3Com Switch 7750 Configuration Guide Guide

DHCP Relay Configuration 579
DHCP server directly. The following are the mechanism of option 82 supporting on
DHCP relay.
1 A DHCP client broadcasts a request packet when it initiates.
2 The DHCP relay 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 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 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.
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.
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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
DHCP Relay
Configuration Tasks
Table 449 DHCP relay configuration tasks
Configuration task Remarks 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 the DHCP relay
agent to broadcast the
response packet from the
server to the clients
Optional
“Configuring a DHCP Relay
Agent to Broadcast Responses
to Clients”
Configure DHCP relay security Optional
“Configuring DHCP Relay
Security”
Configure option 82
supporting
Optional
“Configuring Option 82
Supporting”