Active Directory Replication Traffic

I was monitoring network traffic on our Exchange 2007 server when I noticed that every night between 2 and 3 AM the traffic spiked. The traffic went from 25MB/hour to 25GB/hour, a 1000 time increase.
Slightly nervous I did some packet sniffing and found that all that traffic was traffic between the Exchange server and the main domain server, the two active directory servers we have.
Is this kind of traffic normal?

Thank you
/Mats