Server Load Balancer:FAQ about backend servers

Can I increase or decrease the number of backend ECS instances for a CLB instance when the CLB instance is running?

Yes.

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?

Yes.

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 the same operating system.

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

CLB does not support backend servers that are in different regions. You can use Global Traffic Manager (GTM) to deploy CLB instances in different regions. GTM can switch network traffic between the CLB instances. For more information, see Use GTM and CLB instances to implement cross-region load balancing.

Application Load Balancer (ALB) and Network Load Balancer (NLB) support backend servers in different regions. For more information, see the following topics:

• Specify an ECS instance in a VPC as a backend server of ALB in a different region

• Add on-premises servers to an ALB instance within the same region

• Add backend servers in VPCs to NLB across regions

• Add on-premises servers to an NLB instance within the same region

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 system CIDR block of CLB is 100.64.0.0/10, which is reserved by Alibaba Cloud. To prevent security issues, this CIDR block is not allocated to other network elements. As a result, the backend ECS instances are accessed by IP addresses that start with 100.

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?

Yes, chunked transfer encoding is supported if your backend ECS instances use HTTP/1.0.

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

Problem:

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 UA string is KeepAliveClient.

Possible causes:

TCP listeners are used, but HTTP is used to perform health checks. By default, TCP listeners use the GET method to perform HTTP health checks.

Solutions:

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

Can I change the ports of servers in the default server group?

Problem:

If you modify the configurations of an existing listener, the ports of servers in the default server group cannot be changed.

You can specify server ports for the default server group only when you create a listener. On the same listener, the ports of servers in the default server group must be the same.

Solutions:

To specify different server ports on the same listener, select a vServer group when you configure the listener on the configuration wizard. For more information about vServer groups, see Create and manage a vServer group.

Does Layer 4 CLB support an ECS instance working as both a backend server and a client?

No, Layer 4 CLB does not support this. However, Network Load Balancer (NLB) instances can support this scenario. You can deploy an NLB instance and disable the client IP preservation feature for the NLB instance. Then, an ECS instance in the backend server group can work as both a backend server and a client. For more information, see FAQ about NLB.