Classic Load Balancer (CLB) provides Layer 4 and Layer 7 load balancing. The tables below list the features supported at each layer. "✔" means supported and "–" means not supported.
Traffic management
| Feature | Description | Layer 4 | Layer 7 |
|---|---|---|---|
| Routing algorithms | CLB supports three algorithms: round-robin (RR), weighted round-robin (WRR), and consistent hashing (CH). Layer 7 does not support CH. | ✔ | ✔ |
| Session persistence | Routes all requests from the same session to the same backend server. | ✔ | ✔ |
| Domain name-based and URL-based forwarding | Routes requests to different backend servers based on domain name or URL path. Configured on Layer 7 listeners only. | – | ✔ |
High availability and security
| Feature | Description | Layer 4 | Layer 7 |
|---|---|---|---|
| High availability | Distributes traffic across zones, with cross-zone deployment supported in most regions. If the primary zone fails, traffic automatically shifts to the secondary zone. | ✔ | ✔ |
| Network access control lists (ACLs) | Controls access using allowlists and denylists. | ✔ | ✔ |
| DDoS protection | Integrates with Apsara Stack Security to protect against DDoS attacks of up to 5 Gbit/s. | ✔ | ✔ |
| Certificate management | Centrally manages certificates for HTTPS listeners. CLB decrypts requests before forwarding them to backend servers, reducing backend processing load. | – | ✔ |
| Server Name Indication (SNI) | Associates multiple certificates with a single HTTPS listener to route requests from different domain names to different backend servers. | – | ✔ |
| HTTP-to-HTTPS redirection | Redirects HTTP requests to HTTPS. | – | ✔ |
Protocols and network
| Feature | Description | Layer 4 | Layer 7 |
|---|---|---|---|
| Network types | Supports Internet-facing instances for public traffic and internal-facing instances for traffic within a virtual private cloud (VPC). | ✔ | ✔ |
| IPv6 support | Forwards requests from IPv6 clients to IPv4 backend servers. IPv6 backend servers are not supported. | ✔ | ✔ |
| WebSocket (WS) and WebSocket Secure (WSS) | Supports full-duplex communication between clients and servers. | – | ✔ |
| HTTP/2 | Supports HTTP/2, which is backward compatible with HTTP/1.X. | – | ✔ |
Monitoring and logging
| Feature | Description | Layer 4 | Layer 7 |
|---|---|---|---|
| Health checks | Monitors the health of backend servers and stops routing traffic to unhealthy servers. | ✔ | ✔ |
| Monitoring | Provides connection counts and the volumes of inbound and outbound traffic through CloudMonitor. | ✔ | ✔ |
| Health check logs | Retains health check logs for the last 3 days by default. To store all logs, enable Object Storage Service (OSS) first. | ✔ | ✔ |