# Network Load Balancers
<a name="arc-zonal-shift.resource-types.network-load-balancers"></a>

## Using zonal shift for Network Load Balancers
<a name="using-nlb-zs"></a>

To use Network Load Balancers with zonal shift, you must enable ARC zonal shift integration in the Network Load Balancer attributes. Network Load Balancer supports zonal shift with cross-zone enabled or cross-zone disabled configurations. 

You can choose which resources to opt-in to use zonal shift and zonal autoshift, and when you would like to fail away from an impaired Availability Zone. Both internet-facing and internal Network Load Balancers are supported. 

To enable zonal shift for your cross-zone enabled Network Load Balancer, all target groups attached to the load balancer must meet the following requirements.
+ Cross-zone load balancing must be enabled, or set to `use_load_balancer_configuration`.
  + For more information on target group cross-zone load balancing, see [Cross-zone load balancing for target groups](https://docs.amazonaws.cn//elasticloadbalancing/latest/network/edit-target-group-attributes.html#target-group-cross-zone).
+ Target group protocol must be TCP or TLS.
  + For more information on Network Load Balancer target group protocols, see [Routing configuration](https://docs.amazonaws.cn//elasticloadbalancing/latest/network/load-balancer-target-groups.html#target-group-routing-configuration).
+ Connection termination for unhealthy targets must be disabled.
  + For more information on target group connection termination, see [Connection termination for unhealthy targets](https://docs.amazonaws.cn//elasticloadbalancing/latest/network/edit-target-group-attributes.html#unhealthy-target-connection-termination).
+ Target group must not have any Application Load Balancers as targets.
  + For more information on Application Load Balancers as targets, see [Use Application Load Balancers as targets of a Network Load Balancer](https://docs.amazonaws.cn//elasticloadbalancing/latest/network/application-load-balancer-target.html).

You can start a zonal shift for a Network Load Balancer by using the Amazon CLI, the Amazon Web Services Management Console, or the Elastic Load Balancing widget. When an Application Load Balancer is the target of a Network Load Balancer, you must start the zonal shift from the Network Load Balancer. If you start the zonal shift from the Application Load Balancer, the Network Load Balancer will not stop sending traffic to the Application Load Balancer and its targets.

------
#### [ Console ]

**To enable zonal shift on a load balancer (Console)**

1. Open the Amazon EC2 console at [https://console.amazonaws.cn/ec2/](https://console.amazonaws.cn/ec2/).

1. On the **Navigation** page, under **Load balancing**, choose **Load balancers**.

1. Select the Network Load Balancer name.

1. On the **Attributes** tab, choose **Edit**.

1. Under **Availability Zone routing configuration**, for **ARC zonal shift integration**, choose **Enable**.

1. Choose **Save**.

------
#### [ Amazon CLI ]

**To enable zonal shift on a load balancer (Amazon CLI)**
+ Enter the following command:

  ```
  aws elbv2 modify-load-balancer-attributes --load-balancer-arn my-nlb-arn --attributes Key=zonal_shift.config.enabled,Value=true
  ```

------

For more information about starting a zonal shift, see [Starting, updating, or canceling a zonal shift](arc-zonal-shift.start-cancel.md).

## How zonal shift works for Network Load Balancers
<a name="howt-it-works-nlb-zs"></a>

ARC creates a health check failure for the registered Network Load Balancer so that the Network Load Balancer node in the impaired AZ is removed from the DNS when you start a zonal shift. The Network Load Balancer disables the targets in the impacted zone so that they stop receiving traffic, and Elastic Load Balancing treats these targets as disabled targets for zonal shift. Targets in the disabled state continue receiving health checks. When the targets are healthy and the zonal shift expires (or is canceled), routing to targets in the previously impaired zone resumes.

During zonal shift on Network Load Balancers with cross-zone load balancing enabled, the zonal load balancer IP addresses are removed from DNS. Existing connections to targets in the impaired Availability Zone persist until they organically close, while new connections are no longer routed to targets in the impaired Availability Zone.

 For more information see [Zonal Shift for your Network Load Balancer](https://docs.amazonaws.cn//elasticloadbalancing/latest/network/zonal-shift.html) in the *Network Load Balancer User Guide*.