Jorge 's Quest For Knowledge!

This is so simple and people still ask questions in the newsgroups about it. It has been answered sooooooooo many times it almost hurts typing this over and over again....

Concerning the Infrastructure Master FSMO role and the GC role look at it from the domain perspective. If all DCs in a certain domain are GC then it does not matter if the Infrastructure Master FSMO of that domain is on a GC. You don't have any other choice though. If you at least have one OTHER DC (besides the Infrastructure Master FSMO itself) in that same domain that is/will NOT be a GC then do not put the Infrastructure Master FSMO of that same domain on a GC.

For more info on this read: MS-KBQ248047_Phantoms, Tombstones and the Infrastructure Master.

Short and sweet rule of thumb:
No matter what forest structure is, the following rules apply for EACH domain in the forest
(1) If all DCs in a domain are GC, there is no other choice where to put the Infrastructure Master FSMO. So no issue here!
(2) If at least one or more other DCs in a domain (besides the Infrastructure Master FSMO itself) are not a GC, then the Infrastructure Master FSMO should NOT be on a GC. (If the Infrastructure Master FSMO is the ONLY DC within the domain that is not a GC, make it a GC as there is no other DC that is not a GC!)