Here's some info that might be helpful:
It is important to look at timers and exports as aggressive timers will lead to frequent purging of netflow table and frequent export of netflow entries to the collector.
Default timers result in optimal cpu usage, but it also depends on the traffic that you have in the network.
For example:
One must understand several characteristics of the traffic in the network, including average packets/second of throughput and average number of packets/flow, to estimate the amount of data Netflow will generate.
For example, if the average throughput on a Cisco
7600 router is 150kpps and the average number of packets per flow is 20, the device will
forward about 7500 flows per second. One can then expect 250 flow export datagrams (v5 in
this example) per second (7500 flows/30 flows per export datagram), or about 250 x 1500
bytes/datagram = 375KB/second of flow export traffic from one router toward the collector.
So a lot depends on traffic flow in the network , so it is hard to say how much CPU will
increase by enabling netflow. It can vary from one device to another.
