asterisk users - Dec 2007 - Catalyst 2950 series with Asterisk + FAX

Hello to all.
would like to know if somebody already had problems of transmission of
FAX through interconnection two asterisk with protocol IAX2 and codec
Ulaw with siwtches Cisco Systems 2950.
I explain: If I place this family of switches(Catalyst 2950) passes
through voice, normally, but the FAX is not transmitted.
If I place another Switch (3com for sample) the voice passes through
normally and the FAX is transmitted and received normally.
Somebody already passed previously for this problem?
Best Regards

Josu?