ALB vs ELB: Which AWS Load Balancer Should You Use and Why?

Comparing ALB vs ELB? You’re not alone. Many AWS users find it tricky to choose between these two load balancing options. One is a legacy solution with limited features, while the other is built for modern, scalable cloud environments. So, which one is the right fit for your architecture?

In this guide, we’ll break down the key differences between ALB and ELB in plain language. You’ll learn when to use each, how they affect performance and scalability, and which option helps you future-proof your infrastructure without adding unnecessary complexity or cost.

Quick Comparison Table – ALB vs ELB at a Glance

Feature	ALB	ELB (Classic)
OSI Layer	Layer 7 (Application)	Layer 4 (Transport)
Routing Methods	Host, Path, Query String	IP Address, Port Only
Protocol Support	HTTP, HTTPS, WebSockets, HTTP/2	TCP, SSL
Target Types	EC2, Containers, Lambda, IP	EC2, IP
Security Features	AWS WAF, Shield, SNI	SSL Termination Only
Monitoring	Detailed Access Logs, CloudWatch	Basic Metrics, CloudWatch
Use Cases	Microservices, APIs, Modern Apps	Legacy Applications

What Is a Load Balancer and Why It Matters?

Picture your application like a busy airport. Without someone directing incoming planes, you’d have chaos on the runway. Load balancers work the same way, directing traffic to the right resources so your applications stay fast, reliable, and ready for anything.

Load balancers are critical components in AWS systems, and selecting the most suitable option might prove confusing for some users. Choosing the right option enables users to distribute various tasks across resources, resulting in an optimized process. Operating a network without load balancers may result in significant delays in web services during a spike in user requests.

The modern digital age has led to a significant increase in user requests from social media use and IoT operations, increasing the importance of load balancers as critical components in web traffic management. 

Essentially, a suitable load balancer serves as the gatekeeper or contact point between client devices and backend servers, driving application responsiveness, scalability, and availability while reducing the risk of traffic overload (i.e., increased fault tolerance). 

Behind the scenes, they follow preset algorithms to decide where traffic goes, using methods like round-robin, least response time, or custom rules. The right load balancer keeps your systems stable and sets you up for long-term scalability and cost control.

What Is an Application Load Balancer (ALB)

ALBs operate from the application layer or the seventh layer of the OSI (Open Systems Interconnections) model, which drives communications among multiple systems. An ALB receives the request and evaluates listener (a process that checks for connection requests) rules through prioritized order, essentially routing requests based on content to a specific subgroup. 

Users can choose to route the algorithm of listener rules specifically to different target groups. Additionally, system administrators can conveniently add or remove target groups according to the changing priorities and demands of a project or organization without causing disruptions to the overall requests received by the application.  

Users can combine ALB with various other AWS services to optimize the availability and scalability of applications. These services may include AWS Certificate Manager, Amazon EC2, Amazon Route 53, AWS WAF, and Amazon CloudWatch. 

For instance, Amazon CloudWatch offers users real-time application monitoring capabilities, providing quick error detection and troubleshooting in response to system anomalies or performance delays. With Amazon Route 53, users can create an alias record, listing multiple IP addresses for responding to DNS requests, an effective web solution for managing geographically distributed servers. 

How ALB Works

ALB primarily distributes network load in a public cloud to optimize availability and stability. The ALB monitors the health of applications within the seventh layer of the OSI model and will specifically route traffic to healthy registered targets. 

Specifically, ALB assesses data packets identified with HTTP and HTTPS headers, providing developers with detailed reports of each check that zooms in on the specific code and HTTP-related errors encountered. 

AWS users can apply ALB through internal load balancing in front of AWS EC2 instances, applications (through Rest API), or containers. Multiple services in a complex environment may share a single ALB load balancer through path-based routings, such as routing it to different server clusters based on application needs. Users can route up to 10 applications behind a single ALB.

Key ALB Features You Should Know

Advanced Routing

ALB works at the application layer (Layer 7), which means it can make smart routing decisions based on content. You can direct traffic using host-based, path-based, and even query string parameters. Perfect for managing complex microservice architectures or directing users to region-specific content.

WebSockets & HTTP/2 Support

Modern applications need modern protocols. ALB supports both WebSockets for real-time, two-way communication and HTTP/2 for faster, more efficient network connections. This helps improve application responsiveness without requiring significant backend changes.

Security with AWS WAF & Shield

ALB integrates directly with AWS Web Application Firewall (WAF) and AWS Shield to protect your applications from common web exploits and DDoS attacks. You can easily apply security policies and monitor traffic for potential threats, all without extra infrastructure overhead.

