H3C Technologies H3C S5120 Series Switches - Manual (Page 242)

1-2
No matter whether a relay agent exists or not, the DHCP server and client interact with each other in a
similar way . The following describes the forwarding process on the DHCP relay agent.
Figure 1-2 DHCP relay agent work process
As shown in Figure 1-2 , 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.
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.
If a client’s
requesting
message has…
Handling
strategy
Padding format The DHCP relay agent will…
Drop Random Drop the message.
Keep Random
Forward the message without changing
Option 82.
normal
Forward the message after replacing
the original Option 82 with the Option 82
padded in normal format.
verbose
Forward the message after replacing
the original Option 82 with the Option 82
padded in verbose format.
Option 82
Replace
user-defined
Forward the message after replacing
the original Option 82 with the
user-defined Option 82.