In a single domain environment, global catalog servers are not required to process a user logon request.
However, you should assign global catalog servers in a single domain environment for the reasons listed below:
Clients still seek global catalog servers for search operations.
Having global catalog servers already in place makes the system more scalable if you later choose to add more domains.
As always, the cost of global catalog replication is weighed against the need for speedier logon and response to global catalog queries.
None of the objects contained in an application partition are replicated to the Global Catalog.
Even if a domain controller that holds a replica of an application partition is also a Global Catalog server, the domain controller will not return any objects
from the application partition during a Global Catalog search.
This is registered only if the domain controller is also a Global Catalog server.
You can query
with nslookup to obtain a list of all the Global Catalog servers in the forest.
One interesting thing to note about SRV records is the seventh field, which designates the port used to contact the service on that host. In all of these cases,
3268 is used, which corresponds to the Global Catalog port. You may have also noticed the entries that contain Default-First-Site-Name.
Each Global Catalog server
registers site-specific records so clients can find the optimal Global Catalog based on their site membership.