AWS Classic Load Balancer vs Application Load Balancer vs Network Load Balancer

• Elastic Load Balancing supports four types of load balancers:
  • Classic Load Balancer – CLB (Previous Generation)
  • Application Load Balancer – ALB
  • Network Load Balancer – NLB
  • Gateway Load Balancer – GWLB
• While there is some overlap in the features, AWS does not maintain feature parity between the different types of load balancers.

Usage Patterns

• Classic Load Balancer (Previous Generation)
  • provides basic load balancing across multiple EC2 instances and operates at both the request level and connection level.
  • is intended for applications that were built within the EC2-Classic network. EC2-Classic was fully retired in August 2023.
  • is ideal for simple load balancing of traffic across multiple EC2 instances.
  • AWS recommends migrating to ALB or NLB for all new workloads.
• Application Load Balancer
  • is ideal for microservices or container-based architectures where there is a need to route traffic to multiple services or load balance across multiple ports on the same EC2 instance.
  • operates at the request level (layer 7), routing traffic to targets – EC2 instances, containers, IP addresses, and Lambda functions based on the content of the request.
  • is ideal for advanced load balancing of HTTP and HTTPS traffic, and provides advanced request routing targeted at delivery of modern application architectures, including microservices and container-based applications.
  • simplifies and improves the security of the application, by ensuring that the latest SSL/TLS ciphers and protocols are used at all times.
  • supports Mutual TLS (mTLS) authentication to verify client certificate-based identities.
  • supports native JWT (JSON Web Token) verification for secure service-to-service authentication.
• Network Load Balancer
  • operates at the connection level (Layer 4), routing connections to targets – EC2 instances, microservices, and containers – within VPC based on IP protocol data.
  • is ideal for load balancing of TCP, UDP, TLS, and QUIC traffic.
  • is capable of handling millions of requests per second while maintaining ultra-low latencies.
  • is optimized to handle sudden and volatile traffic patterns while using a single static IP address per AZ
  • is integrated with other popular AWS services such as Auto Scaling, ECS, CloudFormation, and AWS Certificate Manager (ACM).
  • now supports security groups (since August 2023) for centralized access control.
  • supports QUIC protocol in passthrough mode (since November 2025) for low-latency mobile and real-time applications.
• Gateway Load Balancer
  • operates at Layer 3 (network layer), providing a transparent network gateway and distributing traffic to virtual appliances.
  • is ideal for deploying, scaling, and managing third-party virtual appliances such as firewalls, intrusion detection/prevention systems (IDS/IPS), and deep packet inspection systems.
  • combines a transparent network gateway (single entry and exit point for all traffic) with load balancing of virtual appliances.
  • uses the GENEVE protocol on port 6081 to exchange traffic with registered virtual appliance instances.
  • maintains flow stickiness using 5-tuple (default), 3-tuple, or 2-tuple.
• AWS recommends using Application Load Balancer for Layer 7 and Network Load Balancer for Layer 4 when using VPC.

Supported Protocols

• Classic ELB operates at layer 4 and supports HTTP, HTTPS, TCP, SSL
• ALB operates at layer 7 and supports HTTP, HTTPS, HTTP/2, gRPC, WebSockets
• NLB operates at the connection level (Layer 4) and supports TCP, UDP, TLS, QUIC, TCP_QUIC
• GWLB operates at Layer 3 and listens for all IP packets across all ports

Load Balancing to Multiple Ports on the same instance

• ALB, NLB, and GWLB support Load Balancing to multiple ports on the same instance
• CLB does not support load balancing to multiple ports on the same instance

Host-based Routing & Path-based Routing

• Host-based routing use host conditions to define rules that forward requests to different target groups based on the hostname in the host header. This enables ALB to support multiple domains using a single load balancer.
• Path-based routing use path conditions to define rules that forward requests to different target groups based on the URL in the request. Each path condition has one path pattern. If the URL in a request matches the path pattern in a listener rule exactly, the request is routed using that rule.
• Only ALB supports Host-based & Path-based routing.

URL and Host Header Rewrite (ALB – New 2025)

• ALB now supports URL and Host Header rewrite capabilities (October 2025).
• Enables modification of request URLs and Host Headers using regex-based pattern matching before routing requests to targets.
• Useful for URL normalization, path rewriting, and host header transformation without application code changes.
• Only ALB supports URL and Host Header Rewrite.

