H3C Technologies H3C SecPath F1000-E - Manual (Page 211)

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Figure 106 DHCP replay agent work process
As shown in Figure 106 , 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, letting the administrator locate the DHCP
client for security control and accounting purposes.
If the DHCP relay agent supports Option 82, it handles a client's request according to the contents
defined in Option 82, if any. The handling strategies are described in Table 31 .
If a reply returned by the DHCP server contains Option 82, the DHCP relay agent removes the Option 82
before forwarding the reply to the client.
Table 31 Handling strategies of the DHCP relay agent
If a client's
requesting
message has…
Handling
strategy
Padding format The DHCP relay agent will…
Option 82
Drop Random Drop the message.
Keep Random
Forward the message without changing
Option 82.
Replace
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.
user-defined
Forward the message after replacing the
original Option 82 with the user-defined
Option 82.
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)