Question : Exchange and Global catalog

If I understood, in GC infrastructure we can have at least one domian controller configured as Global Catalog for a domain.

In a forest with multiple domains (Parent-Child),  and each domain with exchange  mail server, but all exchange servers belong to one  exchange Organization.
Now I wonder where the location of the global catalog that holds the GAL is ?

thanks

Answer : Exchange and Global catalog

Every domain controller that is acting as a Global Catalog server, in your case all 4 DC's, has a copy of the Global Catalog which it constantly synchronizes with every other Global Catalog server.  The Global Catalog is built from a small set of the active directory data from each domain, so it has a record of every domain user in all domains, their email address, phone number, etc.  Each Exchange server automatically queries the Global Catalog server it is using (usually the closest and quickest to respond) on a regular schedule.  Exchange then creates a Global Address List from the Global Catalog.  (Actually, it creates several lists by default, and can create custom Global Address Lists if you set them up).  Every time relevant information is updated in a domain, the Global Catalog server updates the Global Catalog, and that change is synchronized to the other Global Catalog servers.  Exchange then will see the new information when it queries its Global Catalog server and will update the Global Address List.

The short answer to your question is that since the Global Catalog is the same in every domain, and since the Global Address list comes from the Global Catalog, by default the information will be the same on every Exchange server.

Here is an example in your scenario:

Let's call the DC's MemphisDC01, MemphisDC02, MiamiDC01, and MiamiDC02, and the Exchange servers MemphisMail01 and MiamiMail01.  A Miami user opens his Outlook and opens his address book, and at the same time a Memphis user does the same thing.  Even though they are in different domains, they see the same Global Address list, including users, groups, etc. from both Memphis and Miami domains.  Why?  Because when the Miami user opened his address book, the information came from the Global Address List on MiamiMail01, which it got from the information in the Global Catalog held by MiamiDC01 or MiamiDC02.  When the Memphis user opened his address book, the information came from the Global Address List on MemphisMail01, which it got from the information in the Global Catalog held by MemphisDC01 or MemphisDC02.  But remember, MiamiDC01, MiamiDC02, MemphisDC01, and MemphisDC02 all have IDENTICAL copies of the Global Catalog, so when MiamiMail01 and MemphisMail01 create the Global Address List, they will end up with the exact same lists.

And busbar is correct that you can create additional GAL's and restrict who sees what with permissions.  That is how you would have Miami and Memphis look at different GAL's if desired.