DHCP server and router-on-a-stick are diff devices for dhcp client

[chch6597]

Hello all,

I have been struggling with a problem I kind of create myself, or my knowledge is still not good enough, hopefully some professionals can help me. Look at my picture:

My PC wants to get IP address from DHCP server, but they are in different vlan so I put a router on a stick with sub-interfaces .10 with 192.168.10.254 and .20 with 192.168.20.254, but my PC doesn't get IP address. I also configured default route on PC and DHCP server that points to the Router. I did some research and people suggest to do ip helper-address on Router or Switch, but I tried both, it's still not working. I can see my DHCP server can get DHCP discover but that's it. Am I missing anything? or this is impossible?

Thank you for your answers

[luis_cordova]

Hi @chch6597 ,

Your configurations should be something like this:

Switch L3:

g0/0

switchport mode access

switchport access valn 10

g0/2

switchport mode access

switchport access valn 20

g0/1

switchport trunk encapsulation dot1q

switchport mode trunk

Router

f0/0

no ip address 

f0/0.10

ip address 192.168.10.254

ip helper-address <ip dhcp server>

f0/0.20

ip address 192.168.20.254

Regards

[chch6597]

@luis_cordova 

Thank you for your reply! That's what I have right now but my DHCP server can only get DHCP DISCOVER message. That's it. I wonder if I missed anything else

[saif akbar]

can you ping PC<---->DHCP ?

[chch6597 · Accepted solution]

I kind of figure it out. Instead of the command, 'network 192.168.20.0 255.255.255.0', I put 'network 192.168.10.0 255.255.255.0', I'm not really sure why, but my best guess is the sub-interface of Router is 192.168.10.254 so it will give out the same subnet from the vlan 10