How to configure DHCP RELAY AGENT on CISCO Routers
Dynamic Host Configuration Protocol helps us to address dynamically our hosts on the network. In fact, when a Host is configured to get its IP address dynamically, it will broadcast a DHCP REQUEST on the network searching for a DHCP SERVER.
The problem here is that our DHCP SERVER has to be on the same broadcast domain as the CLIENTS since routers do not forward broadcast packets. I suggest to start with the following architecture:
As our DHCP Client wants to get an IP address, it will send a DHCP Discover message which is a broadcast message. As the Router do not forward broadcast packets, this message will never reach the DHCP Server, and as a result: No IP address for our friend DHCP Client. This is not good. 🙁
To solve this problem, the DHCP Relay Agent feature is used on routers to forward DHCP messages to the DHCP Server, and when the DHCP Server respond, the Router will forward the replies to the Client.
Let’s see how it works:
The DHCP Client broadcasts a DHCP Discover message looking for a DHCP Server. When the Router Receives this message, and with the DHCP Relay Agent feature enabled, it will forward the message to the DHCP Server as a Unicast Packet.
The router will also add to this Unicast Packet a field called giaddr (Gateway IP address) in the DHCP Packet, this field will contains in our case the IP address 192.168.1.1 since the router receives the DHCP Discover message on the Fa0/0, and the source IP address of this Unicast packet will be also 192.168.1.1.
This field is required by the DHCP Server so it can know from which pool it has to select an IP address.
One the DHCP Server receives the DHCP Discover message, it will respond by the DHCP OFFER Message; this message will be sent as a Unicast parcket to the router.
The router will forward the DHCP OFFER Message as a broadcast message.
The DHCP Client has verified the information in the DHCP Offer message, and sends a DHCP REQUEST as a broadcast message. The router will forward this message as Unicast packet to the DHCP SERVER. Note that once again, the giaddr field is added add contains the IP address 192.168.1.1.
As a response to the DHCP REQUEST message, the DHCP SERVER will send a DHCP ACK message to the router, and in turn, the router will broadcast this message on the Fa0/0. Finally, our DHCP Client has its IP address.
Let’s see how we can configure this, are you ready?
We will use 3 routers to see the configuration.
Here is the basic configuration:
ROUTER(config-if)#ip address 192.168.1.1 255.255.255.0
ROUTER(config-if)#ip address 192.168.2.1 255.255.255.0
DHCP_SERVER(config-if)#ip address 192.168.2.2 255.255.255.0
DHCP_SERVER(config)#ip dhcp pool NET1
DHCP_SERVER(dhcp-config)#network 192.168.1.0 255.255.255.0
As you can see, this is the basic configuration of every router. We have also configured the DHCP SERVER service. Now we have to configure the DHCP RELAY AGENT on the router:
ROUTER(config-if)#ip helper-address 192.168.2.2
As you can see, the DHCP RELAY AGENT feature is enabled using the IP helper-address command. 192.168.2.2 is the IP address of the DHCP SERVER. And we enable the feature on the interface connected to the DHCP CLIENT. You can see the feature enabled on the interface fa0/0:
|ROUTER#show ip int fa0/0
FastEthernet0/0 is up, line protocol is up (connected)
Internet address is 192.168.1.1/24
Broadcast address is 255.255.255.255
Address determined by setup command
MTU is 1500 bytes
Helper address is 192.168.2.2
Directed broadcast forwarding is disabled
Outgoing access list is not set
Inbound access list is not set
Proxy ARP is enabled
Before configuring the interface of the DHCP CLIENT to get a dynamic IP address, we have to configure the Server to reach the network 192.168.1.0/24, because the source IP address of the DHCP messages will be 192.168.1.1/24. For this we’ll configure a static router:
|DHCP_SERVER(config)#ip route 192.168.1.0 255.255.255.0 192.168.2.1|
Last step is to configure the interface of the DHCP CLIENT for DHCP:
DHCP_CLIENT(config-if)#ip address dhcp
Now, let’s see if the interface has an IP address:
|DHCP_CLIENT#show ip int br
Interface IP-Address OK? Method Status Protocol
FastEthernet0/0 192.168.1.8 YES DHCP up up
FastEthernet0/1 unassigned YES unset administratively down down
As you can see, our router has successfully received an IP address. This means that the Relay feature is working fine.
This is all that I have for you on DHCP RELAY AGENT feature. If you have any question, just leave a comment.