You are configuring a DHCP server to service a group of clients that are located on a different subnet than the DHCP server itself. What else must you configure to ensure a successful setup?
If a DHCP server needs to service clients in a different subnet, you will need to configure a relay agent. The relay agent service is enable by default but does not function unless you provide the IP address of the remote DHCP server, which is done by executing the ip helper address command on the interface where the address needs to be announced.
The fact that the clients are on a different subnet indicates that there is a router between the DHCP server and the clients. The DHCP discover packet that a client sends out is in the form of a broadcast. Routers do not forward broadcast traffic from one segment to the other. Without a relay agent, the DHCP server would never receive the requests.
A relay agent resides on the same segment as the clients. When a client sends out a discover packet, the relay agent takes the request, converts it to a unicast packet, and forwards the request to the DHCP server on the other network segment.
The relay agent can also be activated on the router that separates the two network segments. To enable the relay agent service on a Cisco router where 172.16.10.2 is the IP address of the DHCP server, use the following command:
Router(config-if)# ip helper-address 172.16.10.2
A relay agent can also be used to assist in the auto configuration of a switch. Auto configuration is a process whereby:
1.A switch boots up.
2.The switch obtains an IP address, subnet mask, and gateway address (optional).
3.The switch uses the DNS server to locate the TFTP server.
4.The switch connects to the TFTP server, downloads the configuration file, and applies it.
When the switch must broadcast to locate the DHCP, DNS, or TFTP server, IP helper addresses can be provided for all of these. When the switch broadcasts, a unicast will be sent to all of these addresses.
In following illustration, the FastEthernet0 interface of the router is connected to the subnet containing the switch and the FastEthernet1 interface of the router is connected to the subnet containing the DHCP, DNS, and TFTP servers. The addresses involved are:
Switch - 10.2.2.2
Router - F0 10.2.21, F2 184.108.40.206
DHCP - 220.127.116.11
DNS - 18.104.22.168
TFTP - 22.214.171.124
The router that is located between the subnet containing the switch and the subnet containing the DHCP, DNS, and TFTP servers should be configured as shown below:
Regardless of whether the ip helper-address command has been used to aid in the DHCP configuration of a switch utilizing auto configuration, or to aid DHCP clients in a different subnet from the DHCP server, the DHCP relay service will provide relay services for the following UDP protocols by default:
Trivial File Transfer Protocol (TFTP) (port 69)
Domain Naming System (DNS) (port 53)
Time service (port 37)
NetBIOS Name Server (port 137)
NetBIOS Datagram Server (port 138)
Boot Protocol (BOOTP) client and server packets (ports 67 and 68)
TACACS service (port 49)
IEN-116 Name Service (port 42)
This default behavior can be altered with the IP forward-protocol udp command executed in global configuration mode.
Multicast routing, unicast routing and access lists do not aid in the DHCP communication process.
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Cisco > IP Addressing: DHCP Configuration Guide > Configuring the Cisco IOS DHCP Relay Agent