Management Connection kills our internet

[tooleyj]

I have a 100D that is being used primarily for URL filtering and some basic firewall rules between our main network and the router for our Network Technology program here at the college.  I set the device up using a USB cable before Christmas and everything was working fine. 

Each classroom for the Net Tech department is on its own /24 connected to the router.

I am now coming back to this to make a few changes and to make the connection for the management interface.  When I plug the cable from the management interface into a switch that is on one of the /24 networks that are behind the firewall, the firewall seems to stop receiving traffic. When I remove the connection and reboot the 100D everything goes back to normal.  Anyone see anything like this before?

[ede_pfau]

Network loop / broadcast storm?

Is your switch guarded against a broadcast storm, i.e. is (R)STP active?

How about the addresses of your ports on the FGT?

[tooleyj]

ede_pfau wrote:

Network loop / broadcast storm?

Is your switch guarded against a broadcast storm, i.e. is (R)STP active?

How about the addresses of your ports on the FGT?

I wouldn't think it could be a broadcast storm as there are only single connections between the router and each room switch.  The only place you might have a network loop would be from the Fortinet, but it only has 3 connections (inside, outside, and management).  I wouldn't think the management connection could cause a broadcast storm.

The only interface on the FGT is the management as it is set up in transparent mode.