Slow Start

• By default, a target starts to receive its full share of requests as soon as it is registered with a target group and passes an initial health check.
• Using slow start mode gives targets time to warm up before the load balancer sends them a full share of requests.
• Only ALB supports slow start mode

Target Optimizer (ALB – New 2025)

• ALB Target Optimizer (November 2025) allows you to enforce a maximum number of concurrent requests on a target.
• Uses an agent installed on each target that tracks concurrent requests and signals the ALB when capacity is available.
• Enables high-efficiency load balanced applications while maintaining low latency and high availability.
• Returns 503 errors during overload rather than overwhelming targets.
• Only ALB supports Target Optimizer.

Static IP and Elastic IP Address

• NLB automatically provides a static IP per AZ (subnet) that can be used by applications as the front-end IP of the load balancer.
• NLB also allows the option to assign an Elastic IP per AZ (subnet) thereby providing your own fixed IP.
• Classic ELB, ALB, and GWLB do not support Static and Elastic IP address

Connection Draining OR Deregistration Delay

• Connection draining enables the load balancer to complete in-flight requests made to instances that are de-registering or unhealthy.
• All Load Balancer types (CLB, ALB, NLB, GWLB) support connection draining/deregistration delay.

Idle Connection Timeout

• Idle Connection Timeout helps specify a time period, which ELB uses to close the connection if no data has been sent or received by the time that the idle timeout period elapses
• Can be configured for CLB & ALB (default 60 seconds)
• Cannot be configured for NLB (350 secs for TCP, 120 secs for UDP)
• GWLB supports configurable TCP idle timeout (60 to 6000 seconds, since September 2024)
• It is recommended to enable HTTP keep-alive in the web server settings for the EC2 instances, thus making the ELB reuse the backend connections until the keep-alive timeout expires.

PrivateLink Support

• CLB and ALB do not support PrivateLink
• NLB supports PrivateLink (TCP, TLS, UDP)
• GWLB supports PrivateLink via Gateway Load Balancer Endpoints (GWLBE)

Zonal Isolation

• NLB and GWLB support Zonal Isolation which supports application architectures in a single zone. It automatically fails over to other healthy AZs, if something fails in an AZ
• CLB and ALB do not support Zonal Isolation.
• NLB supports Zonal DNS Affinity (since October 2023), allowing clients to resolve the load balancer DNS to an IP in their same AZ.

Deletion Protection

• ALB, NLB, and GWLB support Deletion Protection, wherein a load balancer can’t be deleted if deletion protection is enabled
• CLB does not support deletion protection.

Preserve Source IP address

• As the ELB intercepts the traffic between the client and the back-end servers, the back-end server does not know the IP address, Protocol, and the Port used between the Client and the Load balancer.
• Classic ELB (HTTP/HTTPS) and ALB do not preserve the client-side source IP. It needs to be retrieved using X-Forward-XXX.
  • X-Forwarded-For request header to help back-end servers identify the IP address of a client when you use an HTTP or HTTPS load balancer.
  • X-Forwarded-Proto request header to help back-end servers identify the protocol (HTTP/S) that a client used to connect to the server
  • X-Forwarded-Port request header to help back-end servers identify the port that an HTTP or HTTPS load balancer uses to connect to the client.
• CLB (SSL/TLS) uses Proxy Protocol Version 1 and NLB uses Proxy Protocol Version 2 to provide the information.
• NLB preserves the client-side source IP or needs Proxy Protocol allowing the back-end to see the IP address of the client.
  • If targets are registered by instance ID or ECS tasks, the source IP addresses of the clients are preserved and provided to the applications.
  • If targets are registered by IP address
    • for TCP & TLS, the source IP addresses are the private IP addresses of the load balancer nodes. Use Proxy Protocol.
    • for UDP & TCP_UDP, it is enabled by default and the source IP addresses of the clients are preserved.
• GWLB preserves the source IP address as it operates as a transparent bump-in-the-wire.

Health Checks

• All Load Balancer types support Health checks to determine if the instance is healthy or unhealthy
• ALB provides health check improvements that allow detailed error codes from 200-399 to be configured
• ALB supports HTTP, HTTPS, and gRPC health checks
• NLB and GWLB support TCP, HTTP, and HTTPS health checks
• CLB supports TCP, SSL/TLS, HTTP, and HTTPS health checks

Security Groups

• ALB, NLB, and CLB support security groups
• NLB added security group support in August 2023, enabling centralized access control and inbound rule enforcement
• GWLB does not support security groups
• Note: NLB security groups must be associated at creation time; they cannot be added to an existing NLB that was created without them.

Supported Platforms

• Classic ELB supports both EC2-Classic and EC2-VPC — EC2-Classic was fully retired in August 2023. All load balancers now operate in VPC only.
• ALB, NLB, and GWLB support only EC2-VPC.
• CLB now effectively operates only in VPC (EC2-Classic no longer exists).

