Network troubleshooting is the process of identifying the failing layer, testing a clear hypothesis, applying a controlled fix, and verifying service restoration. A repeatable method prevents random configuration changes from making an outage worse.
1. Define the Problem
- Which users, devices, sites, or applications are affected?
- When did it start?
- Is the failure constant or intermittent?
- What changed recently?
- What exact error appears?
2. Check the Physical Layer
Confirm power, cabling, link lights, wireless signal, interface state, speed, duplex, and error counters. An interface that is administratively down requires a different fix from one with no physical carrier.
3. Check VLANs and Switching
Verify access VLAN assignment, trunk status, allowed VLANs, native VLAN consistency, STP state, EtherChannel membership, and MAC address learning.
4. Check IP Configuration
Confirm the IP address, prefix or mask, default gateway, and DNS servers. A self-assigned link-local address can indicate DHCP failure. Look for duplicate addresses and incorrect subnets.
5. Test in a Useful Order
- Ping the local loopback or interface.
- Ping another host on the same subnet.
- Ping the default gateway.
- Ping a remote IP address.
- Test name resolution.
- Test the application and required port.
6. Check Routing and Security
Inspect the routing table, next hop, return path, dynamic-routing neighbors, NAT, firewall rules, and access lists. Successful outbound forwarding does not guarantee that return traffic has a valid path.
7. Verify DNS and the Application
Compare DNS answers with the expected service destination. If IP connectivity works but the application fails, check ports, certificates, authentication, proxies, service status, and server logs.
Document and Close
Record the cause, evidence, change, verification, and prevention action. Monitor after the fix and reverse temporary diagnostic changes.
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