User's Manual

29-71
1. The client broadcasts a DHCPDISCOVER packet, and DHCP relay inserts its own IP address to the
relay agent field in the DHCPDISCOVER packet on receiving the packet, and forwards the packet to
the specified DHCP server (for DHCP frame format, please refer to RFC2131).
2. On the receiving the DHCPDISCOVER packets forwarded by DHCP relay, the DHCP server sends
the DHCPOFFER packet via DHCP relay to the DHCP client.
3. DHCP client chooses a DHCP server and broadcasts a DHCPREQUEST packet, DHCP relay
forwards the packet to the DHCP server after processing.
4. On receiving DHCPREQUEST, the DHCP server responds with a DHCPACK packet via DHCP relay
to the DHCP client.
DHCP Relay Configuration Task List:
1. Enable DHCP relay.
2. Configure DHCP relay to forward DHCP broadcast packet.
1. Enable DHCP relay.
Command
Explanation
Global Mode
service dhcp
no service dhcp
DHCP server and DHCP relay is enabled as the
DHCP service is enabled.
2. Configure DHCP relay to forward DHCP broadcast packet.
Command
Explanation
Global Mode
ip forward-protocol udp bootps
no ip forward-protocol udp bootps
The UDP port 67 is used for DHCP broadcast
packet forwarding.
Interface Configuration Mode
ip helper-address <ipaddress>
no ip helper-address <ipaddress>
Set the destination IP address for DHCP relay
forwarding; the “no ip helper-address
<ipaddress>“command cancels the setting.
29.4 DHCP Configuration Examples
Scenario 1:
Too save configuration efforts of network administrators and users, a company is using switch as a DHCP
server. The Admin VLAN IP address is 10.16.1.2/16. The local area network for the company is divided into
network A and B according to the office locations. The network configurations for location A and B are shown
below.