PlantUML DiagramLayer 1 Troubleshooting Tree
A structured lesson workspace with readable content, hands-on examples, and a clean path to completion.
A user reports intermittent connectivity. Their port shows `up/up`. `show interfaces` shows CRC errors climbing slowly — a few per hour. Nothing obvious. The helpdesk closes the ticket. Two weeks later the port fails completely. The physical layer was degrading the whole time — the warning signs were there, but nobody looked at the counters.
Layer 1 problems are rarely dramatic. They start as intermittent slowness, a few errors per hour, occasional retransmits. If you only look at interface state (up/up), you miss them entirely. Counters tell the real story.
| Status | Protocol | Meaning | Cause |
|---|---|---|---|
| up | up | Working | Normal |
| up | down | Physical OK, Layer 2 issue | Keepalive failure, protocol mismatch |
| down | down | Physical link down | Cable, remote end, SFP |
| administratively down | down | Shut by admin | `shutdown` command |
| up | up (err-disabled) | Protection feature triggered | Port security, BPDU Guard, UDLD |
`1287 CRC` errors with a clean duplex and speed — that's a physical problem. The cable or SFP is corrupting frames. `0 runts, 0 giants, 0 late collisions` rules out duplex mismatch. The rate matters: 1287 over weeks is minor, 1287 per hour is a failing cable. Check the counter rate, not the total.
Layer 1 problems hide in counters. Get into the habit of looking at `show interfaces` on any port you touch — not just the state, but the error counters. A port with clean counters is a port you don't have to think about again. A port with climbing CRC is one that will page you in two weeks if you ignore it.
Finish the lesson once you have worked through the material. This awards ★ 30 XP.