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Spirent TestCenter: 5.04 - MX-10G-S8 card shows just 159 RX packets under Stream Block results but capture is showing 199

Symptoms

User is seeing that Spirent is showing incorrect RX signature packets, 

In the image below, streamblock145 is sending 199 packets, and Rx count is 159. However, did the capture on the receive port 4/1 and the capture shows 199 packets.
 

 
Environment
 
  • MX-10G-S8.
  • SPT-N12U chassis
  • Spirent TestCenter 5.04

 
Explanation/Resolution

We ran the same configuration in-house in back to back (one STC port directly connected to another STC port, no DUT in the middle), and comparing the capture from our test against customer's capture we found a difference:
 
 
  1. We noticed the packets on customer's capture are 130 frame size while on STC it’s configured to be 128 fixed frame size,
  2. Digging into it further, it looks like in the VLAN header, the DUT seems to be inserting two bytes of 0x0000 between the vlan id and type field:
 
 
 
Root Cause

The DUT seems to be inserting two bytes of 0x0000 between the vlan id and type field we are thinking this is throwing STC analyzer off.
  • At this point we are not sure if this behavior was expected or not by customer's DUT.
 


Product : Spirent TestCenter,Capture,Packet Captures,MX2