CCIE Pursuit Blog

June 28, 2007

LFU2 – Know Your Acronyms

Filed under: Lab Fuck Ups,Switching,VTP — cciepursuit @ 10:01 pm

This one is from my first lab rental.  I was just getting my feet wet with lab rentals (especially use reverse telnet from the access router) and decided to do a couple of old CCNP labs.  I decided to warm up with some simple VTP labs. 

For whatever reason I simply could not get the two switches to exchange VTP information.  I’ll spare you the ugly details, but an hour and a half into my “easy” lab I had exhausted my repertoire of VTP troubleshooting (“sh vtp status”, “sh vtp counters”, “debug debug sw-vlan vtp events”, etc.) and I still could not get the damned switches to exchange VTP information.  The links were up and passing frames.  VTP was configured correctly (same VTP domain, no revision conflicts, etc.).  I could not figure out what I had done wrong.

Finally it dawned on me…what does VTP stand for?  VLAN Trunking Protocol.  Well, I had VLANS.  Did I have trunking?  After a quick “show int trunk” I discovered the reason that my switches weren’t passing VTP information.  My links were up, but they were not trunking.  The reason for this was another misunderstanding on my part.  I assumed (always dangerous) that DTP (Dynamic Trunking Protocol) had hooked me up with trunks between these two 3560s.  Wrong!!!  As I mentioned in an earlier post, the default DTP configuration on the 3560 is “switchport mode dynamic auto”.  A connection between two 3560s will NOT form a trunk by default.  The 3550’s default DTP setting is “switchport mode dynamic desirable” which means that it will from a trunk with another switch unless that other switch’s port is in “switchport mode access” or “switchport nonegotiate”.

Even with my confusion over whether DTP would create the trunks for me automatically, I should have verified that trunking was working very early on in my troubleshooting rather than 1.5 hours later. 😦

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