Flexible Target Types

ALB isn’t limited to EC2 instances. You can route traffic to containers (like those running on ECS or EKS), IP addresses, and even AWS Lambda functions. This makes it ideal for modern, serverless, and containerized environments.

Built-in Authentication & Redirects

Save time and reduce backend complexity by handling user authentication directly at the load balancer. ALB supports integration with services like Amazon Cognito, Microsoft AD, and Google, along with built-in redirect and fixed-response actions to simplify traffic management.

What Is an Elastic Load Balancer (ELB)?

Introduced by AWS in 2009, the ELB, also known as the classic load balancer, is a software-based load balance that automates the traffic distribution process across multiple targets. These targets may include containers and IP addresses.

The ELB operates from the fourth layer (i.e., the transport layer) of the OSI model and transfers requests based on the applied protocol of TCP or IP and links with a similar backend target. For instance, when an ELB receives a client request from a TCP port, it routes the request based on the rules pre-configured during a load balancer setup. 

The classic load balancer serves various functions to provide application stacks with added security, easier management, and reliability. 

Specifically, ELB provides web networks with functions that include:

• Traffic distribution across healthy registered instances
• Centralized SSL certificate management
• User authentication using public key infrastructure
• Support for both IPv4 and IPv6 traffic
• Single entry point for managing traffic to EC2 instances

ELB provides a single entry point for users of EC2 instances, efficiently distributing traffic across available targets. With configured health checks, ELBs can closely monitor the health of each registered target and limit traffic distribution to healthy locations, improving fault tolerance.

While AWS looks to have discontinued EC2-Classic Networking in 2022, Classic Load Balancers remain supported. If your Classic Load Balancers are already deployed within a VPC, no changes are needed. If they are associated with EC2-Classic, AWS recommends migrating them to a VPC environment.

How ELB Works  

Classic Load Balancers operate at Layer 4 of the OSI model, managing traffic based on TCP and IP protocols. When you create a Classic Load Balancer, you register backend instances directly with it and assign them to one or more Availability Zones (AZs).

Having multiple servers behind AZs within a region improves the availability of networks, enabling the ELB to reroute traffic to available AZs during inaccessibility. ELB routes traffic evenly among AZs during default configurations. However, the default setting could lead to overload/load imbalance if servers are not responding to the requests. 

The activation of cross-zone load balancing enables each balancer node to distribute traffic across registered targets across all enabled AZs. Alternatively, disabling cross-zone load balancing limits each balancer node to distributing traffic to its specific AZ. As such, cross-zone load balancing mitigates the risks of potential load imbalances. 

Key ELB Features You Should Know

Layer 4 Load Balancing

Classic Load Balancers operate at the transport layer (Layer 4), routing traffic based on low-level network information like TCP and IP protocols. This makes ELB suitable for simple, traditional applications that don’t require content-based routing.

Health Checks for Target Instances

ELB performs regular health checks on registered targets and routes traffic only to healthy instances. This helps maintain application availability and ensures users aren’t directed to resources that can’t handle their requests.

SSL Certificate Management

You can centrally manage SSL certificates on the load balancer, simplifying encryption for secure communications without the need to configure certificates directly on each backend server.

Support for IPv4 and IPv6

ELB supports both IPv4 and IPv6, allowing applications to handle modern IP traffic standards without additional configuration.

Cross-Zone Load Balancing

With cross-zone load balancing enabled, each ELB node can distribute traffic evenly across all healthy instances in every enabled Availability Zone. This helps prevent overloads and improves fault tolerance, especially during instance failures or high-traffic periods.

Elastic Load Balancer vs Application Load Balancer

ALBs and ELBs share several core functions and capabilities despite their specialized features. For starters, they feature high availability and scalability, and users can choose to add or remove resources when required without disrupting the overall request flow from applications.

ALB and ELB support primary functions that include:

• Sticky Sessions: Maintain user sessions by directing repeat traffic to the same server, using cookies or IP tracking. Helpful for apps that need session persistence, like shopping carts.
• SSL Termination: Offload SSL/TLS decryption to the load balancer to reduce the processing burden on backend servers.
• Idle Session Termination: Automatically close inactive connections after a set period to free up resources.
• Connection Draining: Allows ongoing requests to complete before instances are removed from rotation, great for graceful shutdowns during maintenance.
• Health Checks: Continually monitor backend targets and route traffic only to healthy resources.

The Differences Between ALB and ELB

In 2016, AWS improved its original ELB program with ALB, which provides users with additional features and enhanced functions. 

For instance, while ELB enables users to add and remove listeners according to changing priorities, the ALB provides the extra feature of viewing and editing listener routing rules. As such, users can conveniently direct routes to a predefined target group. 

