Data center migration strategy SAME IP SUBNETS IN 2 LIVE ENVIRONMENTS (cold cut over or gradual)

We are consolidating multiple regional data centers into a single data center (Hosted vSphere).  The requirements are that the new data center keeps the same IP addressing schema as they used to have. At the same time they can't decide how they will cut over to the new data center (Cold Cut or gradual migration) In my opinion they need to decide which method they will use as this will alter the Network design at every level..

Please don't ask me why the decision makers are insisting on this I have already fought a political battle about how outrageously stupid this request is and how the Network is not a magical unicorn that can sparkle all the issues away. I still cant get my head around how to allow the same IP subnet to sit in 2 connected routable L3 Networks without doing some NAT voodoo and Host routes.

Their justifications is that they don’t want to have to change any Hard coded IP addresses in the Applications but they are not even sure if there are any hard coded IP's in the first place. They rely on an external vendor to manage this for them and any time you ask them for anything they charge an extortionate amount of money even to look into anything.

So my question is.. Have any of you come across this kind of lunacy and do you have any advice or knowledge about how to approach this migration?

I can't see how both the current IP addressing/DNS scheme can live at 2 physical locations at once without massive L3 issues.

After searching high and low I want to avoid NAT since we don’t know if the applications can live with that. I think possible "Stretching the VLAN" may be a good option that way we can gradually migrate the VM's across and we will not be forced to do a cold cut

All thoughts welcome !
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I still cant get my head around how to allow the same IP subnet to sit in 2 connected routable L3 Networks without doing some NAT voodoo and Host routes.
Several options for this.  Layer 2 tunnel protocol (L2TP) has done this for some time.  OTV will do it, with the benefit of preventing "traffic tromboning".  If you're pure VMware you can also look at NSX as a possible solution.

For a temporary solution, though, I'd suggest L2TP is your best option.  There will be some sub-optimal traffic patterns, but depending on the length of the migration you can probably overcome it.

Reference on L2TP:
ASAVENER had some great suggestions. EoMPLS is another method of accomplishing this task.
Andrew Hancock (VMware vExpert / EE MVE^2)VMware and Virtualization ConsultantCommented:
Stretched VLAN is the simplest solution.
Thirst4KnowledgeAuthor Commented:
Unfortunately NSX & EoMPLS are not an option (Costs)

Do I need any consideration for when we want to "turn off" the Old Data center after we have migrated all the VM's ?

I would have thought some routing would need to be changed
Easiest way is to set up a router at each physical location with HSRP or VRRP enabled, then when you turn down the original site your don't need to change any routing, default gateways, etc.

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