A bridgehead server is a domain controller in each site, 
which is used as a contact point to receive and replicate 
data between sites. For intersite replication, KCC 
designates one of the domain controllers as a bridgehead 
server. In case the server is down, KCC designates another 
one from the domain controller. When a bridgehead server 
receives replication updates from another site, it 
replicates the data to the other domain controllers within 
its site.