, discards the packet, replaces the
original option 82 in the packet with its own, or leaves the original option 82
unchanged in the packet), and forwards the packet (if not discarded) to the DHCP
server.
4 If the packet does not contain option 82, the DHCP relay agent adds option 82 to
the packet and forwards the packet to the DHCP server. The forwarded packet
contains the port number of the switch to which the DHCP client is connected, the
VLAN to which the DHCP client belongs, and the MAC address of the DHCP relay
agent.
5 Upon receiving the DHCP request packet forwarded by the DHCP relay agent, the
DHCP server stores the information contained in the option field and sends a
packet that contains DHCP configuration information and option 82 to the DHCP
relay agent.
6 Upon receiving the packet returned from the DHCP server, the DHCP relay agent
strips option 82 from the packet and forwards the packet with the DHCP
configuration information to the DHCP client.
n
Request packets sent by a DHCP client fall into two categories: DHCP-DISCOVER
packets and DHCP-REQUEST packets. As DHCP servers coming from different
manufacturers process DHCP request packets in different ways (that is, some
DHCP servers process option 82 in DHCP-DISCOVER packets, whereas the rest
process option 82 in DHCP-REQUEST packets), a DHCP relay agent adds option 82
to both types of packets to accommodate to DHCP servers of different
manufacturers.
Configuring DHCP
Relay Agent
Introduction to DHCP
Relay Agent
Configuration Tasks
Table 481 DHCP relay agent configuration tasks
Configuration task Description Related section
Enable DHCP Required “Enabling DHCP”
on page 614
Configure an interface to operate in DHCP relay
agent mode
Required “Configuring an
Interface to Operate
in DHCP Relay
Agent Mode” on
page 614