# Target groups for your Gateway Load Balancers
<a name="target-groups"></a>

Each *target group* is used to route requests to one or more registered targets. When you create a listener, you specify a target group for its default action. Traffic is forwarded to the target group that's specified in the listener rule. You can create different target groups for different types of requests.

You define health check settings for your Gateway Load Balancer on a per target group basis. Each target group uses the default health check settings, unless you override them when you create the target group or modify them later on. After you specify a target group in a rule for a listener, the Gateway Load Balancer continually monitors the health of all targets registered with the target group that are in an Availability Zone enabled for the Gateway Load Balancer. The Gateway Load Balancer routes requests to the registered targets that are healthy. For more information, see [Health checks for Gateway Load Balancer target groups](health-checks.md).

**Topics**
+ [Routing configuration](#target-group-routing-configuration)
+ [Target type](#target-type)
+ [Registered targets](#registered-targets)
+ [Target group attributes](#target-group-attributes)
+ [Create a target group](create-target-group.md)
+ [Configure health checks](health-checks.md)
+ [Edit target group attributes](edit-target-group-attributes.md)
+ [Register targets](target-group-register-targets.md)
+ [Tag a target group](target-group-tags.md)
+ [Delete a target group](delete-target-group.md)

## Routing configuration
<a name="target-group-routing-configuration"></a>

Target groups for Gateway Load Balancers support the following protocol and port:
+ **Protocol**: GENEVE
+ **Port**: 6081

The Gateway Load Balancer encapsulates the original packets using GENEVE. The GENEVE header uses a Type-Length-Value (TLV) format to store information, using Option Class 0x0108. Appliances must decapsulate the TLV pairs to process the original packets. For more information, see the following blog post: [Integrate your appliance with a Gateway Load Balancers](https://amazonaws-china.com/blogs/networking-and-content-delivery/integrate-your-custom-logic-or-appliance-with-aws-gateway-load-balancer/).

## Target type
<a name="target-type"></a>

When you create a target group, you specify its target type, which determines how you specify its targets. After you create a target group, you cannot change its target type.

The following are the possible target types:

`instance`  
The targets are specified by instance ID.

`ip`  
The targets are specified by IP address.

When the target type is `ip`, you can specify IP addresses from one of the following CIDR blocks:
+ The subnets of the VPC for the target group
+ 10.0.0.0/8 ([RFC 1918](https://tools.ietf.org/html/rfc1918))
+ 100.64.0.0/10 ([RFC 6598](https://tools.ietf.org/html/rfc6598))
+ 172.16.0.0/12 (RFC 1918)
+ 192.168.0.0/16 (RFC 1918)

**Important**  
You can't specify publicly routable IP addresses.

## Registered targets
<a name="registered-targets"></a>

Your Gateway Load Balancer serves as a single point of contact for clients, and distributes incoming traffic across its healthy registered targets. Each target group must have at least one registered target in each Availability Zone that is enabled for the Gateway Load Balancer. You can register each target with one or more target groups.

If demand increases, you can register additional targets with one or more target groups in order to handle the demand. The Gateway Load Balancer starts routing traffic to a newly registered target as soon as the registration process completes.

If demand decreases, or you need to service your targets, you can deregister targets from your target groups. Deregistering a target removes it from your target group, but does not affect the target otherwise. The Gateway Load Balancer stops routing traffic to a target as soon as it is deregistered. The target enters the `draining` state until in-flight requests have completed. You can register the target with the target group again when you are ready for it to resume receiving traffic.

## Target group attributes
<a name="target-group-attributes"></a>

You can use the following attributes with target groups:

`deregistration_delay.timeout_seconds`  
The amount of time for Elastic Load Balancing to wait before changing the state of a deregistering target from `draining` to `unused`. The range is 0-3600 seconds. The default value is 300 seconds.

`stickiness.enabled`  
Indicates whether configurable flow stickiness is enabled for the target group. The possible values are `true` or `false`. The default is false. When the attribute is set to `false`, 5\$1tuple is used. 

`stickiness.type`  
Indicates the type of the flow stickiness. The possible values for target groups associated to Gateway Load Balancers are:  
+ `source_ip_dest_ip`
+ `source_ip_dest_ip_proto`

`target_failover.on_deregistration`  
Indicates how the Gateway Load Balancer handles existing flows when a target is deregistered. The possible values are `rebalance` and `no_rebalance`. The default is `no_rebalance`. The two attributes (`target_failover.on_deregistration` and `target_failover.on_unhealthy`) can't be set independently. The value you set for both attributes must be the same. 

`target_failover.on_unhealthy`  
Indicates how the Gateway Load Balancer handles existing flows when a target is unhealthy. The possible values are `rebalance` and `no_rebalance`. The default is `no_rebalance`. The two attributes (`target_failover.on_deregistration` and `target_failover.on_unhealthy`) cannot be set independently. The value you set for both attributes must be the same. 

For more information, see [Edit target group attributes](edit-target-group-attributes.md).