Upgraded Dell 8024F 10GbE switch, connected Cisco became "hub"

I had a very strange problem yesterday.

We have two stacked Cisco 3750E Switches with 2 10GbE uplinks each and a Dell 8024F 10GbE switch. There are one 10GbE link from the stack to the Dell switch.

A couple of days ago I upgraded the Dell switch from firmware version to the newest

I wanted to use private vlan in our environment, and the 5.x fimware supports this.

The problem was that we suddenly saw 900-1000 Mbit/s traffic on all ports on the Cisco switches connected to vSphere hosts (and maybe other trunk ports). I first concluded some problems with the private vlan setup, but the problem did not go away when I removed this config from switches AND DVS.

The strange thing was that the load on the Dell switch was quite low, with high traffic only FROM the Cisco switch.

There are no switching loops, no firewalls/bridges running on VMs etc.

After booting from the old firmware all was back to normal. I checked traffic using wireshark (trunk port), and it was not broadcast, multicast or virus traffic. It was "all types of traffic", including traffic that should only go directly to other switch ports. So all traffic was sent to all trunk ports.

What can be the problem??

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