An elastic network interface (ENI) is a virtual network interface that can be attached to an Elastic Compute Service (ECS) instance in a virtual private cloud (VPC). ENIs can improve the availability of clusters and offer cost savings in service failovers and fine-grained network management. You can connect a backend server to a Classic Load Balancer (CLB) instance by specifying the primary or secondary IP address of the ENI used by the backend server.
Background information
For more information about how to attach an ENI to an ECS instance, see Bind an ENI.

Note
- You can specify the primary and secondary ENIs of ECS instances only for high-performance CLB instances.
- You can specify only secondary ENIs in the console. To specify primary ENIs, make an API request.