ALB also rectifies some of the limitations of ELB, which include:

• The unsupported function of forwarding traffic to more than one port per instance
• Incompatibilities with EKS servers that run on Fargate
• Inability to deliver traffic to IP addresses, which prevents traffic to targets outside AWS
• Lack of support for WebSockets and HTTP/2
• Serves only one permitted domain name

One of the most significant differences between ALB and ELB lies in the system of their routing process. While ELB only routes traffic based on routing number, ALB facilitates context-driven routing based on multiple references, including query string parameter, source IP, port number, hostname, and path. 

Additionally, ALB supports Server Name Indication (SNI), enabling it to bypass the conventional limitations of the ELB in serving a single domain name. ALB offers users native HTTP/2 and WebSocket support via multiple requests delivered through a single connection. 

ALB Provides Built-in Capabilities

ELB only allows routing via a single port, while ALB supports distribution through multiple ports and lambda functions. Lambda functions enable users to manage and run various functions, build websites through serverless coding, and create customized ALB targets through serverless methods.  

ALB offloads and optimizes server functions by performing advanced tasks directly from its program, including a preconfigured redirection or fixed response and user authentication through LDAP, Microsoft AD, and Google. The added load balancer function enables applications to focus on their business logic for increased performance. 

Other notable built-in ALB capabilities include:

• Container-based application support, enabling a single instance to host multiple containers listening for network traffic behind the same target group.
• The capabilities of performing fine-grained health checks at the port level. Specifically, ALB has console support for filtering by tags, resource, and resource-based permissions, so users can use IAM policies to implement the fine-grain controls.
• Providing detailed access logs stored securely in a compressed format.

Access Logs 

ALB’s access logs include a detailed breakdown of information that consists of the original HTTP response and response_processing_time, which determines the time required to transfer a client request, request type, and time stamps. 

Common Use Cases: When to Choose Each

Choose ALB if…

• You’re running modern, cloud-native applications or microservices.
  
• You need advanced routing based on URL paths, hostnames, or query strings.
  
• Your application requires WebSockets or HTTP/2 support for real-time communication.
  
• You’re working with containers (ECS, EKS) or serverless architectures (AWS Lambda).
  
• You want built-in user authentication and easy redirect handling at the load balancer.
  
• Security is a top priority, and you plan to integrate AWS WAF and Shield.
  

Choose ELB if…

• You’re supporting legacy applications that rely on simple TCP or SSL traffic routing.
  
• Your workloads run in EC2 Classic (though keep in mind the network has been phased out).
  
• You don’t require content-based routing or advanced traffic management.
  
• Your environment is relatively static and doesn’t leverage containers or serverless architectures.
  
• You need a straightforward Layer 4 load balancer without additional configuration complexity.

Migrating from ELB to ALB

If you’re still using Classic Load Balancers, migrating to an ALB is a smart move toward modernizing your architecture. ALB offers advanced routing, improved performance, and built-in integrations that better support today’s cloud-native applications. But before you jump in, it’s important to plan the transition carefully.

Start by auditing your current ELB setup. Understand which applications rely on it and document configurations like health checks, SSL certificates, and listener settings. From there, assess whether your workloads can take advantage of ALB’s Layer 7 capabilities, such as path-based routing and support for containers or serverless functions.

During migration, map ELB features to their ALB equivalents. Some features, like health checks and SSL management, are more advanced in ALB and may require configuration updates. You’ll also need to plan for DNS changes, updating Route 53 or other services to point traffic to the new load balancer after testing is complete.

Whenever possible, deploy ALB alongside your existing ELB and run traffic simulations or gradual cutovers to minimize risk. Once you’re confident everything is running smoothly, you can decommission the ELB resources to avoid extra costs.

Modernizing with ALB brings clear benefits: smarter traffic routing, improved support for microservices and serverless apps, better security through AWS WAF and Shield integrations, and enhanced visibility with detailed logs and monitoring. And while the migration takes a bit of planning, the long-term payoff is a more scalable, resilient, and efficient infrastructure.

Confidently Manage AWS Load Balancers with LogicMonitor 

Whether you’re sticking with ELB for legacy applications or moving to ALB to support modern, cloud-native workloads, one thing remains the same: visibility is critical. Without the right monitoring in place, it’s nearly impossible to understand traffic patterns, troubleshoot issues quickly, or manage costs effectively.

LogicMonitor gives you full observability across your AWS load balancers, no matter how complex your environment. From real-time performance monitoring to detailed analytics and automated alerts, LogicMonitor helps you keep your infrastructure healthy, scalable, and ready for whatever comes next.