User Manual
UMN:CLI
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9.6.4
DHCP Relay Agent
A DHCP relay agent is any host that forwards DHCP packets between clients and servers.
The DHCP relay agents are used to forward DHCP requests and replies between clients
and servers when they are not on the same physical subnet. The DHCP relay agent for-
warding is distinct from the normal forwarding of an IP router, where IP datagrams are
switched between networks somewhat transparently.
By contrast, DHCP relay agents receive DHCP messages and then generate a new
DHCP message to send out on another interface. The DHCP relay agent sets the gate-
way address and, if configured, adds the DHCP option 82 information in the packet and
forwards it to the DHCP server. The reply from the server is forwarded back to the client
after removing the DHCP option 82 information.
Fig. 9.37
Example of DHCP Relay Agent
To activate/deactivate the DHCP function in the system, use the following command.
Command
Mode
Description
service dhcp
Global
Activates the DHCP function in the system.
no service dhcp
Deactivates the DHCP function in the system.
Before configuring DHCP server or relay, you need to use the
service dhcp
command
first to activate the DHCP function in the system.
9.6.4.1
DHCP Helper Address
A DHCP client sends DHCP_DISCOVER message to a DHCP server. DHCP_DISCOVER
message is broadcasted within the network to which it is attached. If the client is on a
network that does not have any DHCP server, the broadcast is not forwarded because
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