2 points regarding DAG cross-site failover
1. DAG Cluster Name
NBU backup is based on DAG cluster Name (DAGOS51). if the DAGOS51 is fail-over to another site (MY), then the NBU backup will be failed since the backup network is not routable.
to prevent DAGOS51 auto failover to MY site, it is needed to increase the cluster NodeWeight in the DAG members (MBSOM0D/0E) in MY site .
2. DAG member failover
by default, DAG members are configured to auto failover to any nodes without your notice. if DAG members failover to MY site, the client access will be cross the WAN
to prevent this, it is needed to set DatabaseCopyAutoActivationPolicy to “Blocked” in DAG members in MY site. this is only to prevent auto failover. you are still able to do the manaul failover via EMC/EMS
BUT, when you apply the patches on DAG, Microsoft advise you:
a. before patching, run StopDagServerMaintenance -ServerName “EXSERVER1”
b. after patching, run StartDagServerMaintenance -ServerName “EXSERVER1”
Step b) will auto set DatabaseCopyAutoActivationPolicy back to “Unrestricted” !!!!!!
Thanks the link below highlight how step a) & b) works