ALB vs NLB vs GLB vs CLB: Which Load Balancer Should You Choose?

Key Takeaway

ALB is for HTTP/HTTPS web applications (L7), NLB is for TCP/UDP high-performance workloads (L4), GLB is for third-party security appliances. Choose ALB for web/API services, NLB when you need ultra-low latency or static IPs, GLB for firewall/IDS integration. CLB is legacy and not recommended for new deployments.

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What are AWS Load Balancers?

Elastic Load Balancing (ELB) is a service that automatically distributes incoming traffic across multiple targets (EC2, containers, Lambda, etc.). It provides high availability and fault tolerance with 4 types available.

Client Request → Load Balancer → Target Group (EC2, Containers, Lambda)
                      ↓
              Health Check excludes unhealthy instances

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ELB 4 Types at a Glance

Comparison	ALB	NLB	GLB	CLB
OSI Layer	Layer 7	Layer 4	Layer 3	Layer 4/7
Protocol	HTTP, HTTPS, gRPC	TCP, UDP, TLS	IP	TCP, HTTP, HTTPS
Latency	~10ms	~100μs (ultra-low)	-	Medium
Static IP	❌	✅	❌	❌
Path-based Routing	✅	❌	❌	❌
Host-based Routing	✅	❌	❌	❌
Lambda Target	✅	❌	❌	❌
PrivateLink	❌	✅	✅	❌
Released	2016	2017	2020	2009
Recommended	✅ Yes	✅ Yes	✅ Yes	❌ Legacy

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Application Load Balancer (ALB)

What is ALB?

ALB operates at OSI Layer 7 (application layer). It understands HTTP/HTTPS traffic and can route based on URL paths, host headers, HTTP headers, and more.

Key Features

1. Path-based Routing

example.com/api/*    → API server target group
example.com/images/* → Image server target group
example.com/*        → Web server target group

2. Host-based Routing

api.example.com  → API target group
www.example.com  → Web target group

3. Various Target Types

• EC2 instances
• IP addresses (including on-premises)
• Lambda functions
• ECS/EKS containers

ALB Ideal Workloads

• Web applications (HTTP/HTTPS)
• Microservices architecture
• Container-based applications (ECS, EKS)
• API Gateway alternative
• Lambda function invocation

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Network Load Balancer (NLB)

What is NLB?

NLB operates at OSI Layer 4 (transport layer) and provides ultra-high-performance load balancing at the TCP/UDP protocol level. It can handle millions of requests per second with less than 100 microseconds latency.

Key Features

1. Static IP Addresses

• One static IP per AZ
• Elastic IP attachable
• Useful for firewall whitelisting

2. Ultra-low Latency

• ~100x lower latency than ALB
• Millions of requests per second

3. Source IP Preservation

• Maintains client's original IP address
• No X-Forwarded-For header needed

4. Long-lived TCP Connections

• WebSocket
• Database connections
• Game servers

NLB Ideal Workloads

• Real-time game servers (UDP)
• IoT device connections
• Financial trading systems (ultra-low latency)
• Media streaming
• VoIP services
• Services requiring static IPs
• Direct TCP/UDP protocol usage

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Gateway Load Balancer (GLB)

What is GLB?

GLB operates at Layer 3 (network layer) and is used to deploy and scale third-party virtual appliances (firewalls, IDS/IPS, etc.).

Key Features

1. Transparent Network Gateway

• All traffic passes through appliances
• Single entry/exit point

2. Auto Scaling

• Scale appliances based on traffic
• Health Check excludes failed appliances

3. GENEVE Protocol

• Encapsulates original packets
• Returns after appliance inspection

GLB Ideal Workloads

• Third-party firewalls (Palo Alto, Fortinet, etc.)
• Intrusion Detection/Prevention Systems (IDS/IPS)
• Deep Packet Inspection (DPI)
• Network monitoring

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Classic Load Balancer (CLB)

What is CLB?

CLB is AWS's original load balancer (2009) that provides basic load balancing at both Layer 4 and Layer 7. Not recommended for new deployments.

CLB Limitations

• No path/host-based routing
• Limited WebSocket support
• Single SSL certificate only
• No container targets

Migration Recommended

AWS recommends migrating from CLB to ALB or NLB:

• HTTP/HTTPS → ALB
• TCP/UDP → NLB

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ALB vs NLB: When to Choose Which?

Selection Criteria Summary

Requirement	Choice
HTTP/HTTPS traffic	ALB
URL path-based routing	ALB
Host-based routing	ALB
Lambda function invocation	ALB
gRPC protocol	ALB
TCP/UDP traffic	NLB
Static IP address needed	NLB
Ultra-low latency required	NLB
Millions of requests/sec	NLB
Source IP preservation	NLB
WebSocket (long-lived)	NLB

Real-World Scenario Selection

Scenario 1: Web Application

Requirement: HTTP/HTTPS traffic, separate /api and /web routing
→ ALB (path-based routing)

Scenario 2: Online Gaming

Requirement: UDP protocol, ultra-low latency, massive concurrent connections
→ NLB (L4, UDP support, high performance)

Scenario 3: Microservices

Requirement: Different URL paths per service, ECS containers
→ ALB (path-based routing, container support)

Scenario 4: Financial Trading System

Requirement: TCP, ultra-low latency, fixed IP for partner integration
→ NLB (static IP, ultra-low latency)

Scenario 5: Security Inspection

Requirement: Route all traffic through third-party firewall
→ GLB (appliance integration)

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ELB Common Features

Features available across all ELB types:

Feature	Description
Health Check	Monitor target status, exclude unhealthy targets
High Availability	Auto-distribute across multiple AZs
Auto Scaling Integration	Automatic integration with EC2 Auto Scaling
SSL/TLS Termination	Handle encryption at load balancer
CloudWatch Metrics	Performance monitoring
Access Log	Store request logs in S3
Connection Draining	Remove instances after completing existing connections

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SAA-C03 Exam Focus Points

1. ✅ ALB vs NLB selection: HTTP → ALB, TCP/UDP → NLB
2. ✅ Static IP requirement: Only NLB supports static IP and Elastic IP
3. ✅ Path-based routing: Only ALB supports
4. ✅ Lambda targets: Only ALB supports
5. ✅ GLB use case: Third-party security appliances
6. ✅ CLB migration: ALB/NLB recommended for new deployments

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Frequently Asked Questions

Q: Which is faster, ALB or NLB?

NLB is much faster. NLB operates at Layer 4, forwarding packets directly with ~100 microseconds latency. ALB parses HTTP at Layer 7, taking ~10 milliseconds.

Q: Can I use static IP with ALB?

Not directly. ALB only provides dynamic IPs. If you need static IPs, place NLB in front of ALB, or use Global Accelerator.

Q: Can I use ALB and NLB together on one load balancer?

Yes. NLB → ALB configuration lets you use both static IPs and L7 routing. This pattern is useful when combining PrivateLink with ALB.

Q: Which load balancer should I use for WebSocket?

Both support it. ALB natively supports WebSocket, and NLB handles it as TCP connections. NLB may be more suitable for long-lived connections.

Q: Can I continue using CLB?

Technically yes, but not recommended. AWS only adds new features to ALB/NLB and recommends migrating from CLB. The console provides a migration wizard.

Q: What is Cross-Zone Load Balancing?

A feature that distributes traffic evenly across all targets in all AZs. ALB has it enabled by default (free), NLB has it disabled by default (enabling incurs inter-AZ data transfer costs).

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Related Posts

• Auto Scaling Group Setup and Policies
• Route 53 Routing Policies
• VPC Fundamentals: CIDR, Subnets, Route Tables

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References

• AWS Elastic Load Balancing Features
• ALB vs NLB vs GLB Comparison (AWS)
• Understanding AWS ALB and NLB Differences