2-2
Figure 2-1 DHCP relay agent application
IP network
DHCP server
DHCP relay agent
DHCP client DHCP client
DHCP clientDHCP client
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). The following describes the
forwarding process on the DHCP relay agent.
Figure 2-2 DHCP relay agent work process
As shown in
Figure 2-2, 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)
.
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.
If a reply returned by the DHCP server contains Option 82, the DHCP relay agent will remove the Option
82 before forwarding the reply to the client.
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