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

If multiple ENIs are attached to the ECS instance that you want to add to a CLB instance, you can specify both the primary and secondary ENIs of the ECS instance.

For more information about how to attach an ENI to an ECS instance, see Bind an ENI.

  • 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.


  1. Log on to the CLB console.
  2. In the top navigation bar, select the region where the CLB instance is deployed.
  3. On the Instances page, click the ID of the CLB instance for which you want to create a backend server group.
  4. Click the VServer Groups, Default Server Group, or Primary/Secondary Server Groups tab.
    Note You can add the primary and secondary ENIs of ECS instances to the default server group, vServer group, or primary/secondary server group. A vServer is used in this example.
  5. On the VServer Groups tab, click Create VServer Group.
  6. On the Create VServer Group page, click Add.
  7. In the Select Servers wizard page, select Elastic Compute Service (ECS)/Elastic Network Interface (ENI) from the Select Backend Server Type drop-down list, and turn on Advanced Mode.
  8. Select the backend servers that you want to add to the vServer group and click Next.
  9. In the Configure Ports and Weights wizard page, specify the ports and weights of the backend servers and click Add.

    After you add backend servers to the server group of the listener, you can view the backend servers and ENIs that you added on the Instances page.

    • indicates an ECS instance.
    • indicates a primary or secondary ENI.