Classic Load Balancer (CLB) instance FAQ - Server Load Balancer

This topic answers frequently asked questions (FAQs) about Classic Load Balancer (CLB) instances.

This topic contains the following questions:

Category	FAQ
Instance selection	How do I select a pay-as-you-go instance? Why does a guaranteed-performance instance sometimes fail to reach its performance limits?
Instance type	Can I change the instance type of a guaranteed-performance instance? After I change the instance type of a guaranteed-performance instance, why does the change not take effect immediately?
Instance configuration	Can I change the primary and secondary availability zones after a CLB instance is created? Does the processing time of a CLB instance include the time to receive data from and send data to a client? How do I obtain the public IP CIDR blocks that are automatically assigned to a CLB instance?
Instance O&M	Can I disable Ping for the service address of a CLB instance? Why do security scans report all UDP ports as open on my CLB instance even if the instance listens only on TCP ports?

How do I select a pay-as-you-go instance?

Starting from 00:00:00 (UTC+8) on June 1, 2025, pay-by-specification CLB instances will no longer be available for purchase.

By default, CLB instances use the pay-by-LCU billing method. The performance of an instance automatically scales with usage, and you do not need to specify an instance type. You are charged an hourly fee based on the number of LCUs that you use. For more information about performance limits, see Instance performance limits. For more information about billing, see Instance billing methods.

Failure to reach performance limits

A guaranteed-performance instance does not guarantee that all its performance metrics, including bandwidth, can reach their limits simultaneously. The instance is throttled as soon as one of the metrics reaches its upper limit.

For example, a user selects an slb.s3.small instance. If the QPS of the instance reaches 20,000 but the number of concurrent connections has not reached 200,000, the maximum number of connections may never reach the upper limit of the instance type. This is because new connection requests are dropped when the QPS reaches its upper limit.

Changing the guaranteed-performance instance type

Yes.

You can modify the configuration of a guaranteed-performance instance in the console. For more information, see Modify the configurations of a pay-as-you-go CLB instance.

Note

Starting from 00:00:00 (UTC+8) on June 1, 2025, pay-by-specification CLB instances will no longer be available for purchase. You cannot change the billing method of a pay-by-LCU instance to pay-by-specification. For more information, see End of sale for pay-by-specification Classic Load Balancer (CLB) instances.
You can change only shared-resource CLB instances to high-performance ones. You cannot change high-performance CLB instances to shared-resource ones.
Changing a shared-resource instance to a guaranteed-performance instance does not affect your services or change the IP address of the CLB instance.
Configuration modifications do not change the IP address of the CLB instance.
We recommend that you change a shared-resource instance to a guaranteed-performance instance during off-peak hours. You can also use DNS to implement load balancing among instances before you modify the configuration.

Delayed effect of an instance type change

If you change the instance type and billing method (from pay-by-bandwidth to pay-by-data-transfer or vice versa) simultaneously, the changes take effect at 00:00:00 on the next day.

Changing availability zones after creation

No.

You cannot change the primary availability zone after a CLB instance is created. The system automatically selects the most appropriate secondary availability zone based on regional resources, so you do not need to configure it manually.

We recommend using Application Load Balancer (ALB) or Network Load Balancer (NLB), as both support multi-availability zone deployment. For more information, see Introduction to the SLB product family, What is Application Load Balancer (ALB)?, and What is Network Load Balancer (NLB)?.

What CLB processing time includes

Yes, the processing time of a CLB instance includes the time to receive client data and send response data.

Time to receive client data: This is read_request_time. It is the total time CLB spends reading a client request and includes the time to receive the HTTP request header (read_header_time) and the request body (read_body_time).
Time to send response data: the time required to return a response to the client.

Obtaining public IP CIDR blocks

CLB instances use dynamically allocated public IP addresses. To prevent access restrictions, obtain the IP CIDR blocks in advance to configure your firewall's allowlist.

Elastic IP Addresses (EIPs) and the public IP addresses of CLB instances are allocated from the same public IP address pool. You can call the DescribePublicIpAddress operation to obtain the public IP CIDR blocks in a specified region.

Note

This API supports pagination (100 entries per page by default). If the result set is large, adjust the pagination parameters to retrieve the full list.

Disabling Ping for the service address

No. You can add the public IP address of an Internet-facing CLB instance or the EIP that is associated with an internal-facing CLB instance to Cloud Firewall and configure an inbound rule to deny ICMP traffic.

A CLB service address responds to Ping only when the CLB instance has at least one running listener.

Security scans reporting open UDP ports

This is a false positive from the scanning tool. For UDP ports without a configured listener, CLB silently drops packets and does not send an "ICMP Port Unreachable" message to the client. Some scanners treat the lack of an ICMP response as an open port. In reality, CLB does not process or forward traffic on any unconfigured port, and this behavior poses no security risk.