DHCP IP helper address questions

[j.vong]

hello, I have some questions on dhcp ip helper address.

Assuming my router has two FE interface (0/0 and 0/1). 0/0 has 10.1.1.1/24 and 0/1 has 10.1.2.1/24.

The LAN connecting to 0/0 via a LAN switch has a DHCP server of 10.1.1.5. There is no ip helper address configured on the 0/0 interface. Current DHCP server serves the 10.1.1.x dhcp clients.

We are now adding 0/1 with 10.1.2.1/24 on the router. We want to have dhcp function to the clients site on this new LAN but we are not going to have another dhcp server on this LAN and we are not configuring the router interface to run dhcp server on it either.

Questions:

1. Can I configure ip helper address of 10.1.1.5 on the FE 0/1 interface with 10.1.2.1 address on it?

2. If yes, when the client on this new LAN sends dhcp broadcast to request ip address, will the router on FE 0/1 "route" the packet to FE 0/0 and onto the dhcp server of 10.1.1.5? I read the ip helper address says "forward the broadcast" and I'm not sure if "forward" = "route on layer 3" as well as the dhcp server is not connected to the FE 0/1 LAN switch. Just not sure if the DHCP server needs to connect to both LAN switches or not?

3. I guess that implies the DHCP server configures two set of pools and serves both sets of clients, right? (just want to make sure as it's my first time dealing with dhcp).

Thanks in advance for your expert advice to help out.

[thisisshanky]

1. You are right. Configure under fa0/1 the following command.

int fa0/1

ip helper-address 10.1.1.5

ip address 10.1.2.1 255.255.255.0

2. The client sends a DHCP broadcast on 255.255.255.255. This broadcast is received by the router, since helper address is configured, it routes that packet with destination address of 10.1.1.5 and sends it to the DHCP server. This is similar to Microsoft DHCP relay agent configuration.

3. YOu need two scopes and also default gateway option defined for these scopes. (You can add, DNS, WINS or other options as required). When the DHCP request from the client is received and before routing the packet to the DHCP server, the gateway also inserts the ip address 10.1.2.1 into the GIADDR field of the DHCP packet. The DHCP server receives this packet, reads the GIADDR field and thus determines what scope to choose from to lease the ip address to the client.

Hope that makes sense!

[j.vong]

Hello Shanky,

Thank you very much for helping out. I understand #1 and #2. #3 is a little lost. May I ask for clarification?

3a. When you mean gateway also inserts the 10.1.2.1 into the GIADDR, do you mean "gateway" = "router" we are referrng to?

3b. What is "scopes"? Does that mean two sets of pools (i.e. 10.1.1.x and 10.1.2.x) on the DHCP server?

3c. What is the default gateway option you are referring to? Are you referring to the dhcp server or the dhcp clients in the new LAN? If you mean dhcp clients in the new LAN, do you mean those clients needs to configure 10.1.2.1 as their default gateway pointing to FE 0/1? How does that related to the dhcp request? Or do you mean something else?

3d. So when the DHCP server receive the request, and see 10.1.2.1 in the GIADDR field, does that mean it will know to select an IP address from the one of two pools of 10.1.2.x and send this packet back to FE 0/0 and the router routes it to FE 0/1 and send back to the client?

3e. Does that mean we are ok that the dhcp server on 10.1.1.x LAN does "NOT" need to have any physical connection to the FE 0/1 LAN switch, is that correct?

Another separate question:

4. The existing FE 0/0, there is no ip helper address configured. Is it because the dhcp server already sits on the same LAN subnet as the router FE 0/0 with the clients, so the router does not need to forward any dhcp broadcast back to the dhcp server as the dhcp server will receive the request directly from the client, is that correct?

Sorry for so many questions, but being the first timer, want to make sure I fully understand this before implementing it on a live network.

Thanks again very much!!!

[ashok_boin · Accepted solution]

Hello,

Answers to your questions....

Your setup is like this one....

DHCP server(10.1.1.5) -- {Fe0/0(10.1.1.1)ROUTER or Gateway Fe0/1(10.1.2.1)}

Your DHCP server 10.1.1.5 is configured with two scopes or pools to assign IP addresses to client requests.

Scope 1 : 10.1.1.0/24 with default gateway option set to 10.1.1.1

Scope 2 : 10.1.2.0/24 with default gateway option set to 10.1.2.1

3a-d. Let's consider the request is originated from a client on Fe0/1 interface. IP helper address makes the Router to forward the DHCP broadcasts to 10.1.1.5 which is DHCP server. While forwarding the request, it will see from which interface it received the request and it will set GIADDR field accordingly. So, in this case it sets to 10.1.2.1 as it received from Fe0/1 interface and forwards to DHCP server.

You need to think from the perspective of the DHCP server, how and which IP address should it assign to the requested clients from different networks. After receving the request, DHCP server will find out the scope or pool from which to assign the IP by comparing GIADDR field in the request and local scopes default gateway values. In this example, it will match to Scope 2 and it will assign one free IP from that pool.

3e-4: Yes, you are absolutely right for 3e and 4.

You are welcome with more questions :-)

All the best...

-Ashok.

[j.vong]

Hello Shanky and Ashok,

Thank you very much for both of your great help. I'm ready to implement this in the network.

Thanks again, you are GREAT!!!