Check this out, matches with a post I put up a few days ago about Cisco (Bridge Icon) Switches being a culprit
200,000 Cisco Switches hacked!
https://www.indybay.org/newsitems/2006/08/16/18297965.php
Check out this video, important info related to Cisco Switches is at 13:07 called the Superloop
https://www.youtube.com/watch?v=3rZWyZxnG80