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Deleting the VTP Configuration From a Cisco Switch

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Joseph Hornsey
I built my first network in 1994 using PowerLAN.  From there I messed around with NetWare and then moved to Windows NT and Cisco.

WARNING:  If you follow the instructions here, you will wipe out your VTP and VLAN configurations.  Make sure you have backed up your switch!!!


I recently had some issues with a few low-end Cisco routers (RV325) and I opened a case with Cisco TAC.  The basic problem was that I couldn't get the routers to route traffic in this kind of environment:



I wasn't using the firewall feature; just routing.  (The firewalls in the diagram were ASAs).

Well, the Cisco engineer couldn't figure out what was wrong, so I pulled a couple of routers out of the network and set up a small lab so the engineer could remote in and play with it.  The lab environment looked like this:



The networks were all connected with a Cisco 2950 24-port switch using VLAN and a Cisco 2601 configured as a router-on-a-stick.


I know... really old hardware, but it was just lying around collecting dust and it could do what I needed, so why not?


When I attempted to blank out the config, I couldn't get rid of the VLANs... which reminded me how frustrating VTP can be.


For example, years ago, I borrowed one of these 2950's from the datacenter where I have a few cabinets.  Before I returned it, I wiped the config.  Six months later, I get a call from their head engineer informing me that I had taken down the entire datacenter.


VTP configuration information is stored in the VLAN database, which is NOT deleted when one clears the config.  I had actually used VTP in my network, but they didn't and the VTP operating mode of all of their switches were still the default - "server".  So, when they put that switch back into production, my VTP config was pushed out across their network and every single VLAN database on every single switch was overwritten with my VLAN config.


The VLAN database is stored as a file in the flash memory.  To see it, go into privileged mode and issue a directory command for flash:



The VLAN database is stored in the file "vlan.dat".


Since Cisco represents the state-of-the-art for networking equipment, one could assume the VTP configuration could be reset by issuing a command such as "clear config vtp".  Of course, one would assume incorrectly.


You actually have to delete the file:

 


Once you've done that, you should be good to go.  Reload the switch and you'll find the VTP (and VLAN) configuration has been removed.


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