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For each USTAT module, the CNA system can effectively send probes to and receive data back from as many as 250 ICMP targets per second and 225 tcp/traceprobes per second. If multiple links are being probed by a single USTAT module, then keep in mind that its capacity will be shared over those links.
However, you may want to probe some targets more often than others. While there is no precise formula for calculating how you should distribute your active probes, in general your aggregate measurement traffic should correlate to the actual traffic in a meaningful way. In other words, you should probe high-traffic prefixes at a higher frequency than low-traffic prefixes.
If you want to disable all active probing, then set active-measurement max-probes-per-second to zero. This parameter is really a maximum average rate, and sets a limit on the probing rate only if the running configuration calls for a rate that exceeds max-probes-per-second. If it does, then each target’s probing period will be extended on a pro-rated basis to reduce the total probing rate to max-probes-per-second. It is possible that the actual probing rate may exceed max-probes-per-second during momentary spikes. The default value is 200.
Be aware, however, that the number of measurements the engine module can handle per second is finite; measurements in excess of that number will be dropped. See Measurements in Chapter 3: Adaptive Path Control of the CNA Adaptive Path Controller Guide for more information about the system’s capacity to measure traffic--including both active probes and passive user traffic tests. When a probe is first enabled, the CNA system will pick the exact starting time at random during the time interval between the moment you enable the probe and the end of the first interval. That is, if you enable probes for ten different targets exactly at noon and specify an interval of once every hour, each of the ten probes will be sent out at random times during the hour between noon and 1 p.m. This is done to spread out the probe times among all of your targets in order to avoid bursts in probing activity.
When using the EFC module as a source for active measurement targets (see Configuring the EFC Module in Chapter 2: Collecting Endpoint Addresses), it is important to note that the measurement frequency is shared by all the targets generated by the EFC. For example, if you have configured an active-measurement group to probe the top 250 web servers (as determined by the EFC), and you wish to probe each web server once a second, then you should set the probing rate to be 250 probes per second.
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