How to Reset a Cisco Catalyst 3550 / 3560 Switch to Factory Defaults

in privileged mode: (#)

delete flash:vlan.dat

Switch#delete flash:vlan.dat
Delete filename [vlan.dat]?
Delete flash:vlan.dat? [confirm]

erase startup-config

Switch#erase startup-config
Erasing the nvram filesystem will remove all configuration files! Continue? [confirm]
[OK]
Erase of nvram: complete
Switch#
*Mar  1 00:11:37.445: %SYS-7-NV_BLOCK_INIT: Initialized the geometry of nvram

reload

If you are prompted to save anything before reloading, respond by saying no.
After the switch reloads you should get the initial config prompt.

Switch#reload

System configuration has been modified. Save? [yes/no]: no
Proceed with reload? [confirm]

Sep 17 21:19:46.518: %SYS-5-RELOAD: Reload requested by console. Reload reason: Reload command
Using driver version 1 for media type 1
. . . 
         --- System Configuration Dialog ---

Enable secret warning
----------------------------------
In order to access the device manager, an enable secret is required
If you enter the initial configuration dialog, you will be prompted for the enable secret
If you choose not to enter the intial configuration dialog, or if you exit setup without setting the enable secret,
please set an enable secret using the following CLI in configuration mode-
enable secret 0 <cleartext password>
----------------------------------
Would you like to enter the initial configuration dialog? [yes/no]: