DHCP Relay Agent
Hosts discover their DHCP server by sending broadcasts. If that server is on
a different subnet, those broadcasts must be routed to the server as unicasts.
You can configure a router to relay DHCP messages with the ip helperaddress
interface command. It is important to understand that this command
must be given on the interface that receives the host broadcasts. A Cisco
DHCP relay agent functions as follows:
Step 1. A client broadcasts a DHCP request, which is seen by the IOS
device (a router, for instance).
Step 2. The router changes the destination address of the packet to the
unicast address of the server. It optionally adds option 82 (relay
agent option) information.
Step 3. The router sends the unicast packet to the server.
Step 4. The server responds with the IP address and other parameters,
such as the default gateway assigned to the client.
Step 5. The router gets the packet from the server, removes any option 82
information, and forwards it to the client.
The ip helper-address command enables the relaying of UDP broadcasts
only. By default, eight broadcast types are enabled:
�¡ Time, port 37
�¡ TACACS, port 49
�¡ DNS, port 53
�¡ BOOTP/DHCP server, port 67
�¡ BOOTP/DHCP client, port 68
�¡ TFTP, port 69
�¡ NetBIOS name service, port 137
�¡ NetBIOS datagram service, port 138
To disable the forwarding any of these protocols, use the interface command
no ip forward-protocol udp port-number. To add UDP protocols to be
relayed, use the interface command ip forward-protocol udp port-number.
Verify your DHCP configuration with the commands show ip dhcp database,
show ip dhcp server statistics, and show ip dhcp binding. Delete
address assignments with the clear ip dhcp binding {address | *}
command.