Dangers of Tunnel vision

I gave myself a real fright today.

I was trying to troubleshoot an issue with a configuration that I've never done before; IPv6 ISIS. It was while I was on a training course, I was trying to do some route summarisation. and I struggled with the lab for about 45 minutes, taking out configuration, checking the routing table, putting it back in, checking it again. The topology was about as simple as you can get; two routers. And I checked and double-checked my configuration and found a little error and that made a difference but not the one I wanted.

Over and over and over again I looked at the routing table.

Except...

That is wasn't the full routing table. I was just looking at the ISIS routes. And as soon as the instructor made one little suggestion I looked at the WHOLE routing table, and there was the route... advertised by another protocol with a lower AD! Hidden in the trees, was the wood!

It turned my blood to ice to realise how easily I fell into the tunnel-vision trap - forgetting to take a pause and a step back; to think about alternatives. So that's why I'm writing about it today. If I read this again in 6-9 months' time I can remember today, and the fear I felt when I thought about how easily that could have happened to me in a lab exam, and how I would have totally blown it.

A long way to go, but food for thought.