|Try disabling spanning tree on the cisco and see if that doesn't fix the issue.|
If I'm not mistaken, from 'global configuration mode' just enter,
and that should kill the spanning tree. If not, read through the spanning-tree command at that level and see what, if anything needs to be added to it to shut of spanning tree on the switch.
Cisco's spanning tree doesn't play well with others and I've found disabling it in a mixed environment is a must.