This topic provides answers to some frequently asked questions about backend servers of Classic Load Balancer (CLB).

Can I increase or decrease the number of backend Elastic Compute Service (ECS) instances for a CLB instance when the CLB instance is running?


You can increase or decrease the number of backend ECS instances and switch between ECS instances anytime. Before you perform these operations, make sure that health checks are enabled and that at least one backend ECS instance is running as expected. This ensures service stability.

Can backend ECS instances run in different operating systems?


Backend ECS instances of CLB can run in different operating systems. However, the applications deployed on the ECS instances must be the same and have the same data. To facilitate maintenance, we recommend that backend ECS instances run in the same operating system.

Can I specify ECS instances deployed in different regions as backend servers for a CLB instance?


CLB does not support cross-region deployment. If you want to specify ECS instances deployed in different regions as backend servers, we recommend that you use Application Load Balancer (ALB) or Network Load Balancer (NLB) in combination with Cloud Enterprise Network (CEN). For more information, see the following topics:

Why are my ECS instances frequently accessed by IP addresses that start with 100?

CLB forwards external requests to backend ECS instances by using the private IP address of the system server. CLB also accesses the ECS instances to perform health checks and monitor service availability.

The IP address range of the CLB system is ( is reserved by Alibaba Cloud. It is not used by any user and therefore causes no security risks) . Therefore, many IP addresses that start with 100 are accessing the ECS instances.

To ensure service availability, you must allow access from the preceding IP addresses.

Why are responses from CLB compressed when compression is not configured for my ECS instances?

A possible cause is that the client web browser supports compression. You can disable the Gzip feature when you create listeners in the console, or use TCP listeners.

Is chunked transfer encoding supported if my backend ECS instances use HTTP/1.0?


Why do my backend ECS instances frequently receive requests whose UA string value is KeepAliveClient?


Backend ECS instances of CLB frequently receive GET requests even if no user is accessing the ECS instances. The source IP addresses of these requests are private IP addresses of Alibaba Cloud, and the value of the User-Agent string is KeepAliveClient.


TCP listeners are used, but HTTP is used to perform health checks. When health checks that use HTTP are performed by TCP listeners, GET requests are used by default.


We recommend that you use the same protocol for listeners and health checks.