SMTP Site Links in Active Directory - When To Use

I'm sure most of you Microsoft Administrators out there have seen SMTP site links before. What are they and how are they different to standard IP site links used by the DRA (Directory Replication Agent - the thing that compresses replication data for inter-site replication).

If you have an extremely unreliable connection with high latency you must use SMTP Site Links and not IP Site Links. These are connections like satellite for example - satellite connections must use SMTP for replication as IP will not function correctly.

One thing about SMTP Site Links is it cannot replicate the SYSVOL, it only replicates things like the Schema Naming Context and the Default Domain Naming context. It can also replicate Global Catalog Data. Because SMTP Site Links cannot replicate SYSVOL, they can only be used between different domains in the same active directory forest (remember active directory sites and services is the physical network topology for all logical domains in a forest - not just for 1 domain like we usually see).

If you have a company with a remote site over satellite links, you need a domain controller on site, as latency in satellite is way to high to authenticate over the WAN. Your only option is to create a child domain for that remote site. This is not a bad thing, because remember child domains have transitive trusts and can access all resources in the parent domain anyway if granted permissions so don't be scared about creating additional domains - it is very easy. Most companies try and get away with just a single active directory domain - but some cases you are required to have additional domains!

For more information please read:

http://technet.microsoft.com/en-us/library/bb742427.aspx
http://support.microsoft.com/kb/244368