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Why is my ISL saturated when the line is not busy?

  1. Last updated
    May 21, 2025
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Last Updated:
5/21/2025, 7:42:11 AM

Applies to

  • AFF systems
  • FAS systems
  • MetroCluster IP
  • Switches
  • TCP
  • Network

Answer

  • When monitoring Network components it is important to understand that most monitoring solutions do not visualize all traffic at every point in time.
  • Simply speaking, most monitoring solutions only show an average (scaled) view of traffic that has been measured over a certain time period
  • In case of NetApp Grafana, the time-stamps available for examination represent an average workload over the previous 30 seconds as a minimum. 
  • Consequently, a brief but intense spike in workload lasting only 10 milliseconds is unlikely to be discernible in the displayed data.
  • Similarly, in case of TCP or network traffic in general, observations are an aggregated average over a specific time frame. 
  • Detailed insights into each millisecond or even finer intervals of activity on the network line are often not visible. 

Example:

  • To illustrate this we have to look at a very much simplified TCP workload graph and what happens within the network.

clipboard_e6ae86ccc3bf6036c4b177d675185a04c.png

 

  • The  red  and green dotted lines show the range of bandwidth available on the network line. 
  • Data starts in flight from the left hand site . 
  • The workload very quickly reaches the maximum that the network line can handle.
  • The TCP Protocol automatically avoids congestion by reducing the throughput of the in flight data. (aka TCP Fast Recovery)
  • Once there is enough headroom, the data throughput increases again until it reaches the line maximum bandwidth. 
  • At this point the TCP protocol once more reduces the throughput.
  • The blue dotted line in the middle illustrates the output from a network monitoring solution showing the average bandwidth in use.


Note: 

  • The overall saturation on a given network can not be measured only by relying on network monitoring solutions which rely on the average bandwidth.
  • To learn if your available bandwidth meats the requirement of data throughput, it is necessary to check on your network devices for packet drops due to queue discards.
  • Those are strong indicators that the given bandwidth might not meeting the requirements for the given data traffic.

Additional Information

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