Isolate a physical-layer fault with link state, counters and a loopback ladder

short · 40 min · Objective 4.1

Task

Diagnose a link that is down and a link that is up but broken using only physical-layer evidence: link state, carrier, negotiated speed and duplex, and error counters. Then test outward from the machine itself -- loopback, own address, neighbour -- to find the first step that fails, which is where the fault is.

Steps

  1. Save ip -s link show <if> and ethtool <if> for lin-srv's lab adapter to lab/link/healthy.txt.
  2. In the hypervisor, disconnect the adapter's virtual cable. Save the same two outputs to lab/link/down.txt.
  3. Still disconnected, run the ladder: ping 127.0.0.1, ping lin-srv's own address, ping win-srv. Record each result in lab/link/ladder.csv with header step,target,result, where result is ok or fail.
  4. Reconnect, and repeat the ladder, appending the results.
  5. Record in lab/link/isolation.txt the order in which you would swap parts on a real server with the same symptom -- cable, switch port, transceiver, adapter -- and why that order.

Verify

These checks run in a POSIX shell: Terminal on macOS or Linux, and on Windows Git Bash (it comes with Git for Windows) or WSL. A stock Windows PowerShell or Command Prompt has no awk or grep, so there the first line fails.

grep -Ec 'Link detected: yes' lab/link/healthy.txt
grep -Ec 'Link detected: no|NO-CARRIER' lab/link/down.txt
grep -Ec 'Speed|Duplex' lab/link/healthy.txt
awk -F, 'NR>1 {print $1" "$2" "$3}' lab/link/ladder.csv
awk -F, 'NR>1 && $2=="127.0.0.1" && $3!="ok" {bad++} END {print bad+0" failed loopback test(s)"}' lab/link/ladder.csv
grep -Eic 'cable|port|transceiver|adapter' lab/link/isolation.txt

The healthy capture shows a link and a negotiated speed and duplex; the disconnected one shows no carrier. With the cable out, loopback and the server's own address normally still answer while the neighbour does not, which places the fault outside the network stack. If the server's own address fails too, look at ip addr: some network managers remove the address when the carrier drops. The loopback test must never fail; if it does, the problem is the operating system, not the network.

Notes

Error counters in ip -s link and on the switch port are the evidence for a link that is up but damaged: rising CRC or frame errors point to a cable or transceiver, and late collisions to a duplex mismatch.

This is an independent study companion for CompTIA Server+ SK0-005 and is not produced by or endorsed by CompTIA.