What is BridgeHead Server in AD ?

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.