If you have saved any modification to the running-config file, then there will be no difference and therefore you won't be able to compare. Otherwise, what I would do is the following:

- sho running-config till the last line.

- copy the contents and paste to a text editor.

- reload the device

- sho startup-config

- compare each line in the startup-config file against the contents you have pasted in the text editor(which should be the running-config file).

Hope this helps.

SK

There's a suite of commands just for this:

http://www.cisco.com/en/US/docs/ios/12_4/cfg_fund/configuration/guide/h_diff_ps6350_TSD_Products_Configuration_Guide_Chapter.html

That link is for router IOS but the commands are preset on recent switch software revs. They're "unsupported" so they don't show up in the switch documentation and therefore I can't tell you what minimum rev you need to be running. I also believe you need to be on a switch family >=3550.

Edit: I just checked my Cat6.5k (SUP2/MSFC2) and the command is not present. My Cat4.5k does have it. Bottom line is YMMV. :-)

Edit2: You also need to specify the file system where the files are that you want to diff. In your case, system:running-config and nvram:startup-config