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New Member

Local SPAN / Port Miror limitations - Over subscription

Hi All

I have a question regarding over subscription of a local span/port mirror. Basically I know that if you source monitor a port channel (lets say 2 x 1Gig members) you can only copy that traffic to a physical desitnation port (ie another single 1 gig port).

As I understand is this oversubsciption of the destination port has 2 drawbacks:

1) as the switch has to copy all tx and rx frames seen on source port the likely hood of the destination port dropping or missing packets is very high due to the lower bandwidth

2) I read somewhere that not only will you drop packets on the destination port but you will also affect the performance of the live port channel that you are mirroring.

Does anyone know of a whitepaper that lists both these issues? Im sure I have read it but cant find the doc on Cisco.

Thanks all.

  • LAN Switching and Routing
Cisco Employee

Local SPAN / Port Miror limitations - Over subscription


I am not aware of any white paper. What device/linecard specifically are you setting this up on? Issue 1) is definitely expected if the traffic rate is high enough so the span traffic will be impacted meaning you will likely drop. Issue 2) should not happen but I would like to check the architecture of the linecard you are working with.



New Member

as described at http://www

as described at

a SPAN destination port has the following characteristic...

  • 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.

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