Troubleshoot a network connectivity or performance problem

Help me troubleshoot a network problem. Symptom: [what is failing - e.g. "users in one VLAN can't reach the file server", "VPN drops every 20 minutes", "slow access to a cloud app"] Who and what is affected: [one user / one site / everyone; specific apps or services] When it started and what changed: [recent changes, updates, new devices, config pushes] Environment: [on-prem, AWS, Azure, hybrid; key devices and vendors - firewalls, switches, SD-WAN, VPN, ZTNA/SASE] What I've already checked: [results so far] Output I can share: [PASTE ping, traceroute, logs, or config snippets with IPs, hostnames, and secrets masked] Work through it like this: 1. Restate the problem and the scope, and list the most likely causes ranked by probability given my evidence. 2. A step-by-step diagnosis from the bottom up (physical/link, IP and routing, DNS, firewall and security policy, application), with the exact commands or console checks for my platform and what a good vs. bad result looks like. 3. For each likely cause, the fix and how to confirm it worked. 4. Security checks: whether a policy, certificate, or access rule change could explain it, and fixes that do not weaken security (no "allow any" rules as a permanent fix). 5. Rollback steps for any change, and what to monitor afterward. Only reason from the evidence I give. Mark assumptions clearly and tell me what extra output would narrow it down. Remind me to follow my change-control process before touching production.