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Siprent TestCenter Virtual: Random port load is not working as expected on Spirent TestCenter Virtual running at 10G and 25G speed

Symptoms
When the user configures a random port load the Spirent TestCenter system inserts a random number of bytes in the gap between each frame.  Unless the overall rate of the port is very low, about 10 frames per second, it will be difficult to see this degree of randomness in the counter views.  
At higher rates the port load will appear to be fixed – right at the average.  (Refer to FAQ11895)

However, when configuring 2 STCv ports b2b in 10G or 25G and using random load, Tx L1 Rate (Percent) is always reaching the maximum and stuck there. 
NOTE: Hardware ports works as expected:

Example: 
Port load configuration is minimum 1% and maximum 4%, but looks like load is always 4% - and not 2.5 -> Average
 
Virtual Ports:
Not working as expected Tx L1 Rate (Percent) is getting at maximum
 


Hardware Ports:
Working as expected - Tx L1 Rate (Percent) appear to be fixed at the average (expected)
 


 
Environment
  1. STCv 5.01 (driver mode dpdk / sockets) 
  2. 10G 
  3. 25G
  4. ESXi 6.5
 
Steps to Reproduce:
  1. Connect 2 STCv ports b2b
  2. configure Port Based scheduling mode, with a port load as random using Minimum 1% and Maximum 4%. Average is 2.5 (automatically)
  3. Run continuous traffic. Port Traffic and Counters > Basic Traffic Results -> Tx L1 Rate (Percent) is always 4.
 
Explanation/Resolution
  • The issue is fixed within 5.04 SpirentTestCenter release - November 2019
Root Cause
  • bug
  • CR raised:
    • CR-01407272   
    • JIRA_id: CIPCD-15571
    • Subject: 5.01 STCv: Port Load "Random" not working as expected
    • Opened Date: 9/27/2019 
    • Closed Date: 11/27/2019
    • Fix Release: 5.04
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Product : STC Virtual,Spirent TestCenter