There isn't really any document for this and no quick answer.
Your DC is currently advertising site's 3,4 and 5 subnets to site 1. You probably still need this as a backup (although you didn't answer my question in the last post) but what you need to do is -
1) make sure that site 2 is advertising site's 3,4 and 5 to site 1 as well
2) make site 2's advertisements the preferred routes rather than the routes received from the DC.
I'm assuming that if you want site 1 to go via site 2 to get to sites 3,4,5 then you also want return traffic from those sites to go back via site 2 and not the DC. If so -
3) you are also going to have advertise site 1's subnets from site 2 into the Sprint cloud so that sites 3,4 and 5 know where to send return traffic.
4) following on from 3) you will need to make sure that if site 1's Sprint link is up then sites 3, 4 and 5 go direct to site 1 and not via site 2.
This could get quite complex as you can hopefully see. It will involve BGP configuration and EIGRP configuration and the EIGRP configuration could get quite tricky.
I would suggest firstly looking at all the routing tables to see exactly what is where and you certainly need a full understanding of how everything works now before you can begin to make any changes.
Jon
