New feature in Windows Server 2008 R2 to reduces replication traffic across the forest when a modification was made to one of the universal group membership. This is Windows 2008 Universal Group Caching.
By enabling Windows 2008 Universal Group Caching , global catalog server will not be needed for each site since data being cached to local domain controller and make use in data whenever a request comes for the group membership. Because of that, users that have no local global catalog server in site can login in a reasonable time speed.
To enable Universal Group Caching (for local site, should be done for each site):

1. Open “Active directory sites and services” < Browse to “Sites”\<YourSiteName>.

2. Right click “NTDS Site settings” < press “Properties” < mark the “Enable Universal Group Membership Caching” check box or choose a site to “refresh the cache from”

3. Click “OK”.