Exchange 2010 DAG Network Configuration

I have seen many DAG deployments with mis-configuration for DAG networks, especially when the DAG is accross multiple sites. I am posting an example configuration here to show how the DAG networks should be set up.

The example makes the following assumption:

• 2 AD Sites: Site1 & Site2
• 4-node DAG with each Site having two DAG nodes

IP address Configuration  

Server	Network\IP
MBX1	MAPI: 192.168.10.11 Replication: 10.1.10.11
MBX2	MAPI: 192.168.10.12 Replication: 10.1.10.12
MBX3	MAPI: 192.168.20.13 Replication: 10.1.20.13
MBX4	MAPI: 192.168.20.14 Replication: 10.1.20.14

By default, the DAG network will be set up like this: 

DAG Network Name	Subnets	Interfaces
DAGNetwork01	192.168.10.0/24	192.168.10.11 192.168.10.12
DAGNetwork02	10.1.10.0/24	10.1.10.11 10.1.10.12
DAGNetwork03	192.168.20.0/24	192.168.20.13 192.168.20.14
DAGNetwork04	10.1.20.0/24	10.1.20.13 10.1.20.14

The problem of this configuration is the cross-site log repliation will use the MAPI NIC on the servers instead of using the dedicated Replicaiton NIC because there is no DAG network that has been defined between 10.1.10.0/24 and 10.1.20.0/24.

	DAGNetwork01	DAGNetwork02	DAGNetwork03	DAGNetwork04
DAGNetwork01	MAPI	-	MAPI\Replication	-
DAGNetwork02	-	Replication	-	-
DAGNetwork03	MAPI\Replication	-	MAPI	-
DAGNetwork04	-	-	-	Replication

The solution is to collapse default DAG networks as below:

DAG Network Name	Subnets	Interfaces	Replication Enabled
DAGNetwork01	192.168.10.0/24 192.168.20.0/24	192.168.10.11 192.168.10.12 192.168.20.13 192.168.20.14	No
DAGNetwork02	10.1.10.0/24 10.1.20.0/24	10.1.10.11 10.1.10.12 10.1.20.13 10.1.20.14	Yes

Make sure log replication is disabled on DAGNetwork01 so that log shipping will always use the replicatio NICs first, but will still fail over to the MAPI NICs. If you don't disable replication on DAGNetwork01, DAGNetwork01 will have the same weight as DAGNetwork02 when the servers choose to ship logs.