"Only traffic that enters or leaves source ports or traffic that enters or leaves source VLANs can be monitored by using SPAN; traffic routed to a source VLAN cannot be monitored. For example, if
incoming traffic is being monitored, traffic that gets routed from another VLAN to the source VLAN cannot be monitored; however, traffic that is received on the source VLAN and routed to another VLAN can be monitored."
Packets will be forworded to destinations before modification (routing(modified TTL ,MAC-address) or QoS) or doping (ACLs , ingress QoS policing ,VLAN ACLs or egress Qos policing).
the packets sent by the source interface after all modification(TTL,MAC-address,QoS values) and processing(ACLs and egresss QoS) is performed by the switch.
3.packes like ping the sourece interfaces,will not be monitored.