One question I hear all the time is “If I enable the exporting of Netflow on my router or switch, will it impact performance?” Yes it will, but usually not by enough to discourage you from including Netflow datagrams in your network analysis plans.

According to this document, released by Cisco, if you have…

• 10000 (ten thousand) active flows in the cache you can expect no more than a 4% increase in CPU utilization.
• 45000 (forty-five thousand) active flows in the cache you can expect no more than a 12% increase in CPU utilization.
• 65000 (sixty-five thousand) active flows in the cache you can expect no more than a 16% increase in CPU utilization.

Also, sampled Netflow will significantly decrease CPU utilization to the router. According to Cisco:

On average sampled NetFlow 1:1000 packets will reduce CPU by 82% and 1:100 sampling packets reduce CPU by 75% on software platforms. The conclusion is sampled NetFlow is a significant factor in reducing CPU utilization.

That being said, sampling Netflow won’t give you the whole picture, just a tiny piece of the flow puzzle.

More information can be found here and here.