Exchange 2010 allows us to have Database Availability Group (DAG) members in several AD Sites. For every subnet a DAG member’s MAPI NIC is in, we must obtain a DAG IP. This DAG IP is a separate IP than is located on the MAPI NICs themselves. We take this DAG IP to the DAG using the Set-DatabaseAvailabilityGroup command.
Multiple DAG IPs
Let’s take a look at an example of how the architecture may look.
Taking a look at the above Visio diagram, we have two sites, Primary Site and DR Site, with one node in each. The MAPI NIC in the Primary Site has an IP Address of 172.17.24.200. That means that we’ll need to have a DAG IP that lives in this same subnet. We choose a DAG IP of 172.17.24.120. The MAPI NIC in the DR Site has an IP Address of 172.16.24.200. That means that we’ll need to have a DAG IP that lives in this same subnet. We choose a DAG IP of 172.16.24.120.
In order to add these MAPI IP Addresses, we’ll need to run the following the command.
Note: IPs on Replication NIC’s subnet do not get added to the Database AvailabilityGroupIPAddresses. Only MAPI NIC Subnets get added.
Keep in mind, when adding additional IPs in the future, it is important that you include all existing DAG IPs. The Set-DatabaseAvailabilityGroup -DatabaseAvailabilityGroupIPAddresses property is not additive.
To verify the DAG IPs were added successfully, let’s check out our DAG Properties.
In Exchange 2010 SP1, we have the ability to add our DAG IPs via the GUI. If we go to the DAG Properties, we now see we can manage our Witness Server and Alternate Witness Server.
This allows us to do our IP Address configuration right from the GUI instead of needing to use Set-DatabaseAvailabilityGroup with the DatabaseAvailabilityGroupIPAddresses property and needing to worry about all previous IP Addresses being included since the property isn’t additive.
So, let’s take a look at what really happens to the cluster resources and what determines which DAG IP is active. Let’s open the Failover Cluster Manager. Start > Administrative Tools > Failover Cluster Manager.
After selecting our DAG, let’s take a look at the cluster resources. We can see from here that we have two Network IP Resources.
But let’s take even a deeper look.
Select the DAG from within the Cluster Core Resources > Right-Click > Choose Properties.
Now let’s take a look at the Dependencies Tab.
As we can see, the two DAG IPs are set up with an OR dependency which means that the cluster can activate either DAG IP at any given time. As we saw earlier, the 172.16.24.120 IP is the existing DAG IP that is online which means the DRSiteNode’s DAG IP is currently the online Network IP resource.
Let’s run a cluster command so we can failover the default “Cluster Group” from one cluster node to another.
We now see the PrimarySiteNode is the node that has the “Cluster Group.” Let’s go ahead and take a look at the Cluster Resources again and see which Network IP Resource is online.
Looks like the PrimarySiteNode’s DAG IP is now Online instead of the DRSiteNode’s DAG IP. This means that the Network IP Resource that is online depends on which DAG Node has the “Cluster Group.” If you recall from my previous articles, the DAG Node that has the “Cluster Group” is the DAG Node that acts as the Primary Active Manager. The Primary Active Manager is the DAG Node responsible for choosing what databases get activated in a failover. For more information on Active Manager, click here.