Diagnosing Problems
This domain tests a structured troubleshooting approach and the commands used to isolate faults. Questions present a symptom and ask for the most likely cause or the next best check. Work from the physical layer upward and verify each layer before moving on.
Method
Identify the problem, gather information, analyze it, eliminate causes, test the likely cause, and document the result. Start with power, link lights, cable, and port status. Then check Layer 2: VLAN assignment, MAC learning, and STP state. Then Layer 3: IP address, mask, default gateway, and routing. Finally check DHCP, DNS, and the application.
Commands
Use ipconfig or ifconfig, including ipconfig /all, to inspect addressing. Use ping for reachability, traceroute or tracert for path, nslookup for DNS, arp -a for local mappings, and netstat for connections. On switches and routers, use show interfaces, show ip route, show vlan brief, and show mac address-table. On wireless, check SSID, signal strength, and channel utilization.
Worked example
A user can ping 8.8.8.8 but cannot open www.example.com. IP connectivity works, so DNS is the likely fault. Run nslookup www.example.com; if it fails, check the DNS server settings shown by ipconfig /all. If ping to the default gateway fails, the problem is local: cable, VLAN, or IP configuration. If the gateway responds but a remote IP does not, check routing, NAT, or firewall policy.
Common symptoms
Duplex mismatch causes late collisions, CRC errors, and poor throughput. A cable fault causes no link or intermittent connectivity. Wireless interference causes retries and low signal-to-noise ratio. A DHCP failure often leaves a 169.254 address. A wrong subnet mask makes some local hosts unreachable. A VLAN mismatch lets a host reach local devices but not its gateway.
Traps
Do not replace hardware before testing it. A successful ping to a name does not prove every client has correct DNS settings. Traceroute showing * * * can mean a firewall blocks ICMP, not that the path is dead. A link light does not prove the cable is good. Always document the fix.