This topic describes the features of Classic Load Balancer (CLB). CLB provides load balancing services at Layer 4 and Layer 7 and supports features such as health checks, session persistence, and domain name-based forwarding to ensure the high availability of backend services.

In the following table, "√“ indicates that the feature is supported and "-" indicates that the feature is not supported.

Feature Layer 4 CLB Layer 7 CLB
Scheduling algorithms

CLB supports the round-robin (RR), weighted round-robin (WRR), and consistent hashing (CH) scheduling algorithms.

Note Layer 7 CLB does not support the CH scheduling algorithm.
Health checks

CLB checks the health status of backend servers. When a backend server is declared unhealthy, CLB stops distributing inbound traffic to the backend server. Network traffic is distributed to other backend servers that work as expected.

Session persistence

CLB supports session persistence. After session persistence is enabled, CLB distributes requests from a client to the same backend server during a session.

Access control

CLB uses whitelists and blacklists to control access to your applications.

High availability

CLB distributes inbound traffic to backend servers that are deployed in different zones. CLB supports cross-zone deployment in most regions. If the primary zone fails, network traffic is automatically distributed to the secondary zone to avoid service disruptions.


You can integrate CLB with Alibaba Cloud Security to defend your applications against DDoS attacks of up to 5 Gbit/s.

Network types

Alibaba Cloud provides Internet-facing and internal-facing CLB instances. To process network traffic in a virtual private cloud (VPC), you can create an internal-facing CLB instance. To process network traffic from the Internet, you can create an Internet-facing CLB instance.


Cloud Monitor allows you to view the monitoring data of CLB instances such as the number of connections and the volumes of inbound and outbound traffic.


CLB can forward requests from IPv6 clients.

Health check logs

By default, CLB stores health check logs that are generated in the last three days. After you enable Object Storage Service (OSS), CLB can store all health check logs in OSS. This allows you to troubleshoot the issues that cause health check failures.

Domain name-based forwarding and URL-based forwarding

You can configure domain name-based forwarding rules and URL-based forwarding rules for the Layer 7 listeners of CLB . Listeners distribute requests to different backend servers based on forwarding rules.

Certificate management

CLB manages certificates for HTTPS in a centralized way. You do not need to upload certificates to backend servers. Requests are decrypted on CLB before the requests are sent to backend servers. This reduces the CPU load on backend servers.


CLB allows you to associate multiple certificates with an HTTPS listener. This way, the listener can distribute requests to different backend servers based on domain names.


CLB can redirect HTTP requests to HTTPS.

WebSocket Secure (WSS) and WebSocket (WS)

WebSocket is an HTML5 protocol that provides full-duplex communication channels between clients and servers. You can use WebSocket to save server resources and bandwidth, and enable real-time communication.


HTTP/2 is the second major version of the HTTP protocol and is backward compatible with HTTP/1.x.In addition, HTTP/2 improves performance by optimizing the flow of content.