Per cisco docs on span for the 2950.
SPAN Configuration Guidelines
Follow these guidelines when configuring SPAN:
* SPAN sessions can coexist with RSPAN sessions within the limits described in the "SPAN and RSPAN Session Limits" section.
* The destination port cannot be a source port; a source port cannot be a destination port.
* You can have only one destination port.
* An EtherChannel port can be a SPAN source port; it cannot be a SPAN destination port.
* For SPAN source ports, you can monitor sent and received traffic for a single port or for a series or range of ports.
* When you configure a switch port as a SPAN destination port, it is no longer a normal switch port; only monitored traffic passes through the SPAN destination port.
* You can configure a disabled port to be a source or destination port, but the SPAN function does not start until the destination port and at least one source port is enabled.
* A SPAN destination port never participates in any VLAN spanning tree. SPAN does include BPDUs in the monitored traffic, so any spanning-tree BPDUs received on the SPAN destination port for a SPAN session were copied from the SPAN source ports.
* When SPAN is enabled, configuration changes have these results:
*
o If you change the VLAN configuration of a destination port, the change is not effective until SPAN is disabled.
o If you disable all source ports or the destination port, the SPAN function stops until both a source and the destination port are enabled.
