Kepner Tregoe Network Troubleshooting

Kepner-Tregoe for Network Troubleshooting

What this skill is for

This skill gives a network-troubleshooting agent a disciplined method so it reaches a verified root cause before acting, instead of pattern-matching to a familiar cause and swapping components. The entire value is sequencing and separation: appraise before diagnosing, specify before theorizing, verify before fixing, decide before acting, protect before executing.

You will usually have tools that can read network state (telemetry, device configs, routing/ARP/session tables, logs, packet/flow data, inventory/CMDB). Use those tools to gather the evidence that fills the specification — do not reason from memory when you can observe. The method tells you which evidence matters and what to conclude from it.

Non-negotiable principles

1. Specify before you hypothesize. Never name a cause until you have drawn the fault's boundary (what IS affected vs. what IS-NOT). The boundary is where the cause hides; skipping it is why troubleshooting drifts from symptom to symptom.
2. The IS-NOT carries the answer. Recording what is broken eliminates almost nothing. Recording the nearest comparable thing that could be broken but isn't eliminates whole classes of cause in one line.
3. Test every candidate cause against the whole specification. A cause that explains what IS but contradicts an IS-NOT is wrong, however plausible. Prefer the cause that fits with the fewest assumptions.
4. Verify in the real world before you fix. A cause proven on paper is a hypothesis until a log line, a counter, a table entry, or a controlled test confirms it. Fixing an unverified cause "fixes" a symptom and leaves the root.
5. Separate the incident fix from the permanent fix. Stop the bleeding with the fastest reversible known-good action; the proper fix is a separate, planned decision. Conflating them extends outages.
6. Protect every change before executing it. Prevention + contingency with explicit triggers.
7. Do not take state-changing actions without authorization. Diagnose and recommend freely. Before any config change, failover, reboot, or traffic-affecting action, confirm with the human/operator unless you have standing authorization for that specific action. Read-only investigation needs no confirmation.

Step 0 — Route to the right process

Ask these in order and stop at the first "yes". Running the wrong process is the most common misuse (e.g. debating fixes before the cause is known).

A single incident often flows SA → PA → DA → PPA: triage the mess, find the cause, choose the fix, protect the fix. See references/integrated-workflow.md for a full worked end-to-end incident.

Problem Analysis — the core loop (run this most often)

This is the process you will use most. Full detail and worked examples: references/problem-analysis.md. Execute these steps and emit the specification as a structured record (template below).

1. State the problem. One object, one defect: "what is wrong with what." Two defects = two problems; split them.
2. Build the specification (IS / IS-NOT) across four dimensions. Use your tools to gather each cell. For every IS, find the nearest comparable case that could show the fault but doesn't.
   • WHAT — which object/service; which exact defect (and which defect it is not, e.g. resolution vs. reachability, drop vs. error, SYN-drop vs. mid-session hang).
   • WHERE — site / VLAN / interface / peer / zone / region / path; and which comparable locations are clean.
   • WHEN — first seen; last known-good; time-of-day/load pattern; and comparable times that are clean.
   • EXTENT — how many / how much / what proportion / trend; and how much is unaffected / not spreading.
3. Distinctions & changes. For each row: what is different/unique about the IS side vs. the IS-NOT side? Within each distinction, what changed and when? Changes anchored to the WHEN boundary are the strongest leads — but the most recent change is a candidate, not a conviction.
4. Generate possible causes from distinctions and changes only (not folklore, not the last incident, not the component you distrust).
5. Test each cause against the full spec: "If this were true, would it produce everything that IS and nothing that IS-NOT, with the fewest assumptions?" Eliminate any cause that contradicts an IS-NOT. Rank survivors by assumption count.
6. Verify the most probable cause with a targeted, ideally read-only observation before proposing a fix. Name the exact command / query / test that confirms it.

