That depends. In a single domain forest, or a domain in a multi-domain
forest where all DCs are GCs, there is no issue and you can have them
The only time you can't is when you have a multi-domain forest and there are
DCs in a domain, that aren't GCs, and the IM sits on a GC. Under such
circumstances the phantom cleanup thread cannot run, as it needs to compare
the DN and SID of foreign objects against those of the phantoms it holds.
It does this by cracking the GUID held locally against a GC.
See this for more info:
Microsoft MVP - Windows Server - Directory Services
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