DHCP Server change

Hi Guys,
I have a network that currently has the DHCP Service running in the firewall (172.27.187.1) and it is giving out IPs on the 172.27.187.0/24 range. All of this is happening in Vlan1

I will move this service to a Windows Server (10.10.88.100) currently running in Vlan88.

Both Vlans exist in the same Cisco Stack and there is intra vlan routing up and running well right now.

The new DHCP server will be giving away IP addresses in the 10.10.70.0/24 range, and the old DHCP service (172.27.187.0/24) will be disabled.

I have already added a secondary IP to my Cisco Stack (10.10.70.1) to Vlan1 and I have tested with a static IP configuration in a test PC.

IP: 10.10.70.30
SM: 255.255.255.0
GW: 10.10.70.1
DNS: 10.10.88.100

This is a 24/7 production environment so I can't test this new DHCP server, but I think that all I have to do now during the window I will request is to disable the current DHCP service in the Firewall, (172.27.187.1) and enable the new DHCP Service in the Windows Server(10.10.88.100).

Then go PC by PC running the following commands:

C:\ipconfig /release
C:\ipconfig /renew

That action should trigger a new DHCP request, and since the only DHCP Server running is my new 10.10.88.100 it will find it and get an IP from the new IP range (10.10.70.0/24)

My questions are:
Am I missing something here?
I want to see this also from the routing perspective, my thinking is that as long as there is intra vlan routing the DHCP request from a PC in Vlan1 should go into Vlan88 and find the DHCP Server, am I correct?
cargexAsked:
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cargexAuthor Commented:
Hi Guys,
I'm reading some articles and it seems that I need to put in my dhcp server IP address in the "helper IP" configuration of my switch.

Does this sound like I'm going in the right direction to you?
I have never configured anything related to this "helper IP".
Muhammad MullaSystems AdministratorCommented:
Yes, you will need an "IP helper-address" configured for each VLAN on the switch, pointing to the IP address of your DHCP server.

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cargexAuthor Commented:
Hi OfflineGeek,
Can you please confirm if the following code will work in my case?

SW# config t
SW (config)# interface vlan1
SW1(config-if)# ip helper-address 10.10.88.100
SW1(config-if)# ^Z
SW1#

And this will forward all DHCP requests from vlan1 to the server in vlan88.
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Muhammad MullaSystems AdministratorCommented:
Yes, it looks right to me.
cargexAuthor Commented:
Ok, I will test this configuration as soon as I can get a Window to do so, and I'll come back to close the case.
cargexAuthor Commented:
Good Morning Guys,
I just added the "IP Helper"  to my vlan and it didn't work.
The client PCs can't find the DHCP Server.

Please read my complete description of my configuration above.

This is my current vlan1 and vlan88 configuration

!
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
!
interface Vlan88
 ip address 10.10.88.1 255.255.255.0
!

Please help before my Window for testing closes at 8:30AM !!!!!
Benjamin Van DitmarsSr Network EngineerCommented:
You need to put the helper address at the vlan witch need the dhcp server.
i think you put it in the wrong vlan.

and it's normal that you cant ping the dhcp. because dhcp is a broadcast protocol.
when a dhcp request is detected the switch tell the helper address to respond.
and then youre client will get an dhcp offer
cargexAuthor Commented:
Ok, so in that case my configuration is right, and the issue might be the DHCP Server itself.
The new DHCP Server is running in a brand new installation of Windows Server 2012, and I have already Authorize the server in Active Directory and I have Activated the new scope.

I will try connecting a PC to a port with access to the vlan88 and I will comeback.

In the meantime if you know why my DHCP server isn't working please comment.

Thanks.
Benjamin Van DitmarsSr Network EngineerCommented:
What vlan is going to get ip address from you dhcp server ?
what is can see, this is vlan 88 and vlan88 must get the helper

Benjamin
cargexAuthor Commented:
I have my users in vlan1

!
 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 configured the DHCP Server to give out IPs in the 10.10.70.0/24 network.

My DHCP Server is in vlan88

!
 interface Vlan88
  ip address 10.10.88.1 255.255.255.0
 !

So, according to the comments above my ip helper should be in vlan1 the way it is right now
Benjamin Van DitmarsSr Network EngineerCommented:
yes, that's correct. and that should work. what options did you configure at the dhcp scope
and is the server that runs dhcp directly connected in vlan88 ?
cargexAuthor Commented:
Guys, it was my DHCP Server, since my scope is giving out IPs in a different IP range (10.10.70.0), I had to create a superscope and it began to work inside the same vlan.

Now I need to test this ip helper.
Since I can't disable the current DHCP Server right now because is needed in production, how can I test this?

Can I force a PC to get an IP address from a particular DHCP Server?
Muhammad MullaSystems AdministratorCommented:
Depending on how long your lease is, you should be safe to disable your old DHCP server for a short while to test. Generally, DHCP leases are configured to 8 days in my experience.

You will only need to disable for a minute to test and can then change back and migrate when you're ready.
cargexAuthor Commented:
Hi Guys, OfflineGeek was right all along, as confirmed by Benjamin. The IP Helper did the trick, the problem was my DHCP Server that wasn't working properly. Once I got my DHCP Server to work, I created a new vlan, added the ip helper command  and connected the test computer there, everything worked beautifully. Thank you very much.
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