Had a customer call in the other day with a NetFlow Billing question. Apparently his Cisco router is sending over 50,000 flows per second during peak hours.  This is some serious high volume NetFlow. His fancy expensive NetFlow collector couldn’t handle the volume and so we were one of the vendors he turned to.

I asked if he had considered getting away from exporting all of the flows and tried exporting only counters per subnet.

We lead him to a Flexible NetFlow configuration (aka FnF) that solved his problems.  With FnF, you can use source/destination prefix as a key field(match).    To understand the following, watch this video on setting up Flexible NetFlow. Here is the FnF configuration we used:

Step 1:
flow record subnets
match ipv4 source prefix
match ipv4 destination prefix
collect counter bytes
collect counter packets

Step 2:
flow monitor subnets
description app traffic analysis
record subnets
exporter export-to-mikek  (Step 2)
cache timeout active 60

Step 3:
int fa0/0
ip flow monitor subnets input
ip flow monitor subnets output

Within 60 seconds, the template came into Scrutinizer

NetFlow Billing
using Flexible NetFlow for billing


The above reduced the flow volume down to a few hundred flows per minute.  Most NetFlow collectors will drop all NetFlow that doesn’t contain certain fields:

Src Interface
Dst Interface
Src Port
Dst Port
Src IP Address
Dst IP Address

Scrutinizer NetFlow Analyzer is not limited to any fields. This is why we can collect and display things like NetFlow option templates containing interface names, NetFlow NBAR, exporter statistics, etc.  Make the switch to Scrutinizer and stop over paying for NetFlow traffic analysis.

Mike Patterson author pic


Michael is one of the Co-founders and the former product manager for Scrutinizer. He enjoys many outdoor winter sports and often takes videos when he is snowmobiling, ice fishing or sledding with his kids. Cold weather and lots of snow make the best winters as far as he is concerned. Prior to starting Somix and Plixer, Mike worked in technical support at Cabletron Systems, acquired his Novell CNE and then moved to the training department for a few years. While in training he finished his Masters in Computer Information Systems from Southern New Hampshire University and then left technical training to pursue a new skill set in Professional Services. In 1998 he left the 'Tron' to start Somix which later became Plixer.


Big Data

Sankey Flow Graph

One of the greatest benefits of NetFlow collection for traffic analysis, is we’re provided with the ability to visualize the…

Leave a Reply