Some key characteristics to know about SPAN destination ports:
- A destination port can be a physical port that is assigned to an EtherChannel group, even if the EtherChannel group has been specified as a SPAN source. The port is removed from the group while it is configured as a SPAN destination port.
- The port does not transmit any traffic except that traffic required for the SPAN session unless learning is enabled.
- The state of the destination port is up/down by design. The interface shows the port in this state in order to make it evident that the port is currently not usable as a production port.
- If ingress traffic forwarding is enabled for a network security device. The destination port forwards traffic at Layer 2.
- A destination port does not participate in spanning tree while the SPAN session is active.
- When it is a destination port, it does not participate in any of the Layer 2 protocols (STP, VTP, CDP, DTP, PagP).
- A destination port that belongs to a source VLAN of any SPAN session is excluded from the source list and is not monitored.
- A destination port receives copies of sent and received traffic for all monitored source ports. If a destination port is oversubscribed, it can become congested. This congestion can affect traffic forwarding on one or more of the source ports.
- When you configure a port as a SPAN destination, it is dedicated for use only by the SPAN feature.
- Destinations, by default, cannot receive any traffic. With Release 12.2(33)SXH and later releases, you can configure Layer 2 destinations to receive traffic from any attached devices.
- Destinations, by default, do not transmit anything except SPAN traffic. Layer 2 destinations that you have configured to receive traffic can be configured to learn the Layer 2 address of any devices attached to the destination and transmit traffic that is addressed to the devices.
Some of the points above seem like repeats and that was done on purpose to emphasize the point. Probably the most important point is the one about the port being in an Up/Down status. To me, this would naturally indicate that it is doing nothing outside of its SPAN function.