Spirent TestCenter: Not able to run at line rate when streamblocks on multiple ports are running at the same time.

Doc ID    KNI16320
Version:    1.0
Status:    Published
Published date:    10/07/2020


Snippet: Transmitting Rate (TX Rate (fps)) counter starts reducing as soon as you starts streamblocks from all the ports within the same card.

If starts only a particular stream in a PORT 1 then it is sending the same rate as configured, If send one more streamblock on the same PORT1, each stream block is sending rate as what is configured, until this point everything is ok.

Once starts sending a stream block from other port (PORT2), then TX FPS for existing ones (PORT 1 and PORT2) starts reducing

Once starts sending more and more stream blocks from all the ports, first two ports sending rate reduced.


  • Line rate traffic
  • DX3-100GO-T12
  • TX Rate (fps) counter
  • Stream Only Generation
  • High Speed Results Analysis (HSRA)


When Soft streams are configured (High Speed Results Analysis is DISABLED), the streams will be generated by the Software PGA, this limits the rate at which these can be generated at, all streamblocks will fight for CPU time, and then that is why other streams in other ports are affected. Hence you won’t be able to reach the line rate as configured.
To be able and get that independence on the ports so each port streams don’t affect others, you will need to enable HSRA on all the streams, when HRSA is enable, the ​streams will be generated by the FPGA's, allowing these streams to be sent at line rate.
The DX3-100GO-T12 has a limitation on the number of hardware modifiers you can configure, so for you to enable HRSA on all streamblocks, so, depending on your configuration you may need to first disable Stream Only Generation option for all streams, then you can enable HRSA.
  • You can first try to just enable High Speed Result Analysis and apply, if you are not allowed to apply due to a message about modifiers exceeded you will to disable Stream Only Generation first:
  • Once HRSA is enabled you can validate each streamblock is sending at line rate and other streams don’t affect the other ports:

Be aware that by turning Stream Only generation OFF will configure Spirent TestCenter's Generators to use a single Stream ID for all flows in that stream block   So you will get statistics for all the streams in aggregate, which I don’t think this affects the test since you are referring to StreamBlocks results view and checking just for the rate.

enlightenedRefer to the following KB articles for detailed information about Stream Only Generation and High Speed Results Analysis behavior:

Root Cause

  • Hardware Limitation on DX* cards.

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