You cannot have loops on a Layer 2 network, or it will eventually take down the LAN.
The reason is that it results in a broadcast storm, as individual broadcast packets get duplicated and repatedly travel the network, amplifying itself, as every broadcast frame gets flooded out all ports (except the port on the switch it was received on).
Broadcast storm (which kills the LAN) is what you get if you have switches that don't support spanning tree protocol, and you plug a switch into your LAN twice.
If the "ACT" light on your Ethernet ports stay solid _on_ where they're normally just flashing, then you've got a problem..
Link aggregation (using two ports as one link) is a special feature, and you need to configure that special feature on both sides before plugging the two ports in.
Otherwise, you get something very bad, since your powerconnects don't have the spanning-tree protocol safety net to stop a loop from forming
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by: MysidiaPosted on 2009-03-03 at 22:04:14ID: 23792045
I believe you're creating a loop.
From what I see in the documentation, the Powerconnect 27xx do not seem to have any loop-avoidance features like STP support.
If you want to setup a second port for redundancy, you need to first setup the link aggregation on both switches, before connecting the two ports.
See the user's guide
http://support.dell.com/su
Page 53
''
Adding a port to a LAG
1. Open the LAG Aggregation Configuration Page
2. Toggle the button under the port number to assign the LAG number
3. Click Apply Changes
''
Essentially, you want to setup a "LAG" (aggregate group) on both switches, in the proper VLANs, etc.
Make sure all 4 ports have an identical config on both switches.
Then on both switches, move the two ports into the LAG at both ends.
_Then_ once aggregation is properly configured, you may connect them both at once.