This issue is due to the hardware limitations of Cisco Catalyst switches. Traffic monitoring in a SPAN session has these restrictions:
1. You can monitor incoming traffic on a series or range of ports and VLANs.
2. You can monitor outgoing traffic on a single port.
3. You cannot monitor outgoing traffic on multiple ports.
4. You cannot monitor outgoing traffic on VLANs.Cisco bug ID CSCef76791 addresses the addition of more egress (Tx) direction SPAN sessions.
Native Cisco IOS allows only one Tx direction source interface to be monitored. However, it is allowed to monitor ingress (Rx) traffic from more interfaces.
For more information about SPAN restrictions on the Cisco Catalyst 3550 switch, refer to Configuring SPAN and RSPAN.For more information about SPAN restrictions on the Cisco Catalyst 6500 switch, refer to Configuring Local SPAN, Remote SPAN (RSPAN), and Encapsulated RSPAN.