Most students hide mistakes. Engineers document them. Every fault I hit — in labs or in real life — gets an entry here: the symptom, what I checked, the root cause, the fix, and the lesson. This page is the most honest thing on the site, and it will become gold for interviews.
Newest first. Each entry follows the same format — copy the template at the bottom of this page for new entries.
Date: (example entry — replace with my first real one)
Topic: VLAN trunking (Lab 06)
What failed: PCs in VLAN 10 on two different switches could not ping each other, even though PCs in VLAN 10 on the same switch could.
Symptom: ping timed out only when crossing the inter-switch link. No error messages anywhere — just silence.
What I checked:
show vlan brief → VLANs existed on both switches ✓ show interfaces trunk → EMPTY on SW1. There's the smell. show running-config → inter-switch port was switchport mode access
Root cause: the link between the switches was still in access mode, so it carried only one untagged VLAN instead of tagging traffic for all VLANs.
Fix: configured the inter-switch port as a trunk on both sides: switchport mode trunk, then confirmed with show interfaces trunk.
What I learned: VLANs cannot pass between switches unless the inter-switch link is trunking. "Same switch works, different switch doesn't" now instantly means: check the trunk first.
Command that saved me: show interfaces trunk
labs/<level>/labXX/notes.md in the repo) every time something breaks.
If a week passes with no new mistake, I'm not labbing hard enough.
Same rhythm every time. Less chaos, more engineering. (Also saved in the repo at templates/mistake-template.md.)
#0XX — Short title of what broke Date: Topic: (and which lab, if any) What failed: (one or two sentences) Symptom: (exact error / behavior observed) What I checked: (commands, in the order I ran them) Root cause: (the real reason, not the first guess) Fix: (exact change that solved it) What I learned: (the rule I'll remember) Command that saved me: