Sounds like a test question doesn't it? Well unfortunately it happens often enough in real production networks. A new switch is added with VTP server mode turned on and a higher revision number than the current VLAN database. This can cause a totally bogus VLAN database to be propagated to the network via VTP if it is enabled on the production switches. While there are plenty of ways to prevent this from happening, even the best network team can occasionally have a bad day.
Cisco's EEM provides a handy way of backing up your vlan.dat file so that you can quickly and relatively easily restore your VLAN database.
event manager session cli username "user" ! Determines the user that the script runs as. If you use TACACS+ command authentication this is important. event manager applet backup-vlan event timer cron cron-entry "0 23 * * *" maxrun 60000 ! Schedules the script to run at 23:00 every day. action 1 cli command "enable" action 2 cli command "configure terminal" action 3 cli command "file prompt quiet" ! Eliminates the "Are you sure?" prompts. action 4 cli command "end" action 5 cli command "copy const_nvram:/vlan.dat scp://user:password@FQDN/vlan.dat" ! Copies vlan.dat to a SCP server. action 6 cli command "configure terminal" action 7 cli command "no file prompt quiet" ! Restores the "Are you sure?" prompts. action 8 cli command "end"