If I were you I would think about doing this in a slightly different manner. Aquisitions
are a point where most enterprise networks take a turn for the worse because speed of integrating the purchased company takes precedence over doing it right...so far in my consulting life I've seen a LOT of them. It can be slightly less painful to do it quickly, but all you're doing is kicking the can down the road and it's often WAY more painful after a couple years when you start having major problems.
My approach would be something like:
1. Determine if resources on the old network still need to be accessed (ie: servers & data).
2. Figure out what you need to do in regards to partner and vendor connectivity of the purchased company.
3. Design the new network for the purchased company - sites, datacenters, etc.
4. Build a new infrastructure at the sites that has connectivity to your existing enterprise network.
5. Create a L3 connection between the new network and the old network.
6. Migrate users off of the old network (handful of test users first), and make sure the PCs conform to your companies policy.
7. Migrate applications off of the old network as needed, but you should try to have them use your existing stuff if possible.
8. Move partner and vendor connections as needed.
9. Shut down the connection to the old network.
10. Remove and retire the old equipment after a waiting period.
Obviously there are MANY more steps in there, and it really all depends on what needs to be done. My experience though, is no matter what you need to make the new company conform to your existing enterprise standards, or you are just guaranteeing that you'll have more problems down the line.
