The latest Packet magazine features a tip from Aurelio DeSimone mentioning the show controllers utilization command. You can specify the SPAN port in your syntax to see how much traffic it is carrying to your sensor. The current Packet issue also features excellent articles on new modularity features of Cisco IOS and an overview of 10 GB Ethernet and its seven (yes, seven) Ethernet variants. If anyone knows of a similar set of information via SNMP, please let me know via a comment here.”]
Source: https://taosecurity.blogspot.com/2005/09/measuring-bandwidth-utilization-on.html