DHCP

Changing DHCP scopes with zero downtime for your users

fixitDave Networking , , , , , , , ,

Changing scopes for user PCs with no downtime is easier than you may think.

Process;

  • Find devices with static IPs within the current scope
  • Create new DHCP scope
  • Configure the gateway device
  • Deactivating the old scope

Find devices with static IPs within the current scope

You will need to locate all the devices with static IP and make plans to port them to the new scheme.

There are many tools that you can use but I would recommend Lansweeper, use the free trial or even purchase a 12 month subscription…it will be worth the cost.

Once you find devices with static IPs, make your plan to move them to the new scheme.

Create new DHCP scope

Open the DHCP MMC and create your new scope, remember to match any extra options that is configured in the existing scope and any exclusions that need to be configured.

Right click on the DHCP server within the MMC and select Create Superscope, within the wizard give the superscope a name and select the current and new scopes (ctrl+mouse click).

This will join the two scopes into the superscope and make sure the new scope is deactivated.

Configure the gateway device

We use Hewlett Packard Procurve L2/L3 switches which don’t have the option of using a secondary IP address on a VLAN.

This creates an issue that the lowest IP address configured on the VLAN interface is the one that will be used as the source IP when connecting to the IP Helper (DHCP server).  Take a look at the following VLAN configuration, devices will get an IP from the 192.168.36.0 scope and NOT 192.168.80.0 scope as the 36 scheme is the lowest IP on the interface.

vlan 80
Name Users
ip helper 192.168.1.10
ip address 192.168.36.0 255.255.255.0
ip address 192.168.80.0 255.255.255.0
exit

Knowing this, if the new scheme is LOWER than the current one, the new scheme will be used as soon as it’s configured on the VLAN – reason why you should make sure the scope is deactivated.

At this point you can test by using a statically assigned IP to a device to make sure the new scheme works as expected.

Deactivating the old scope

Before you can deactivate the old scope, you need to activate the new one…remember if the new scope is LOWER it will be used straight away.

If the new scope is higher, you need to deactivate the old one before it will be used.  Once deactivated keep an eye on the allocations for the new scope and once some IPs have been allocated make some checks to make sure all is ok.

Once you’re confident all is ok, you can force the move of devices to the new scope quicker by deleting all the allocated addresses on the old scope.

A device using DHCP will request an extension to the allocated address three times before it asks for a new address;

  1. After 50% of the lease time – on an 8 day lease, the device will ask for an extension after 4 days
  2. After 87.5% of the lease time – on an 8 day lease, it will request a renewal after another 3 days
  3. The last request is at 100% and will send out a new address request for any available DHCP server

As you can see, deleting the allocations and disabling the the old scope will speed up the transition to the new scope.

Once there are no devices on the old range, remove the scopes from the superscope and delete the old scope and the old IP address from the VLAN interface.