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I have a set of IP's for dedicated servers which I will be losing soon and have a new block of IP's that I need to assign. The plan is to use a cisco router with dhcp to assign new IP's by mac address. We will have a new dedicated line coming in that will go through the router and I intend to use the IP dhcp pool command to set the ip per mac address. The servers will then be switched from static set IP's to dynamic to hopefully seamlessly change ip's without interruption.

My question is will this work without conflicting with the current IP's/connection? And also would using the dhcp pool and host commands work for assinging the new IP's?

Thanks in advance for any advice.

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There are a lot of factors to consider for something like this. Do you have overlap between when you lose the current addresses, or do you need to cut over all at once? What amount of downtime is acceptable? Do you have physical access or another alternate means to deal with issues if connectivity is lost?

The most seamless way to manage this change would require overlap - the ability to have both blocks of addresses coming in to the router at the same time would be a great help in cutting downtime and allowing you to move devices over to the new addressing scheme at a pace that may fit your needs better than a mass cutover.

If that's workable, then you can add a secondary IP to your router:

int g0/0
 ! old network block (existing config):
 ip address
 ! new address space:
 ip address secondary

This makes both of these subnets available in the same broadcast domain; you can have a device statically configured as with its gateway pointing to, alongside a device asking for a DHCP address and receiving one in the subnet, pointing to as its gateway.

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I appreciate your response. We do have overlap and if necessary we could have physical access (its just less than ideal). If were using a config file to assign the new ip addresses through the router once the server switches to dhcp will it then pick up the new ip? – user82663 May 27 '11 at 23:08
Not sure if I quite understand your meaning, but it sounds like you are on the right track. You'll want to have the DHCP set up and working before switching any devices, then you can switch them over to DHCP configuration and they'll grab one of the new addresses. It's just a little worrying to flip that switch remotely, since you could potentially lose access if the device doesn't get a lease for some reason. – Shane Madden May 28 '11 at 0:32

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