Follow these steps in order to capture traffic on Dell PowerConnect switch series:
SW(config)# interface ethernet g23
SW(config-if)# port monitor g21
In the following example I have configured Ethernet port g23 to monitor and capture all traffic which traverses through port g21. In Cisco terminology port g23 is the SPAN port while mirroring the traffic from port g21.
The default behavior is to capture traffic on both directions, rx and tx, but we have the option to choose only one direction:
SW(config-if)#port monitor g21
rx Monitor received frames only
tx Monitor transmitted frames only
If you ever configured some other port before you may encounter a situation where more then one port is sending traffic to the monitor port, in order to validate this use the command:
SW-2# show ports monitor
Source Port Destination Port Type Status
----------- ---------------- ------- ----------
g19 g23 RX,TX active
g21 g23 RX,TX active
Note that monitor too many ports to a single port may lead to overwhelming of data rate which can lead to packet drop by the monitor port.