DHCP Relay Issue

[OddievenInc]

Hi everyone, 

I'm having issues with obtaining DHCP from a different subnet. Did some troubleshooting, but got to the point where I had to reach out for support. 

I'm trying to obtain a DHCP IP for the client directly connected to the Branch Office Core switch. I can ping the DHCP Server IP from the branch office core switch. 

Attached is the topology, and configuration related to the DHCP relay etc. Is this viable? If it's wrong what is the correct way of setting things up? Appreciate your time and comments.

[OddievenInc]

Hi Harold, Good day.

Thank you ever so much for the response. This is well Noted. 

However, I made some changes, would this work? I want to have the DHCP Pool for the Branch Office (vlan 10) configured in the Head Office Core Switch, rather than having it locally. (this is actually due to a future change; where there will be a centralized DHCP server located in the Head Office premises, for both head office and Branch Office.)

I have configured 'interface vlan 10' in both branch office and head office. Pls refer below. Will the branch office send the DISCOVER Packet out of it's exit interface? and reach the Head Office core switch where the DHCP Pool is located, and ultimately client connected to the branch office in vlan 10 access port getting an IP address. 

[Harold Ritter · Accepted solution]

Hi @OddievenInc ,

Do not configure vlan10 on the head office side. Just the pool. You also need to add the following command on the head office side:

ip dhcp excluded-address 10.140.48.1
ip dhcp pool vlan10
network 10.140.48.0 255.255.252.0
default-router 10.140.48.1
dns-server 8.8.8.8

no interface vlan10

and on the branch side:

interface vlan10

ip address 10.140.48.1 255.255.252.0

ip helper-address 10.3.0.1

Regards,

[OddievenInc]

Hi @Harold Ritter 

Thank you so much. This is well understood. 

I will follow the above steps.

Also, any references/configuration guides to solidify the statement "Do not configure vlan10 on the head office side" I would greatly appreciate if you could provide any if you have please. Would be handy to know the reasoning. 

[Harold Ritter]

Hi @OddievenInc ,

Configuring interface vlan10 (same subnet) on both the head office and the branch office does not make sense and would break your routing scheme. The head office router would consider subnet 10.140.48.0/22  to be directly connected and would therefore never send traffic to the host connected to that same subnet in the branch office.

Regards,