Network Troubleshooting Guide for IT Professionals - Paessler Blog

**Intermittent failures** can't be reproduced on demand. Packet captures and alerts only happen after the fact. Without continuous monitoring and historical baselines for comparison, you're troubleshooting in the dark. **Asymmetric routing** can cause traffic flows to work one direction but fail in the other, or use different paths with different network performance characteristics. Your basic ping tests succeed but applications timeout. **Bandwidth and performance problems** first show as degraded application performance, higher latency, and packet loss during load. But identifying which traffic is consuming capacity, and whether that usage is legitimate or a problem caused by malware or other bad actors, is difficult. **Configuration drift** occurs slowly. Over months and years your actual network configurations begin to diverge from your network diagram, making troubleshooting exponentially more difficult. … |**Symptom**|**Likely Cause**|**First Diagnostic Step**| |--|--|--| |**One direction works, other doesn't**|Asymmetric routing or stateful firewall issues|Run `traceroute` in both directions and compare paths| |**Slow performance for specific applications**|QoS misconfiguration, application server issues, or port blocking|Test application ports specifically and check QoS policies| |**Everything worked until recent change**|Configuration error or incompatible firmware|Review change logs and consider rollback| |**No valid IP address assigned**|DHCP server failure, IP address pool exhaustion, or DHCP relay issues|Check DHCP server logs and verify scope availability|