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Cisco Netflow to tell who is using Internet bandwidth

When working with telecom circuits that are slow and “expensive” (relative to lan circuits), the question frequently comes up “What is using up all of our bandwidth?”.  Many times this is asked because an over-subscribed WAN or Internet circuit is inducing latency/packet drops in mission critical applications such as Citrix or VoIP.  In other cases a company may be paying for a “burstable” Internet connection whereby they are paying for a floor of 10 megabits, but they can utilize up to 30 megabits and just be billed for the overage (at the 95th percentile generally).

So how do you tell which user/server/application is chewing up your Internet or WAN circuits?  Well Cisco has implemented a technology called “netflow” that allows your router to keep statistics on each TCP or UDP “flow” and then periodically shove that data into a logging packet and ship it off to some external server.  On this server you can run one of a variety of different software packages to analyze the data and understand what is using up your network bandwidth.

The question is, what software package should you utilize?  I have not gone and evaluated all of the available options, but I do have experience with a couple of them.  I have used Scrutinizer from Plixer in the past and not been very impressed.  Part of it may have been that the machine it was running on was not very fast, but I just did not like the interface or capabilities much.

More recently I have downloaded and run NetFlow Analyzer from ManageEngine and I have been very impressed!  It is free for only two interfaces and they have an easy-to download and install demo that will run unlimited interfaces for 30 days.  It runs on Linux or Windows (I tried the Linux version) and is is dirt simple to install and configure.  There really is nothing of note to configure on the server itself, you just need to point your router at the server’s IP and it will automatically start generating graphs for you.

I should also mention that Paessler has some kind of netflow capabilities (in PRTG), but I have not checked it out.  I note it here since I use their snmp monitoring software extensively and I have been happy with it.

To get your router to send NetFlow data to a collector, you need to set a couple of basic settings (including which version of NetFlow to use and where to send the packets), and then enable sending flows for traffic on all interfaces.  Note that it used to be you could only collect netflow data upon ingress to an interface and so in order to collect data on bi-directional traffic you needed to enable it on every single router interface in order to see the traffic in the opposite direction.  This was done with the “ip route-cache flow” command on each interface.

Now “ip route-cache flow” has been replaced with “ip flow ingress” and you can also issue “ip flow egress” command if you were to not wanting to monitor all router interfaces.  I have just stuck with issuing “ip flow ingress” on all my interfaces since I wanted to see all traffic anyway (and I am not quite sure what would happen if you issue both commands on two interfaces and then had traffic flow between them, it might double count those flows).

Here are the exact commands I used on plunger to ship data to Netflow Analyzer 7:

plunger#conf t

Enter configuration commands, one per line.  End with CNTL/Z.

plunger(config)#ip flow-cache timeout active 1

plunger(config)#ip flow-export version 5

plunger(config)#ip flow-export destination x.x.x.x 9996

plunger(config)#int fastEthernet 0/0

plunger(config-if)#ip flow ingress

plunger(config-if)#int fastEthernet 0/1

plunger(config-if)#ip flow ingress


plunger#write mem

Building configuration…



Happy NetFlowing!


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  1. July 5th, 2009 at 00:03 | #1

    I’m sorry to hear that you didn’t have good luck with Scrutinizer. We are working on Scrutinizer 7. http://www.plixer.com/blog/scrutinizer/netflow-analysis-advancements-the-new-features-of-scrutinizer-version-7-part-1/


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