Core issue
In the switch, there areonly two monitor (SPAN) sessions available. If the switch has a VPN module inserted into a chassis with Switch Fabric Module, then the VPN module uses one of the monitor (SPAN) sessions. This Switched Port Analyzer (SPAN) session is configured to forward multicast by the VPN module. This feature of forwarding multicast traffic by SPAN sessions is called SPAN reflector feature.
This assignment of monitor session to VPN module is handled by the software when the VPN module along with the switch fabric module is inserted into the chassis.
Note: Supervisor 720 contains the switch fabric module. If the VPN module is inserted with Sup720 module, then the same problem occurs.
Similarly, when you have a Firewall Service Module (FWSM), for each FWSM in a switch, the SPAN reflector feature is enabled, and this feature uses one SPAN session.
Resolution
In order to delete the SPAN session used by the VPN or FWSM module, issue this command in the global configuration mode:
switch(config)#no monitor session servicemodule
NOTE: When this command is issued, the IP Security (IPsec) VPN module drops all multicast traffic.
Connected to same switch or different switches
Same switch
Error message
% Session 1 used by service module
