DHCP & Router on a Stick Config Issues

[MattPanton]

Hi, 

So I've got a 2960 switch that has a trunk link to an 1841 router, the router is in an OSPF network with two other 1841's and this all works fine, the 2960 switch has 2 VLANs, VLAN 35 is Server Data and VLAN 30 is PC Data, I can ping across the VLANs using the router on a stick, but DHCP doesn't work at all, I've added an IP helper onto the interface VLAN 30 on the switch which is the DHCP servers IP address, and on the sub interface on the router for VLAN 30 I've also added the same IP in the IP Helper address. But my clients cant pick up dynamically assigned addresses, please tell there is something really silly I've not done? If you need configs, happy to provide.

[Jon Marshall]

If you are routing the vlans off the router then you don't need the ip helper address on the switch SVI.

What is the DHCP server ie. is it the router or a server ?

If it's the router probably a good idea to post the router configuration.

Jon

[MattPanton]

Its a server attached to port fa0/24 of my 2960 in question

I'll give removing the helper address a go and see what happens

[Jon Marshall]

I doubt that will make it work, I just wanted to clean up your configuration.

So if you give a client a static IP from the right subnet then you can ping both of subinterfaces on the router ie. it's own default gateway and the IP of the other interface ?

Jon

[MattPanton]

Hi Jon,

Yes, can ping both the sub interfaces successfully, I've attached my current configs if this helps?

London is the router in case it isn't obvious

Matt

##Edit##

And if I attached another DHCP server fa0/23 of my switch and put the port in access vlan 30 then devices get IP addresses

[Jon Marshall]

I can't see any "ip helper-address ..." command in the router configuration ?

Jon

[MattPanton]

Hello,

I've added the IP helper to the fa0/0.30 sub int, I had it on the switch instead, I gues because I normally work with L3 switches instead on this type of set-up, I understand now that the IP helper needs to go on the devices that does the routing and in this case it was the router itself.

I've added the ip helper to the router and guess what it works!

thanks for the help!!