Cisco’s Catalyst 2900XL/3500XL switches can have only one SPAN session active at a time. The Catalyst 4000/5000/6000 can monitor ports belonging to several different VLANs. The number of destination ports available on the switch is the only limit to the number of SPAN sessions. Since Snort v1.8, Snort has supported decoding 802.1q VLAN tags as well. The bottom line appears to be that SPANning is not a problem, but there is also limits as to what data available regarding where the packets come or go.”]
Source: https://taosecurity.blogspot.com/2003/04/ive-been-trying-to-understand-how-to.html