WebSockets

• CLB does not support WebSockets
• ALB, NLB, and GWLB support WebSockets

Cross-zone Load Balancing

• By default, Load Balancer will distribute requests evenly across its enabled AZs, irrespective of the instances it hosts.
• Cross-zone Load Balancing help distribute incoming requests evenly across all instances in its enabled AZs.
• CLB → Cross Zone load balancing is disabled, by default, and can be enabled and free of charge.
• ALB → Cross Zone load balancing is always enabled at the load balancer level, but can be turned off at the target group level (since November 2022). Free of charge.
• NLB → Cross Zone load balancing is disabled, by default, and can be enabled but is charged for inter-az data transfer.
• GWLB → Cross Zone load balancing is disabled, by default, and can be enabled.
• Zonal Shift & Autoshift: Both ALB and NLB (with cross-zone enabled) now support zonal shift and zonal autoshift (2024) to move traffic away from an impaired AZ.

Sticky Sessions (Cookies)

• Sticky Sessions (Session Affinity) enables the load balancer to bind a user’s session to a specific instance, which ensures that all requests from the user during the session are sent to the same instance
• CLB, ALB, and NLB support sticky sessions to maintain session affinity
• CLB and ALB maintain session stickiness using cookies.
• NLB supports sticky sessions using a built-in 5-tuple hash table to maintain stickiness across backend servers.
• NLB idle timeout for TCP connections is 350 seconds. Once the timeout is reached or the session is terminated, the NLB will forget the stickiness and incoming packets will be considered as a new flow and could be load balanced to a new target.
• NLB QUIC protocol uses QUIC Connection IDs for session stickiness, which is resilient to client IP/NAT changes.

SSL Termination/Offloading

• SSL Termination helps decrypt requests from clients before sending them to targets and hence reducing the load. SSL certificate must be installed on the load balancer.
• CLB, ALB, and NLB support SSL Termination.
• GWLB does not support SSL offloading (operates at Layer 3).
• ALB and NLB now support Post-Quantum Key Exchange for TLS (November 2025), using hybrid post-quantum key agreement with ML-KEM algorithm for quantum-resistant encryption.

Mutual TLS (mTLS) Authentication (ALB – New 2023)

• Mutual TLS extends standard TLS by requiring clients to present X.509 certificates for authentication.
• ALB can authenticate client certificates from third-party Certificate Authorities or AWS Private Certificate Authority (PCA).
• Supports certificate revocation checks to restrict access for compromised certificates.
• Offloads client authentication to the load balancer, eliminating the need for custom authentication in applications.
• Only ALB supports Mutual TLS authentication.

JWT Verification (ALB – New 2025)

• ALB now supports native JSON Web Token (JWT) verification (November 2025) for secure service-to-service (S2S) or machine-to-machine (M2M) communications.
• Validates token signatures, expiration times, and claims without requiring application code changes.
• Useful for OAuth 2.0 client credentials flow.
• Only ALB supports native JWT verification.

Server Name Indication

• CLB only supports a single certificate and does not support SNI
• ALB and NLB support multiple certificates and use SNI to serve multiple secure websites using a single TLS listener.
  • If the hostname provided by a client matches a single certificate in the certificate list, the load balancer selects this certificate.
  • If a hostname provided by a client matches multiple certificates in the certificate list, the load balancer selects the best certificate that the client can support.

Back-end Server Authentication

• Back-end Server Authentication enables authentication of the instances.
• Load balancer communicates with an instance only if the public key that the instance presents to the load balancer matches a public key in the authentication policy for the load balancer.
• Classic Load Balancer supports Back-end Server Authentication
• ALB does not support Back-end Server Authentication

Capacity Unit Reservation (New 2024)

• ALB and NLB support Load Balancer Capacity Unit (LCU) Reservation (November 2024).
• Allows proactively setting a minimum capacity for the load balancer to prepare for planned traffic spikes.
• Complements existing auto-scaling, useful for product launches, sales events, or traffic migrations.
• GWLB added LCU Reservation support in April 2025.
• ALB, NLB, and GWLB support LCU Reservation.
• CLB does not support LCU Reservation.

Weighted Target Groups (NLB – New 2025)

• NLB now supports weighted target groups (November 2025).
• Allows distributing traffic across multiple target groups with configurable static weights.
• Enables blue/green and canary deployment strategies with zero downtime without needing multiple load balancers.
• ALB has long supported weighted target groups via listener rules; NLB now supports this natively.

