3Com Switch 4200G Configuration Guide

DHCP Relay Configuration 395
3 If the packet contains option 82, the DHCP relay 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 adds option 82 to the
packet and forwards the packet to the DHCP server. The forwarded packet contains
the MAC address of the switch port to which the DHCP client is connected, the VLAN
to which the DHCP client belongs, and the MAC address of the DHCP relay.
5 Upon receiving the DHCP request packet forwarded by the DHCP relay, 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.
6 Upon receiving the packet returned from the DHCP server, the DHCP relay strips
option 82 from the packet and forwards the packet with the DHCP configuration
information to the DHCP client.
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 adds option 82 to both types of
packets to accommodate to DHCP servers of different manufacturers.
DHCP Relay
Configuration
If a switch belongs to a fabric, you need to enable the UDP-helper function on it
before configure it to be a DHCP relay.
DHCP Relay
Configuration Tasks
Enabling DHCP Be sure to enable DHCP before you perform other DHCP relay-related configuration,
for other DHCP-related configurations cannot take effect with DHCP disabled.
Configuring an Interface
to Operate in DHCP
Relay Mode
There may be multiple DHCP servers deployed in one network. This increases the
reliability. Here, you can configure a DHCP server group containing one or multiple
DHCP servers.
Table 337 DHCP relay configuration tasks
Operation Description Related section
Enable DHCP Required Enabling DHCP
Configure an interface to operate in
DHCP relay mode
Required Configuring an Interface to Operate
in DHCP Relay Mode
Configure DHCP relay security Required Configuring DHCP Relay Security
Table 338 Enable DHCP
Operation Command Description
Enter system view system-view —
Enable DHCP dhcp enable Required
By default, DHCP is disabled.