Azure Load Balancer

Azure Load Balancer is a high-performance, Layer 4 (TCP/UDP) load balancer that distributes inbound traffic across healthy backend resources. It is a core networking service used to improve the availability, scalability, and reliability of your applications.

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Layer 4 Load Balancing

Azure Load Balancer operates at Layer 4 of the OSI model — the transport layer. It makes forwarding decisions based on:

• Source IP and port
• Destination IP and port
• Protocol (TCP or UDP)

It does not inspect HTTP headers, URLs, or application-layer content. For Layer 7 (HTTP/HTTPS) load balancing, use Azure Application Gateway instead.

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SKUs

Azure Load Balancer comes in two SKUs:

Feature	Basic	Standard
Backend pool size	Up to 300 instances	Up to 1,000 instances
Health probes	TCP, HTTP	TCP, HTTP, HTTPS
Availability Zones	Not supported	Zone-redundant and zonal
SLA	No SLA	99.99%
Security	Open by default	Secure by default (requires NSG)
Global load balancing	No	Yes (cross-region)
Pricing	Free	Pay per rule + data processed

Important: Basic Load Balancer is being retired. Always use Standard SKU for new deployments.

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Types of Load Balancer

Public Load Balancer

A public load balancer has a public IP address as its frontend. It distributes internet traffic to backend VMs or instances.

Internet --> Public IP --> Load Balancer --> Backend Pool (VMs)

Use cases: web applications, APIs, any workload serving internet clients.

Internal (Private) Load Balancer

An internal load balancer has a private IP address from a VNet subnet as its frontend. It distributes traffic within the VNet.

Web tier --> Internal LB (private IP) --> App tier VMs

Use cases: multi-tier applications, internal APIs, database traffic distribution.

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Components

Component	Description
Frontend IP	The IP address clients connect to (public or private).
Backend pool	The set of VMs or instances that receive traffic.
Health probe	Monitors backend health; unhealthy instances are removed from rotation.
Load-balancing rule	Maps a frontend IP:port to a backend pool and health probe.
Inbound NAT rule	Forwards traffic from a specific frontend port to a specific backend instance (e.g. for RDP/SSH).
Outbound rule	Configures SNAT for outbound internet connectivity from backend pool members.

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Health Probes

Health probes determine whether a backend instance is healthy and should receive traffic.