A Single DAG extended across two active directory sites(Site A & Site B). 6 mailbox Servers in Primary site & 3 mailbox servers in DR Site having 150 databases in total. All are installed with Exchange 2010 SP3 & Update Rollup 6. Over a period of time, high bandwidth utilization was noticed between these sites. Upon investigation, it appeared that the reason is because of content indexing traffic. As we all know, replication traffic consists of both log shipping & Content indexing but Content indexing uses Mapi N/W & log shipping uses replication n/w. We started working with Microsoft Premier Support.
Though the actual problem is the N/W design between the sites, there was also a design behavior in Exchange which we were not aware until that day. MS said “When you have more than 50 empty databases part of DAG extended across sites, Exchange will utilize more content indexing traffic. Hence create databases only when required” though it was weird.
Issue did get resolve after creating test mailboxes in all the empty databases as suggested by MS but Support person did not want to write this as a bug in EMail.