DHCP Configuration
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31.3
DHCP Relay Configuration
When the DHCP client and server are in different segments, DHCP relay is required
to transfer DHCP packets. Adding a DHCP relay makes it unnecessary to configure a
DHCP server for each segment, one DHCP server can provide the network configuration
parameter for clients from multiple segments, which is not only cost-effective but also
management-effective.
DHCP Server
DHCP Client
DHCPDiscover(Broadcast)
DHCPOFFER(Unicast)
DHCPREQUEST(Broadcast)
DHCPACK(Unicast)
DHCPDiscover
DHCP Relay
DHCPOFFER
DHCPREQUEST
DHCPACK
Fig 31-2 DHCP relay
As shown in the above figure, the DHCP client and the DHCP server are in different
networks, the DHCP client performs the four DHCP steps as usual yet DHCP relay is
added to the process.
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.