Hi David,
As long as the ports are capable of autonegotiation, you do not have to set the speed or duplex statically. I was asking, though, because I have had an experience with a Dell blade switching module that was fixed at 1Gbps/full duplex and was not able to work with any other combination of port speeds and duplexes.
How to check whether a loop has formed? First, I would go after the "show spanning-tree" output on all switches that are connected in a physical loop. Check all the ports that connect the switches together and whose sequence forms a physical loop. At least one of these interconnecting ports among the switches must in Blocking (or Discarding) state. If it is not, you most probably already have a Layer2 loop. An existence of a loop will always have a detrimental effect on the performance of the network and devices but the actual symptoms may vary wildly, from LEDs just blinking crazily and feeling that the network "just does not feel well" up to a network crash and loss of entire connectivity. In any case, if a frame starts looping in a network, you will notice an exorbitant number of received/sent frames on an interface (using MRTG/RRD to gather average load data for individual ports can help here significantly), potentially an increased load on the CPU and various other indications of an unusually high load in the network.
That being said, the STP protocol, when deployed properly (which, ironically, is often deployed properly if it is not overconfigured), will prevent your network from creating Layer2 loops.
Best regards,
Peter
