5Diagnosing Problems

• 5.1Troubleshooting methodology and the help desk

  Covers working along a troubleshooting methodology (confirm the symptom -> gather information -> isolate the cause -> act -> verify) instead of poking devices on a hunch, recording work as a ticket with accurate and complete documentation, and prioritizing multiple cases by impact, as the discipline of support work.

• 5.2Packet capture with Wireshark

  Covers the purpose of a packet analyzer (notably Wireshark) that records communication packet by packet so you can see its contents, and how captured traffic is saved as a .pcap file and reopened later for analysis, as a tool for "confirming with your own eyes what is actually flowing."

• 5.3Basic diagnostic commands and interpreting results

  Covers when to use the staple commands—ping for reachability, ipconfig (Windows) / ifconfig and ip (Linux) for local IP settings, tracert/traceroute to trace the path, and nslookup for name resolution—plus correctly interpreting results, such as how a firewall blocking ICMP can make ping fail even though the host is actually reachable.

• 5.4Accessing devices and collecting data

  Covers means of accessing devices and collecting data by situation: RDP/SSH/Telnet for remote operation (Telnet is unencrypted and discouraged; SSH is secure), VPN to safely connect into the office from outside, a console connection and terminal emulator for direct attachment, NMS and cloud management (Meraki) to monitor many devices at once, and scripts to automate repetitive work.

• 5.5Basic show commands on Cisco devices

  Covers the basic show commands for reading a Cisco device's state (show ip interface brief for the interface list and up/up, show cdp neighbors for adjacent Cisco devices, show ip route for the routing table, show mac address-table for MAC learning, show version for IOS/model), the privilege levels of user EXEC mode (>, view-oriented) vs. privileged EXEC mode (#, full), and the ? help and Tab completion aids.