I found this web forum on the same subject.  He turned off spanning tree on the Dell switch and fixed the issue. www.experts-exchange.com/.../Q_26997120.html You will need to scroll all the way to the bottom of the page to see the dialogue without registering for the site. Let us know how it responds.  
Morning all I have recently expanded my fibre connections by adding a 2nd 6224F switch and stacked them; this is all working fine. I now want to expand the bandwidth to the desktop and have purchased 2 Netgear GS748TS 48port 10/100/1000 switches and am having real trouble in getting to 6224 to talk to it. I am using  2 new Dell SX transceivers in both the Dell and Netgear Switches.  I have connected them across the fibre and they seem to be seeing each other; the lights go green and in the GUI on both switches the links are saying they are up, but the problem is that no traffic is flowing down the link.  I have tried connecting my laptop to the netgear and pinging the DC but get timeouts. Has anyone connected these before?  I believe that the issue in the in Dell as 2 netgears connected work fine. I hope someone can help me. Wayne
Connecting 6224F 