Cisco ip helper address doesn't work for DHCP Packets

cargex
cargex used Ask the Experts™
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Hi Guys,
I have a Cisco 3750 Stack.
My goal is to change the IP range for Vlan1.

OLD IP RANGE: 172.27.187.0/24  (current)
NEW IP RANGE: 10.10.70.0/24 (new IP range)

I just configured the following

interface Vlan1
 ip address 10.10.70.1 255.255.255.0 secondary
 ip address 172.27.187.254 255.255.255.0
 ip helper-address 10.10.88.100
!

I have a new DHCP Server in the server 10.10.88.100 (Vlan88) giving out IP addresses in the 10.10.70.0/24 IP range that works, I have tested it with  a test vlan and connected a test computer to the test port and the ip helper worked on that test.

Now I disabled the current DHCP Server in Vlan1 (production environment) that is giving out IP addresses in the range (172.27.187.0/24) and added the ip helper line as shown in the configuration above.

But this time it didn't work, meaning that none of the computers in vlan1 were able to see the DHCP Server.

Now that I'm writing this I'm thinking that it could be that the 10.10.70.1 is a secondary IP address in Vlan1, may be it needs to be the primary IP address of that Vlan1 so that the DHCP Server identifies that the computers in Vlan1 can receive IPs in the 10.10.70.0/24 range. But all this are just ideas.

Can somebody please shed some light on this issue?

Thanks.
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Top Expert 2015

Commented:
ip dhcp smart-relay

Configure the above line under vlan  interface config.. This will solve ur issue

Author

Commented:
Hi NetExpert,
I have DHCP running on the cisco switch and configured for the voip vlan, obviously I don't want to mess that up.

Can you please give me the details?

Is this a command that I would run on the Vlan1 interface?
Top Expert 2015
Commented:
Yes. ip dhcp smart-relay will change the gateway address on the DHCP packets.

You need to configure this command under vlan 1
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Author

Commented:
Actually I need your opinion on a second way to do this.

interface Vlan1
  ip address 10.10.70.1 255.255.255.0 secondary
  ip address 172.27.187.254 255.255.255.0
  ip helper-address 10.10.88.100

Currently Vlan1 has an IP in the 172 range as primary, and the IP in the 10 range as secondary.
And I think that is why the DHCP request didn't went through.

What if I make the 10.10.70.1 the primary
and the 172.27.187.254 the secondary

Would that work?

The 172 range will disappear after this works.

And this way I don't have any additional delays for the DHCP request.

What do you think?
Top Expert 2015
Commented:
Yes.

That good be a good option too as you are going to wipe out the 10.x network range.

When you make 172.x network as primary the new DHCP request packets will process quite fast and there won't be any relay.

Again, before changing the IP address on the vlan interface, you might need to clear the old ip DHCP pools as the machines will have their old ip
Range for 8hrs in usual scenario.

Author

Commented:
NetExpert,

Ok.
Since this is a production environment I just requested a Window, and I will try changing the primary and secondary IP addresses on Vlan1.

I'll be back with the results tomorrow morning, if it works you will have the correct solution and I will close the case, thanks!

Author

Commented:
Netexpert,
It worked like a charm.
Thank you very much for all your help.

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