Exchange 2010 SP1 Stretched DAG node

Just completed an interesting deployment and I thought I would capture a few key points of interest.

The scenario is extending an existing Exchange Database Availability Group to include a copy in an offsite datacenter located over a 10mb WAN with 50ms latency from the local DAG HA pair. Note that this particular scenario does not include a DAG pair in the DR datacenter, just a single node. So we will not be configuring a separate witness fileshare for the DR side.

First off, this KB article from Microsoft goes straight to the heart of the configuration, so if you are looking to go in depth, just click and start reading.

After going through the process, I think I can boil down the process to the following key points

  • When adding new nodes to an existing DAG cluster, especially in another datacenter, server builds may vary. It is exceedingly important that the DAG members match each other in the area of NIC count. They all have to have the same amount of NICs visible to the OS.
  • If you are a straight Exchange admin who does not normally work with network routing, this process does require you to work some with NETSH if you have broken out your replication network from your MAPI network (and you should be doing that!!). Since a given server should only have 1 default gateway (on the MAPI network) your replication network wont have a default gateway. NETSH static routes will establish this for your replication network.
    • Syntax is NETSH interface ipv4 add route <remote replication subnet> “REPLICATION NIC NAME”  <local replication network default gateway>
      • netsh interface ipv4 add route “DAG REPLICATION”
    • this gets run on every DAG node so they can talk to other replication subnets
  • Unless you are in an unusual situation and have a flat network covering both your primary and your DR site, you will be working with 2 different subnets for your MAPI network. For each MAPI NIC you have in a unique subnet, you must have a unique DAG virtual IP assigned in that network as well, and you must add the DAG IP to the DAG in Exchange Management Console (pictured below)



  • Add your DAG node from the Exchange management console, not the failover cluster manager.
  • Reboot the new DAG member after adding it to the DAG and make sure to restart an existing Exchange management console session.

That’s it! Pretty easy process once you have gone through it once and very handy for replicating your Exchange data to a safe location. Just be sure your latency is not over 500ms and expect that large mail databases are going to take a while to seed. You have ZERO control over throttling the seed process from Exchange, so if you have to throttle it, you will need to involve your network team and throttle it from the firewall.


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