Your network consists of one Active Directory domain. The network contains one Active Directory
site. All domain controllers run Windows Server 2008. You create a second Active Directory site and
plan to install a domain controller that runs Windows Server 2008 in the new site. You also plan to
deploy a new firewall to connect the two sites. You need to enable the domain controllers to
replicate between the two sites. Which traffic should you permit through the firewall?

A.
LDAP

B.
NetBIOS

C.
RPC

D.
SMTP

Explanation:
You should permit RPC traffic through the firewall to enable the domain controllers to replicate
between the two sites because the Active Directory relies on remote procedure call (RPC) for
replication between domain controllers. You can open the firewall wide to permit RPC’s native
dynamic behavior.

Active Directory Replication over Firewalls
http://technet.microsoft.com/en-us/library/bb727063.aspx