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    Server Load Balancer:CLB listeners

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    Server Load Balancer:CLB listeners

    Last Updated:Jun 02, 2026

    A CLB listener checks for incoming connection requests and distributes them to backend servers based on configured scheduling algorithms and forwarding rules. Each CLB instance requires at least one listener. CLB supports TCP, UDP, HTTP, and HTTPS listeners.

    Listener types and scenarios

    CLB supports Layer 4 listeners (TCP and UDP) and Layer 7 listeners (HTTP and HTTPS). Choose a listener type based on your requirements.

    Type

    Protocol

    Description

    Scenario

    Layer 4 listener

    TCP

    • Connection-oriented protocol. Requires an established connection before data transmission.

    • Reliable data transmission at lower speed.

    • Scenarios that require high data consistency and integrity, such as file transfer, email, and remote logon.

    • Web applications without custom requirements.

    For more information, see Add a TCP listener.

    UDP

    • Connectionless protocol. No handshake required. Does not support error correction or retransmission.

    • Faster transmission with lower reliability.

    Scenarios that prioritize speed over reliability, such as video conferencing and real-time financial data feeds.

    For more information, see Add a UDP listener.

    Layer 7 listener

    HTTP

    • Application-layer protocol for packaging data.

    • Supports cookie-based session persistence.

    • Uses the X-Forwarded-For header to preserve client IP addresses.

    Applications that need to identify data content, such as web applications and mobile games.

    For more information, see Add an HTTP listener.

    HTTPS

    • Encrypts data in transit to prevent unauthorized access.

    • Supports centralized certificate management. Upload certificates to CLB to offload decryption from backend servers.

    Applications that require encrypted data transmission.

    For more information, see Add an HTTPS listener.

    Port configuration limitations

    Port

    Description

    Limitations

    Listener port (front-end port)

    Receives client requests.

    For a CLB instance:

    • TCP, HTTP, and HTTPS listener ports cannot overlap.

      For example, if a TCP listener uses port 80, no other TCP, HTTP, or HTTPS listener can use port 80.

    • UDP listener ports are independent of TCP, HTTP, and HTTPS listener ports.

      For example, a UDP listener on port 81 does not conflict with a TCP, HTTP, or HTTPS listener on port 81.

    Server port (backend port)

    The port on which backend servers provide services.

    For a CLB instance:

    • If the default server group is associated with a listener, the listener port can only forward requests to the same port on backend servers.

    • If a vServer group or primary/secondary server group is associated with a listener, the listener port can forward requests to various ports on backend servers.

    • Listeners that use different protocols can forward requests to the same port on backend servers. For example, both a TCP listener listening on port 80 and an HTTP listener listening on port 81 can forward requests to the same port on a backend server.

    Note

    • HTTP and HTTPS listeners do not support primary/secondary server groups.

    • Up to two backend servers can be added to a primary/secondary server group.

    Listener feature comparison

    Feature

    Layer 4 listener

    Layer 7 listener

    TCP listener

    UDP listener

    HTTP listener

    HTTPS listener

    Backend protocol

    TCP

    UDP

    HTTP

    HTTP

    Scheduling algorithm

    • Weighted Round Robin (WRR)

    • Round Robin (RR)

    • Consistent Hashing (CH)

    • Weighted Round Robin (WRR)

    • Round Robin (RR)

    • Consistent Hashing (CH)

    • Weighted Round Robin (WRR)

    • Round Robin (RR)

    • Weighted Round Robin (WRR)

    • Round Robin (RR)

    Certificate

    Not applicable. Deploy SSL certificates on backend servers if needed.

    Not applicable

    Not applicable

    Add certificates to CLB.

    Access logs

    Not supported

    Not supported

    Supported

    Supported

    Second-level monitoring

    Supported

    Supported

    Not supported

    Not supported

    Primary/secondary server group

    Supported

    Supported

    Not supported

    Not supported

    Forwarding rule

    Not supported. Configure forwarding rules on backend servers if needed.

    Supported. Configure forwarding rules for listeners.
    Note

    Specific regions do not support Layer 7 listeners. For details, see CLB regions and zones.

    Bandwidth sharing among listeners

    CLB instances have limited bandwidth. You can allocate exclusive bandwidth to listeners for critical services and let the remaining listeners share the rest. This improves stability and availability for key services.

    How it works

    Assume a CLB instance has 10 Mbit/s bandwidth with three listeners: A, B, and C. Listener A has an exclusive bandwidth of 4 Mbit/s:

    • Listener A always has 4 Mbit/s available, regardless of traffic on listeners B and C.

    • Listeners B and C share the remaining 6 Mbit/s bandwidth (10 Mbit/s - 4 Mbit/s).

      • If listener B has no traffic, listener C can use the full 6 Mbit/s, and vice versa.

      • If both receive traffic, the 6 Mbit/s is split proportionally. For example, equal traffic gives each listener roughly 3 Mbit/s.

      • If one listener uses all 6 Mbit/s, the other receives no bandwidth, causing packet loss.

    • No packet loss occurs as long as each listener's traffic stays within its allocated bandwidth.

    Note

    If your CLB service cannot reach the maximum bandwidth you specified when purchasing the instance, refer to Failure to reach peak bandwidth

    Enable bandwidth throttling for a listener

    When you create a listener, go to the Protocol & Listener step, click Modify next to Advanced Settings, and enable Bandwidth Throttling for Listeners.

    • If enabled, specify an exclusive bandwidth for the listener. The value must not exceed the instance's maximum bandwidth minus the bandwidth allocated to other listeners.

      Note

      All forwarding policies on the listener share its exclusive bandwidth.

    • If disabled, the listener shares the remaining instance bandwidth with all other non-exclusive listeners.

    References

    CLB provides basic Layer 4 (TCP/UDP) and Layer 7 (HTTP/HTTPS) load balancing.

    Alibaba Cloud also offers Application Load Balancer (ALB) for Layer 7 with advanced forwarding and QUIC support, and Network Load Balancer (NLB) for Layer 4 with higher performance and auto scaling. ALB and NLB are recommended over CLB.