A Classic Load Balancer (CLB) instance receives client traffic and distributes requests to backend servers. Choose the right network type, IP version, zone deployment, and performance specification before you create an instance.
CLB supports both Layer 4 (TCP/UDP) and Layer 7 (HTTP/HTTPS) load balancing. For dedicated Layer 7 load balancing with content-based routing, see Application Load Balancer (ALB) For high-performance Layer 4 load balancing with dual-stack support, see What is NLB?
Network types
CLB supports Internet-facing and internal instances. The network type cannot be changed after creation.
To change the network type, create a new CLB instance.
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Attribute |
Internet-facing |
Internal |
|
Instance type setting |
Set to Internet during creation. |
Set to Intranet during creation. |
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IP address |
A system-assigned static public IP bound to the instance. Cannot be independently associated or disassociated. |
A private IP address from the vSwitch CIDR block of the specified VPC. |
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Internet access |
Accessible from the Internet through the assigned public IP. |
Not directly Internet-accessible. Associate an Elastic IP Address (EIP) for public access. EIPs can be flexibly bound or unbound and added to an Internet Shared Bandwidth instance to reduce costs. |
|
Private network access |
No private IP access. |
Accessible by any client that can communicate with the VPC. |
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Diagram |
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IP versions
CLB supports IPv4 and IPv6 as separate instance types. Dual-stack is not supported. Use NLB or ALB for dual-stack support.
|
IP version |
Address allocation |
Client access |
|
IPv4 |
Internet-facing: public IPv4 address. Internal: private IPv4 address. |
IPv4 addresses (for example, |
|
IPv6 |
Internet-facing: public IPv6 address. Internal instances do not support IPv6. |
IPv6 addresses (for example, |
IPv6 listener requirements
IPv6 headers are larger than IPv4. Configure listeners accordingly:
|
Listener type |
Requirement |
|
UDP |
Set each backend server's NIC MTU to 1,200 bytes or higher. Oversized packets are dropped. Some applications may require MTU adjustments. |
|
TCP |
No configuration required. TCP negotiates MSS automatically. |
Migrate to IPv6
Migrate services to IPv6 incrementally without modifying the existing system:
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Add servers with IPv4 addresses as backend servers to an IPv6 CLB instance.
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When traffic increases, add an IPv6 ingress and scale out backend servers. Existing IPv4 services remain unaffected.
Zone deployment
CLB supports multi-zone and single-zone deployment. Multi-zone deployment provides automatic failover for high availability.
Multi-zone deployment
In multi-zone regions, CLB instances deploy across a primary zone and a secondary zone. This setting is fixed at creation.
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Primary zone: Selected during creation. To use a different primary zone, create a new instance.
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Secondary zone: Automatically selected by the system.
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Failover: If the primary zone fails, traffic automatically switches to the secondary zone.
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Latency: Place the primary zone in the same zone as your backend servers to reduce latency.
Single-zone deployment
In single-zone regions, CLB instances deploy in the only available zone. If that zone fails, the service becomes unavailable. Use single-zone deployment only for testing or non-critical workloads.
Performance specifications
Performance metrics
CLB performance is measured by three metrics:
|
Metric |
Description |
Applies to |
|
Maximum concurrent connections |
Maximum concurrent connections. Excess requests are dropped. |
Layer 4 and Layer 7 |
|
Connections per second (CPS) |
New connections established per second. Excess requests are dropped. |
Layer 4 and Layer 7 |
|
Queries per second (QPS) |
HTTP/HTTPS requests processed per second. Excess requests are dropped. |
Layer 7 only |
Pay-as-you-go instances
All new CLB instances use pay-as-you-go billing with these performance limits:
|
Maximum concurrent connections |
CPS |
QPS |
|
1,000,000 |
100,000 |
50,000 |
Pay-by-specification instances (discontinued)
Alibaba Cloud has discontinued the sale of pay-by-specification CLB instances. Existing instances continue to operate normally. For details, see End of sale for pay-by-specification CLB instances.
Pay-by-specification instances are categorized as guaranteed-performance or shared-resource. When selecting a specification, focus on Maximum concurrent connections for Layer 4 listeners and QPS for Layer 7 listeners.
Guaranteed-performance specifications
Guaranteed-performance instances provide dedicated resources with these performance tiers:
|
Specification |
Maximum concurrent connections |
CPS |
QPS |
|
Small I (slb.s1.small) |
5,000 |
3,000 |
1,000 |
|
Standard I (slb.s2.small) |
50,000 |
5,000 |
5,000 |
|
Medium II (slb.s2.medium) |
100,000 |
10,000 |
10,000 |
|
Large I (slb.s3.small) |
200,000 |
20,000 |
20,000 |
|
Large II (slb.s3.medium) |
500,000 |
50,000 |
30,000 |
|
Super Large I (slb.s3.large) |
1,000,000 |
100,000 |
50,000 |
Guaranteed-performance vs. shared-resource
Guaranteed-performance instances support all features listed below. Shared-resource instances support none.
|
Feature |
Guaranteed-performance |
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Exclusive resources |
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SLA |
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IPv6 |
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SNI (multiple certificates) |
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Access control lists |
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ENI attachment |
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Secondary private IP addresses of ENIs |
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HTTP-to-HTTPS redirection |
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Consistent hashing |
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TLS security policies |
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HTTP/2 |
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WebSocket and WebSocket Secure |
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Guaranteed-performance instances include a service level agreement (SLA). For details, see .
Instance status
|
Status |
Description |
Can be deleted |
Can be modified |
|
Running |
The instance operates normally. |
Yes, if deletion protection is disabled. |
Yes, if configuration read-only mode is disabled. |
|
Locked |
Locked due to overdue payment or security risk. Renew the instance to remove a financial lock. To remove a security lock, submit a request on the Security Control page. |
No |
No |
|
Stopped |
The instance is stopped. |
Yes |
No |