Layer 1 – The Root of All Evil

The title is actually a play on the title Cabling: It Ain’t Sexy, But It’s Got Teeth!, a short but excellent post on the often-overlooked issues that can be caused by cabling. I just want to expand on it a little bit.


I work for a service provider and I monitor our data equipment. I also check all of the tickets I open and read their resolutions. I can generally determine the root cause of any problem. I work exclusively with WAN equipment. The moment something touches a LAN, I stop. I don’t work on LAN equipment. I occasionally open a ticket on a Layer 3 switch, but it’s very rare and is usually only done if the entire switch is down.

Root Causes

With the above out of the way, I can tell you that 90% of my tickets are the result of a Layer 1 problem. Layer 1 seems to be the most common trouble in any situation at our company. I can’t stress the importance of checking Layer 1 before you even look at a config. Just look at the interface and see how many errors it’s taking or the last time it went down. If it coincides with your problem, chances are you have a bad cable. Maybe it’s just working well enough to maintain service and as traffic increases it just falls apart. Maybe it’s cut, as is often the case when working for a service provider (fiber cuts all over the place!).

If your routing protocols bounce (drop and re-establish), chances are that your physical link (whether it’s fiber or copper) either went completely down or took so many errors that your routing protocols weren’t able to reliably transmit/receive hellos.


Again, I’d like to thank @JJRinehart for his original post which inspired me to basically say the same thing he did. 🙂 Thanks!

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