Cisco IOS DHCP Relay Agent

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.

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



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:

dhcp relay agent topology

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:

dhcp relay discover

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:

dhcp relay discover unicast

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:

dhcp relay offer unicast

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…

dhcp relay offer broadcast

The router, being a good relay will forward the DHCP offer on its FastEthernet0/0 interface as a broadcast.

dhcp relay request 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:

dhcp relay request unicast

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…

dhcp relay dhcp ack

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?

Configuration

I will be using 3 routers for this, the topology is the same as the one I just used for my explanation:

dhcp relay 3 routers example

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:

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Forum Replies

  1. Hi Rene,

    Will the ip helper-address always be the address of the interface on the the DHCP server that connects to the DHCP relay?

  2. Hi Diana,

    A router becomes a DHCP relay when it “relays” DHCP broadcasts to another interface. We do this with the “IP helper-address” command.

    This command, however, does more than just forwarding DHCP broadcasts, here’s a list:

    • UDP 69 - TFTP
    • UDP 67 - BOOTP Client
    • UDP 68 - BOOTP Server
    • UDP 37 - Time Protocol
    • UDP 49 - TACACS
    • UDP 53 - DNS
    • UDP 137 - NetBios
    • UDP 138 - NetBios Datagram

    So by default, it will relay all broadcasts to these UDP destination ports. If you want you can also enable relaying for other destination ports. For example:

    ip forward-pr

    ... Continue reading in our forum

  3. Rene

    Hope you are well - hopelijk heb je veel snoepies gehad van dag !

    Anyway, I tried the lab above for the DHCP but it din’t seem to work … so my client hasnt received an IP address seemingly here is what I got any ideas what is happening many thanks in advance:

    Client#sh ip int br
    Interface                  IP-Address      OK? Method Status                Protocol
    FastEthernet0/0            unassigned      YES DHCP   up                    up
    
    Client#sh ip int f0/0
    FastEthernet0/0 is up, line protocol is up
      Internet address will be negotiated using DHCP
    

    C

    ... Continue reading in our forum

  4. Hi Shree,

    Nothing will change. The router that is configured for DHCP relay will create these unicast packets. Other routers only have to route these packets to the DHCP server, that’s it. Here’s a packet capture of these unicast packets btw:

    DHCP Relay Unicast Packets

    Rene

  5. Hello Juan

    When the DHCP server is in the same subnet, the following communications take place:

    • DHCPDISCOVER is broadcast on both layer 2 and layer 3 (MAC and IP)
    • DHCPOFFER as a response to the discover is unicast. It uses the MAC address of the original sender as the destination MAC and the proposed IP address as the destination IP (even though the DHCP client does not yet have an IP address assigned. This doesn’t matter since communication is happening at Layer 2 for now since we are on the same subnet)
    • DHCPREQUEST is also broadcast on both Layer 2 and Lay
    ... Continue reading in our forum

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