Remember a few weeks back when I had a bad day?  I was actually at HQ that day to do some work for a project, but that got put off due to the extenuating circumstances.  When we finally got back around to do the work, we wound up adding a switch in the data center to extend a VLAN over to a rack.

I had a really, really bad day this past Tuesday.  I mean, a really bad day.  I guess I should have seen it coming since the last #stabbytuesday was uneventful.  Here’s what said cosmos had in for me and the lessons I took away.  Most of these are things we’ve all lived before, but, for various reasons, I got blindsided.  I expected more from myself.

VLAN Trunk Protocol (VTP) is a little gem on Cisco switches that allows you configure VLANs in one place and have them appear on all of your switches. This is great for large enterprises with 8457839 switches all trunked together because who wants to configure the new VLAN for that one-off application on all 8457839 switches?

VTP works by having designated VTP servers (not real servers like your Linux box, but a switch) tell the rest of the switches in the network with what VLANs they should be configured. All the designated VTP clients say “OK” and configure themselves with those VLANs. When you take a VLAN out of the server, all the clients take it out; when you add a new VLAN, all the clients add it as well. The server and client designation is known as the VTP mode, and there’s one more to mention. When a switch is in VTP transparent mode, he will see VTP from the servers but will ignore them and pass them on to the next switch as if nothing ever happened.