NetFlow VoIP Monitoring often requires keeping track of QoS or DSCP values. Here are a few statistics that should be gathered when performing network traffic monitoring with Flexible NetFlow.

Monitor VoIP with NetFlow

  • Round Trip Time: the time to get from host A to host B.  The router measures this by watching the TCP hand shake between SYN, SYN ACK and ACK.
  • Time To Live: This metric indicates the maximum router hops a datagram is permitted to cross. It is decremented by each router it passes through.  When it becomes zero, it will not be forwarded.
  • Jitter: This statistic is a variation in packet delay caused by queuing, contention and/or serialization effects on the path through the network. Buffers can be used to smooth out variations in delay. However, too much buffering adds delay and prevents effective interactive video.
  • Packet Loss: Loss of packets is highly visible and hearable. Due to the nature of video compression, a single packet loss can result in multi-second artifacts visible on the screen. Loss may be caused by poor links (last mile access link generally, or wireless links), a routing change within the network or many other unplanned outages.

NetFlow Monitoring VoIP

Above you can see traffic between the phone and the PBX for each call through the Cisco router. Notice the corresponding VoIP jitter and packet loss per call. The Trans Event Packet Loss (TEPL) is a series of missed packets during the call: not a good thing!

Configure Flexible NetFlow
So, is your company looking to monitor VoIP with NetFlow? Well, forget about using traditional NetFlow exports.  To get these new metrics you have to configure Flexible NetFlow. Also, make sure you are running IOS 15.2.2T or more recent for the most capable Flexible NetFlow exports.

Cisco isn’t the only vendor with VoIP details in their flow export.  The SonicWALL NetFlow configuration (actually it’s IPFIX) provides similar details to Cisco but, includes Caller ID.  Sorry, I had to blur out all of the phone numbers our sales person was calling.

SonicWALL NetFlow Support


Our nBox provides similar details with the addition of Codec information.  Codec is useful as some phone system setups will use a lower quality codec when the PBX runs our of licenses.


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.


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