SCCM 2012 Client is smart enough to do not connect to pre-2012 site servers even being set with SMSSITECODE=AUTO. In reality it may happen even when you specifically set the site name but have pre-2012 sites in the same boundaries.
Of course, the best solution is do not have overlapping boundaries for sites (at least for site assignment, as it is supported in 2012 now), but in some situations (especially during migration) it is not possible unfortunately…
Another approach is to force SCCM Client Site assignment via GPO.
Microsoft provides us with two templates located on installation media in \SMSSETUP\Tools\ConfigMgrADMTEmplates folder. One of them is used to force site assignment to SCCM Client. The idea is to use this template to create a GPO with WMI filters to get it applied to machines with SCCM 2012 client installed only.
1. Create a new GPO and Add Admin Template to it:
2. Enable the site assignment settings and save the GPO
3. Create WMI filter and assign it to the GPO. Note: the filter looks for the ccmexec.exe file in c:\Windows\CCM folder. If you have the Client installed somewhere else please change accordingly.
4. Link the GPO to an OU containing your computers
5. Run GPRESULT (GPRESULT –r for Windows 7) to see if GPO is applied
IT Consultant Everyday Notes 