If a cause that should be right contradicts an IS-NOT, the specification is wrong or incomplete (a mis-recorded IS-NOT, a blank dimension) — re-specify, don't re-guess. If the best cause leaves exactly one boundary unexplained, suspect a second, interacting cause and re-specify that boundary.

Fast elimination: domain signature library

Before generating causes, check the fault's signature against references/domain-signatures.md. It maps common observable signatures to the cause classes they eliminate or point to, per domain (routing/BGP, wireless/RF, firewall/security, SD-WAN/overlay, DNS/DHCP, cloud/hybrid, performance). Example: interface errors are zero eliminates physical-layer causes; auth succeeds but clients drop, band-specific points to RF/DFS, not RADIUS. This is Problem Analysis compressed into a lookup — use it to prune fast.

Output format — the troubleshooting record

Always produce the reasoning as a structured record so a human (or the next agent) inherits the logic, not just the conclusion. Use this template:

PROBLEM: <object> — <defect>

SPECIFICATION
  WHAT   IS: <...>        IS-NOT: <...>
  WHERE  IS: <...>        IS-NOT: <...>
  WHEN   IS: <...>        IS-NOT: <...>
  EXTENT IS: <...>        IS-NOT: <...>

DISTINCTIONS / CHANGES: <what differs about the IS side; what changed and when>

CANDIDATE CAUSES → TEST
  - <cause>: explains IS? explains IS-NOT? assumptions? → <verdict>
  ...

MOST PROBABLE CAUSE: <cause>
VERIFICATION: <exact read-only command/query/test to confirm, and expected result>

INCIDENT FIX (fast, reversible): <action + how to roll back>   [requires authorization]
PERMANENT FIX (planned decision): <route to Decision Analysis if options exist>

For Situation Appraisal, Decision Analysis, and Potential Problem Analysis, use the record templates in references/worksheets.md.

The other three processes (summaries; see references for full method + examples)

• Situation Appraisal — triage. List concerns (single & specific), rate each on Serious / Urgent / Growth, rank, and route each to PA/DA/PPA with an owner. In an alert storm, collapse alarms sharing a time+location signature into their root, then hunt the one alarm that does not fit the pattern — it is noise or a second incident. Full method: references/situation-appraisal.md.
• Decision Analysis — choosing. Define MUSTs (mandatory, measurable, pass/fail) and weighted WANTs (1–10). Screen alternatives against MUSTs (eliminate failures before scoring), score survivors on weighted WANTs, then assess adverse consequences of the leader before committing — the winner is the best score that survives its own risk review. Full method: references/decision-analysis.md.
• Potential Problem Analysis — protecting a plan. For each vulnerable step: the potential problem, a preventive action (reduces probability, acts on the cause) and a contingent action (reduces impact, acts on the effect) with an observable trigger and a named owner. Golden rule: never remove the fallback until the replacement is verified. Full method: references/potential-problem-analysis.md.

When NOT to run the full method

A known, trivial fault with an obvious fix (port admin-down, full disk, obviously-expired cert) should be fixed, not specified. Run the full apparatus when the cause is genuinely unknown, the blast radius is large, the situation is cluttered, or a wrong move is expensive/hard to reverse. Even then, the habit — "what's the IS-NOT?" and "have I verified this?" — costs nothing and should be reflexive.

Reference files

• references/problem-analysis.md — Full PA method, IS/IS-NOT deep dive, six worked network faults, hard cases.
• references/situation-appraisal.md — Full SA method, Serious/Urgent/Growth, on-call and alert-storm examples.
• references/decision-analysis.md — Full DA method, MUST/WANT, three worked decisions incl. vendor selection.
• references/potential-problem-analysis.md — Full PPA/POA method, change/migration protection, postmortems.
• references/domain-signatures.md — Signature→cause lookup tables for seven network domains.
• references/worksheets.md — Fill-in record templates for all four processes and a facilitation script.
• references/integrated-workflow.md — One incident worked end-to-end through all four processes.