3Com 4800G Series, Configuration Manual

Page: 814 / 1246

814 C
HAPTER
58: DHCP R
ELAY
A
GENT
C
ONFIGURATION
Figure 247 DHCP relay agent application
No matter whether a relay agent exists or not, the DHCP server and client interact
with each other in a similar way (see section “Dynamic IP Address Allocation
Process” on page 792). The following describes the forwarding process on the
DHCP relay agent.
Figure 248 DHCP relay agent work process
As shown in the figure above, the DHCP relay agent works as follows:
1 After receiving a DHCP-DISCOVER or DHCP-REQUEST broadcast message from a
DHCP client, the DHCP relay agent fills the giaddr field of the message with its IP
address and forwards the message to the designated DHCP server in unicast
mode.
2 Based on the giaddr field, the DHCP server returns an IP address and other
configuration parameters to the relay agent, which conveys them to the client.
DHCP Relay Agent
Support for Option 82
Option 82 records the location information of the DHCP client. The administrator
can locate the DHCP client to further implement security control and accounting.
For more information, refer to “Relay agent option (Option 82)” on page 795.
If the DHCP relay agent supports Option 82, it will handle a client’s request
according to the contents defined in Option 82, if any. The handling strategies are
described in the table below.
IP network
DHCP server
DHCP relay agent
DHCP client DHCP client
DHCP client DHCP client
DHCP-DISCOVER
(broadcast)
DHCP-OFFER
DHCP-REQUEST
(broadcast)
DHCP-ACK
DHCP client DHCP relay DHCP server
DHCP-DISCOVER
(unicast)
DHCP-OFFER
(unicast)
DHCP-REQUEST
(unicast)
DHCP-ACK
(unicast)