CloudWatch Metrics

• All Load Balancer types integrate with CloudWatch to provide metrics, with ALB providing additional metrics
• ALB and CLB report request counts, error counts, error types, and request latency
• NLB and GWLB report Active Flow Count, New Flow Count, and Processed Bytes

Access Logs

• Access logs capture detailed information about requests sent to the load balancer. Each log contains information such as request received time, client’s IP address, latencies, request paths, and server responses
• All Load Balancer types provide access logs, with ALB providing additional attributes

Gateway Load Balancer (GWLB)

• Operates at Layer 3 (network layer) as a transparent network gateway combined with load balancing.
• Designed for deploying, scaling, and managing third-party virtual appliances (firewalls, IDS/IPS, deep packet inspection).
• Provides a single entry and exit point for all traffic, distributing it to virtual appliances while scaling based on demand.
• Uses GENEVE encapsulation protocol on port 6081.
• Supports 5-tuple (default), 3-tuple, or 2-tuple flow stickiness.
• Accessible via VPC route table entries (not via VIP like ALB/NLB).
• Target types: IP and Instance.
• Supports cross-zone load balancing, deletion protection, and connection draining.
• Supports configurable TCP idle timeout (60–6000 seconds) since September 2024.
• Supports LCU Reservation since April 2025.
• Does not support: SSL offloading, security groups, SNI, static/elastic IP, or slow start.

AWS Certification Exam Practice Questions

• Questions are collected from Internet and the answers are marked as per my knowledge and understanding (which might differ with yours).
• AWS services are updated everyday and both the answers and questions might be outdated soon, so research accordingly.
• AWS exam questions are not updated to keep up the pace with AWS updates, so even if the underlying feature has changed the question might not be updated
• Open to further feedback, discussion and correction.

1. A company wants to use load balancer for their application. However, the company wants to forward the requests without any header modification. What service should the company use?
   1. Classic Load Balancer
   2. Network Load Balancer
   3. Application Load Balancer
   4. Use Route 53
2. A Solutions Architect is building an Amazon ECS-based web application that requires that headers are not modified when being forwarded to Amazon ECS. Which load balancer should the Architect use?
   1. Application Load Balancer
   2. Network Load Balancer
   3. A virtual load balancer appliance from AWS marketplace
   4. Classic Load Balancer
3. An application tier currently hosts two web services on the same set of instances, listening on different ports. Which AWS service should a solutions architect use to route traffic to the service based on the incoming request?
   1. AWS Application Load Balancer
   2. Amazon CloudFront
   3. Amazon Route 53
   4. AWS Classic Load Balancer
4. A Solutions Architect needs to deploy an HTTP/HTTPS service on Amazon EC2 instances with support for WebSockets using load balancers. How can the Architect meet these requirements?
   1. Configure a Network Load balancer.
   2. Configure an Application Load Balancer.
   3. Configure a Classic Load Balancer.
   4. Configure a Layer-4 Load Balancer.
5. A company is hosting an application in AWS for third party access. The third party needs to whitelist the application based on the IP. Which AWS service can the company use in the whitelisting of the IP address?
   1. AWS Application Load Balancer
   2. AWS Classic Load balancer
   3. AWS Network Load Balancer
   4. AWS Route 53
6. A company needs to deploy inline virtual firewall appliances to inspect all traffic entering and leaving their VPC. The solution must scale automatically with traffic. Which load balancer type should they use?
   1. Application Load Balancer
   2. Network Load Balancer
   3. Classic Load Balancer
   4. Gateway Load Balancer
7. A solutions architect needs to implement mutual TLS authentication for an application behind a load balancer to verify client certificates. Which load balancer supports this natively?
   1. Application Load Balancer
   2. Network Load Balancer
   3. Classic Load Balancer
   4. Gateway Load Balancer
8. A company wants to perform blue/green deployments with their Network Load Balancer by gradually shifting traffic between two target groups. Which NLB feature enables this?
   1. Path-based routing
   2. Slow start mode
   3. Weighted target groups
   4. Cross-zone load balancing
9. An organization wants to prepare their Network Load Balancer for a planned marketing event that will cause a sudden spike in traffic. Which feature should they use?
   1. Cross-zone load balancing
   2. Load Balancer Capacity Unit (LCU) Reservation
   3. Target group health configuration
   4. Slow start mode
10. A company running a mobile gaming application needs ultra-low latency load balancing with session stickiness that survives client IP changes due to mobile network roaming. Which protocol and load balancer combination best meets this need?
    1. ALB with cookie-based stickiness
    2. NLB with TCP 5-tuple stickiness
    3. NLB with QUIC protocol using Connection ID stickiness
    4. CLB with session cookies

References

• AWS Elastic Load Balancing Features
• ALB Document History
• NLB Document History
• What is a Gateway Load Balancer?
• Migrate your Classic Load Balancer