EC2 windows failover cluster with Additional NIC cards - Secondary IPs Already in Use


Hello everyone,

I'm encountering an issue while trying to set up a failover cluster for a high availability SQL Server deployment on AWS. I hope someone here might be able to provide some guidance or insights into resolving this problem.


I have two EC2 instances running Windows Server (different availibitily zone) , each equipped with three Network Interface Cards (NICs). These instances are intended to form a failover cluster for a high availability SQL Server deployment. The primary purpose of this setup is to ensure minimal downtime and data loss in case of hardware or software failures.


When attempting to configure the failover cluster, I encountered an error stating that the secondary IPs for the NICs are already in use. Each NIC is assigned a secondary IP address to facilitate communication within the cluster ( it requires addional NIC because I have over than 30 secondaries IPS) . However, it seems that the IP addresses I've allocated for these secondary NICs are conflicting with existing resources.

Configuration Details:

Each EC2 instance has three NICs. All NICs are configured to operate within private subnets (same subnet than the EC2 instances). I've allocated IP addresses within the available range for each NIC, but the configuration process is failing due to apparent conflicts.

Note: If I remove the secondary IP from the NIC and put it on the failover instance it will work, note I'm not using the 2 first secondary IPs as it is reserved by AWS

if I disable 2 NIC and leave only one NIC running it works as expected, will accept my secondary ips and I can allocate the same ones for the failover isntance

1 Answer

Also, I would like to highlight here the message shows when I'm trying to bring the failover cluster online is that the IP is already in use, because was allocated as secondary ip on the NIC, it is not being used by another resource, once I remove it as secondary IP on the NIC it will work, but for aws setup and our requirements we need to have it as secondary ips into the ec2 instances.

answered 10 days ago

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