On the Catalyst Series switches, there is a feature called SPAN, which is basically a way to monitor transmitted and received traffic on different ports or VLANs on the local switch. (SPAN stands for Switched Port Analyzer.)
When doing capacity planning, it is recommended to physically allocate one 10 Megabit or 100 Megabit Ethernet port on the switch specifically for the use of the SPAN feature. Doing so eases the pain of troubleshooting traffic through a switch by giving one dedicated port for a network analyzer or RMON probe.
The SPAN feature allows network troubleshooters to have one physical network analyzer or probe per switch and to locate it on only one port instead of having to physically move the analyzer ...