A Network Tap creates a permanent access port for passively monitoring all traffic on a link without data stream interference or the introduction a potential point of failure.
Cisco Whitepaper ....“The switch treats SPAN data with a lower priority than regular port-to-port data.” In other words, if any resource under load must choose between passing normal traffic and SPAN data, the SPAN loses and the mirrored frames are arbitrarily discarded. This rule applies to preserving network traffic in any situation.
Knowing that the SPAN port arbitrarily drops traffic under specific load conditions, what strategy should users adopt so as not to miss frames? “the best strategy is to make decisions based on the traffic levels of the configuration and when in doubt to use the SPAN port only for relatively low-throughput situations.”
If span ports are the only option syntax for the various switch vendors is contained here, Switch Port Mirroring Page
For more information about Network Taps, see the article in the Miscellaneous section.