Do you manage a DNS server in an advanced Active Directory Forest?  I recently ran across a great article by Gary Olsen on SearchWinIT that explains a best practice for configuring the delegation of the _msdcs zone.  The reason I point this out is because many people probably don't think too much about their DNS configuration until something doesn't work correctly.  Sometimes it's worth fine tuning the behavior to maximize the performance capabilities.

From the article:

When a client operation, such as authentication, needs to find a Global Catalog server, a DNS server is then queried and a specific global catalog resource record is found and returned to the client. The client subsequently contacts the GC. Other operations also query the GC (such as Exchange Server, cross domain searches, etc.) so locating the GC resource records is definitely an important function. Global Catalog records are located in the GC container under the _MSDCS subzone.

CName or "Alias" records are used specifically for replication. Domain controllers must communicate for replication by using each other's server globally unique identifier (GUID). In order to do that, a CName record that maps the server GUID to the fully qualified domain name (FQDN) of the DC is created in the root of the _MSDCS subzone.

The CName records and GC records are in the _MSDCS zone. So what? Well, in a forest with two or more domains, the _MSDCS zone is different. The first domain created in the forest, of course, has the _MSDCS zone, which, as noted previously, contains the CName records and the GC records. All subsequent domains, however, will not contain CName or GC records (although they will each contain a _MSDCS zone). Thus, those records are only stored in the _MSDCS zone in the first domain in the forest, or the forest root domain.