Q. What effect does InterMapper Flows have on server load? How much memory does InterMapper Flows require?

A. There are no hard and fast rules, but NetFlow processing takes considerably more horsepower than InterMapper's standard monitoring. In general, we don't recommend using a discarded piece of hardware found under a forgotten desk for InterMapper Flows.

Receiving and processing flows deals with summaries of large volumes of packets, so the collection and storage process will usually not be the bottleneck. At one customer, where traffic peaks up to 3Gbps, the server could process every packet on an off-the-shelf laptop, while still having plenty of CPU time to spare.

However, handling queries (e.g., reading the saved data to send information to the GUI) is real work for the server. Depending on the selection criteria, the server sometimes must process millions of records to create the summaries and the graphs. This goes faster if more RAM memory is configured for caching session records. Queries going farther back in time will have to read the session records from disk with resulting slower times.

For example, say you have a 200MBps border, with 15,000 hosts inside, all serviced by netflow-capable equipment. If you use InterMapper Flows only at the border router, a capable modern desktop, dual-core CPU, 4GBytes of RAM would do nicely. However, if you do decide to turn on NetFlow on each of 1000 Cisco devices throughout the network, you might need to get some serious hardware, with 4 CPUs, several fast disks, and 16GB RAM. Another alternative would be to run multiple InterMapper Flows copies, each monitoring a subsection of the flow exporters.

Another factor is what the administrator wants to see. If they only want to see the last hour of data, then the desktop might do fine to monitor all 15,000 hosts. If they need the ability to go back in time regularly (to last week, or even last month) they would be looking at expensive, top-of-the-line server hardware.