This is what I would do in your situation.. -Listing what i have
to do to keep the boss happy and save money for your firm.

1. Keep old IP's at the old datacenter, hitting live servers
at the new datacenter 
2. Prevent the need to a second set of servers with DNS entries
seperate from the originals

My question first - why can't you leave just one server with a "sorry
maintence this weekend" sign up. Its perfectly accetable to allow
say 4 hours downtime. Your boss is probably just being a hardass.
Unless your Yahoo and can't afford any downtime, tell him to chillout,
go back to his boss and be a man.

That being said, I would... 

Get the old router (at the old colo)
to do a gre tunnel to the new router and the new colo. At the new colo
have the default route point to the other end of the tunnel (at the old
colo). The Old colo will be our "ip nat outside", with static
routes for the Servers Private IP's (now at the new colo) via the tunnel
interface.

Now, the new colo, will have NO default route, just a route for the
old colo router, via its normal next hop (probably the new colo's edge
equipment)... All incoming requests will hit the old router, go thru the gre
tunnel, and back out the old router from the servers, after going thru the
tunnel. This should work fine, I did something similar when we blew out of
Exodus one weekend :)


One more gotcha is have your DNS TTL taken as low as you can (say 5 minutes)
so you can just have this stupid thing running for a few hours. I dont think
you can have both at once, unless you can do a policy route for "it goes out
the interface it came in by" so it will be using old colo's and new colo's
ip addresses at once.

You will still have the downtime from the move of the servers.. but
it shouldn't be too much.. if you schedule it right, and have a low TTL in
dns, you can get by, with pulling your servers just when your DNS changes,
buy the time you get to the new site (and your DNS is mostly changed around
the globe, your back up)

Even if you had multiple servers (ones at each colo) you still have to
account for DNS issues.

Bottom line : is a few minutes to hours of downtime, worth buying a new set
of servers.



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