DHCP is often used for hosts to automatically assign IP addresses and uses 4 different packets to do so. Since a host doesn’t have an IP address to start with, we use broadcast messages on the network that hopefully end up at a DHCP server.
The problem with broadcast is that this means that the DHCP server has to be in the same broadcast domain since routers do not forward broadcast packets. Take a look at the following picture:
On the left side we have a client (H1), in the middle a router (R1) and on the right side is our DHCP server. The client wants to get an IP address through DHCP and will send broadcast a DHCP discover message. The router, doing its job will not forward broadcast traffic so the DHCP discover will never reach the DHCP server…ouch!
So how can we solve this? We have to use the DHCP Relay Agent feature. In short, the router will forward DHCP requests from the client towards the DHCP server, when the DHCP server responds it will forward the messages back to the client.
Let me describe this process in detail, step-by-step to you:
The first thing that happens is that our client will broadcast a DHCP discover message, the router will receive this message since its in the same broadcast domain as the client. Here’s what happens next:
The router receives the DHCP discover message on its FastEthernet 0/0 interface and will normally just discard this packet. With the DHCP relay agent feature enabled, it will do something else. It will forward the DHCP discover message as a unicast packet and also inserts a field called giaddr (Gateway IP Address) in the DHCP packet. It will insert IP address 192.168.12.2 in this field since we received the DHCP discover on the FastEthernet 0/0 interface. This giaddr field is required by the DHCP server or it won’t know from which pool it has to select an IP address. Also, the source IP address of this unicast packet will be 192.168.12.2. Let’s continue:
The DHCP server has received the DHCP discover message and in return will send a DHCP offer message. This will be sent as a unicast packet to the router…
The router, being a good relay will forward the DHCP offer on its FastEthernet0/0 interface as a broadcast.
The client likes the content of the DHCP offer message and will create a DHCP request which is broadcasted. The router hears this broadcast and will do this:
Just like the initial DHCP discover message, this DHCP request will be forwarded as a unicast packet. Once again the giaddr field is inserted with IP address 192.168.12.2. The DHCP server receives the DHCP request and will process it…
Last but not least, the DHCP server will send a DHCP ACK in response to the DHCP request. This is sent to the router by using unicast and our router will broadcast it on its FastEthernet 0/0 interface so the client receives it. The client now has an IP address and our mission is a great success.
Now you know how the DHCP relay agent works, let’s take a look at the configuration shall we?
I will be using 3 routers for this, the topology is the same as the one I just used for my explanation:
Let’s start with the configuration of the interfaces:
H1(config)#interface FastEthernet 0/0 H1(config-if)#no shutdown
R1(config)#interface FastEthernet 0/0 R1(config-if)#no shutdown R1(config-if)#ip address 192.168.12.2 255.255.255.0
R1(config)#interface FastEthernet 0/1 R1(config-if)#no shutdown R1(config-if)#ip address 192.168.23.2 255.255.255.0
DHCP(config)#interface FastEthernet 0/0 DHCP(config-if)#no shutdown DHCP(config-if)#ip address 192.168.23.3 255.255.255.0
Nothing special so far…let’s make a DHCP pool for the 192.168.12.0 /24 network. That’s where the client is at: