Introduction
Network Load Balancing (NLB) is essential for ensuring high availability, distributing traffic efficiently across servers, and preventing network congestion. However, load balancing misconfigurations can cause slow performance, uneven traffic distribution, downtime, and connectivity failures.
This guide will help you identify and fix load balancing issues, ensuring optimal traffic distribution across your network.
What Causes Network Load Balancing Issues?
Several factors can lead to NLB failures, including:
✅ Incorrect Load Balancer Configuration – Uneven traffic distribution or failed requests.
✅ Unresponsive or Overloaded Servers – Some backend servers are down or underperforming.
✅ Firewall and Security Restrictions – Blocking traffic to the load balancer or backend nodes.
✅ DNS or Routing Issues – Clients resolving to the wrong IP or experiencing delays.
✅ Session Persistence Misconfiguration – Causing session disruptions for users.
✅ Outdated or Faulty Load Balancer Firmware – Affecting stability and performance.
✅ Health Check Failures – Load balancer not detecting server health correctly.
Now, let’s go through step-by-step troubleshooting to resolve these issues.
Step 1: Verify Load Balancer Configuration
If traffic is not properly distributing, start by checking the load balancer settings.
🔹 Check Load Balancer Status (Windows/Linux):
Windows (NLB Cluster Check):
graphql
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nlb.exe query
Linux (HAProxy, Nginx, or F5 LTM):
lua
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sudo systemctl status haproxy
🔹 Ensure the Load Balancer is Listening on the Correct Port:
perl
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netstat -an | grep LISTEN
🔹 Verify the Load Balancing Algorithm:
- Round Robin – Equal distribution of traffic.
- Least Connections – Routes new requests to the least busy server.
- IP Hash – Ensures consistent routing based on client IP.
If traffic distribution looks incorrect, move to Step 2.
Step 2: Check Backend Server Availability
If some servers are overloaded or not receiving traffic, verify their status.
🔹 Check Server Health (Windows/Linux):
php-template
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ping <server IP>
🔹 Ensure All Backend Servers Are Online:
Windows:
pgsql
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Get-Service | Where-Object { $_.Status -eq “Stopped” }
Linux:
bash
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sudo systemctl list-units –type=service –state=failed
🔹 Check for High CPU/Memory Usage on Backend Servers:
- Windows: Task Manager > Performance
Linux:
css
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top
htop
If backend servers are down or overloaded, restart them and proceed to Step 3.
Step 3: Verify Load Balancer Health Checks
Load balancers use health checks to monitor backend servers. If health checks fail, servers are marked as “unhealthy” and stop receiving traffic.
🔹 Check Load Balancer Health Check Settings:
For HAProxy:
perl
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show servers state
For Nginx:
bash
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cat /etc/nginx/nginx.conf | grep health_check
🔹 Manually Test Backend Response:
nginx
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curl -I http://< backend_server > :80
🔹 Adjust Health Check Parameters (Timeout, Interval, Retries):
pgsql
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timeout check 2s
default-server inter 3s fall 2 rise 3
If health checks fail, fix backend server response issues before proceeding to Step 4.
Step 4: Check Firewall and Security Rules
Firewalls can block load balancer traffic, preventing requests from reaching backend servers.
🔹 Check Firewall Status (Windows/Linux):
Windows:
perl
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netsh advfirewall show allprofiles state
Linux:
nginx
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sudo iptables -L
🔹 Temporarily Disable Firewalls (for testing):
Windows:
vbnet
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netsh advfirewall set allprofiles state off
Linux:
bash
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sudo ufw disable
🔹 Ensure Load Balancer is Allowed to Communicate:
Add firewall rules:
css
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sudo iptables -A INPUT -p tcp –dport 80 -j ACCEPT
- For cloud-based setups (AWS, Azure, GCP), update Security Groups or ACLs.
If security settings are correct, move to Step 5.
Step 5: Verify DNS and Routing Configuration
If users cannot resolve the load balancer’s address, DNS issues may be causing the failure.
🔹 Check DNS Resolution:
lua
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nslookup <load balancer hostname>
dig <load balancer hostname>
🔹 Verify the Correct DNS Records Exist:
pgsql
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A record → Points to the Load Balancer’s IP
CNAME record → Used for subdomains mapping
Example (AWS Route 53, Cloudflare, etc.):
css
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example.com. 300 IN A 192.168.1.100
🔹 Check Default Route and IP Forwarding (Linux):
bash
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ip route show
cat /proc/sys/net/ipv4/ip_forward
If routing or DNS records need adjustments, correct them before moving to Step 6.
Step 6: Adjust Session Persistence and Stickiness
If users experience random disconnections, session persistence might be misconfigured.
🔹 For Nginx (Sticky Sessions):
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ip_hash;
🔹 For HAProxy (Cookie-Based Persistence):
bash
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balance source
cookie SERVERID insert indirect nocache
🔹 For Windows Load Balancing (NLB):
vbnet
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nlb.exe set cluster /Affinity:Single
If session issues persist, move to Step 7.
Step 7: Update Load Balancer Firmware or Software
Outdated software can cause NLB instability or performance issues.
🔹 Check Load Balancer Version:
Windows NLB:
vbnet
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Get-WindowsFeature | Select-String “NLB”
Linux HAProxy:
nginx
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haproxy -v
🔹 Update Load Balancer Software:
Windows:
bash
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dism /online /enable-feature /featurename:NetworkLoadBalancingFull
Linux:
sql
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sudo apt update && sudo apt upgrade -y
If issues persist after updating, move to Step 8.
Step 8: Monitor Network Traffic and Load Balancer Performance
If NLB issues recur, monitoring tools can help detect traffic imbalances or failures.
🔹 Recommended Monitoring Tools:
- Wireshark – Analyze traffic load.
- PRTG Network Monitor – Track load balancing efficiency.
- Grafana & Prometheus – Monitor HAProxy/Nginx performance.
If monitoring reveals uneven distribution, adjust load balancing rules accordingly.
Best Practices to Prevent Future Load Balancing Issues
✅ Use health checks to automatically detect and reroute failed backend servers.
✅ Enable logging and monitoring for real-time insights into load balancer performance.
✅ Optimize firewall and security settings to prevent connection issues.
✅ Test NLB configurations in a staging environment before applying changes in production.
✅ Regularly update load balancing software and backend servers for security and performance improvements.
Get Expert IT Support for Load Balancing and Network Optimization
Still facing load balancing failures, uneven traffic distribution, or NLB performance issues?
🔹 TechNow provides expert IT Support Services in Germany, specializing in network load balancing, traffic optimization, and high-availability configurations.
