> For the complete documentation index, see [llms.txt](https://docs.console.zenlayer.com/api-reference/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.console.zenlayer.com/api-reference/compute/bmc/load-balancing/registerbackend.md).

# RegisterBackend

## 1. API Description

This API (RegisterBackend) is used to create a backend server (vm or bmc instance) and add it to a load balance listener.

## 2. Input Parameters

The following request parameter list only provides API request parameters.

| Parameter Name | Required | Type                                                                                   | Description                                                                                                                               |
| -------------- | -------- | -------------------------------------------------------------------------------------- | ----------------------------------------------------------------------------------------------------------------------------------------- |
| listenerId     | Yes      | String                                                                                 | The listener ID.                                                                                                                          |
| backendName    | Yes      | String                                                                                 | <p>Backend server name.</p><p>This parameter can contain up to 64 characters. Only letters, numbers, - and periods (.) are supported.</p> |
| instanceId     | Yes      | String                                                                                 | Bare metal or Virtual machine instance ID.                                                                                                |
| clientToken    | Yes      | String                                                                                 | Used to ensure the idempotency of the request.                                                                                            |
| instanceType   | No       | [InstanceProductType](/api-reference/compute/bmc/datastructure.md#instanceproducttype) | <p>Instance type.</p><p>Default value: BM</p>                                                                                             |
| portList       | No       | Array of Integer                                                                       | Port list.                                                                                                                                |
| weight         | No       | Integer                                                                                | <p>Weight of the instance.</p><p>Value range: \[1, 100]</p>                                                                               |

## 3. Output Parameters

| Parameter Name | Type   | Description                                                                                                           |
| -------------- | ------ | --------------------------------------------------------------------------------------------------------------------- |
| requestId      | String | <p>The unique request ID, which is returned for each request.</p><p>RequestId is required for locating a problem.</p> |
| backendId      | String | Backend server ID.                                                                                                    |

## 4. Code Example

{% tabs %}
{% tab title="Example" %}
**Add a bare metal instance as a backend server to a load balance listener.**

```json
POST /api/v2/bmc HTTP/1.1
Host: console.zenlayer.com
Content-Type: application/json
X-ZC-Action: RegisterBackend
<Common Request Params>

Request:
{
  "listenerId": "xxxx",
  "backendName": "xxxx",
  "portList": [80],
  "clientToken": "xxxx",
  "weight": 100,
  "instanceId": "xxxx"
}

Response:
{
  "requestId": "TEEFDCA5B-76FB-4E91-A18E-DF7F5D2CE41F",
  "response": {
    "requestId": "TEEFDCA5B-76FB-4E91-A18E-DF7F5D2CE41F",
    "backendId": "xxxx"
  }
}
```

{% endtab %}
{% endtabs %}

## 5. Developer Resources

Zenlayer Cloud API 2.0 integrates [SDKs](/api-reference/api-introduction/toolkit/api-sdk.md)，to make it easier for you to call APIs. More programming languages will be supported.

## 6. Error Codes

The following only lists the error codes related to the API business logic. For other error codes, see [Common Error Codes](/api-reference/api-introduction/instruction/commonerrorcode.md).

| HTTP Status Code | Error Code | Description |
| ---------------- | ---------- | ----------- |