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Spirent TestCenter: Why is IMIX adding extra four bytes when testing with devices that have VLAN or MPLS?

Environment/Versions
  • Spirent TestCenter
  • IMIX
Answer
  • Detail Descriptions:  
    • There is no additional four bytes added when Stream block frame Size configured with Fixed or other frame size option (except iMix) and with VLAN or MPLS added.
    • However, Stream block with iMix frame size with VLAN or MPLS always adding extra four bytes to the data when it received
  • Explanation/Resolution: 
    • Fixed frame sizes and other frame sizes already included the VLAN or MPLS header and will not notice any different in the packet size.
      • The concern is only with IMIX.
    • iMix Frame size is mainly focused on the IP length and is where you set the length values.
      • Also displayed is the length of the packet if the default Ethernet frame is considered.
      • The default Ethernet does not include four byte VLAN or MPLS.
    • A default Ethernet frame can be one of two things.
      • 1. ETH, IP, Padding FCS
        • 64 byte is considered the minimum ethernet frame size, so padding is required if the IP length is not large enough to create a 64 byte frame.
      • 2. ETH, IP, FCS   
        • No padding required if IP length creates a frame that meet or exceeds the minimum ethernet frame size.
    • Since only the default Ethernet size is indicated in the IMIX editor, the user need to be aware of this when setting the IP length for VLAN or MPLS traffic.
      • VLAN Examples:
        • IP length is set for 40 bytes (Default Etherent size shows as 64 bytes)
          • ETH-VLAN-IP-Padding-FCS = 14+4+40+2+4
            • Since normally there would be 6 bytes of padding, adding a VLAN to this will still produce a 64 byte frame. 
        • IP length is set for 46 bytes (Default Etherent size shows as 64 bytes)
          • ETH-VLAN-IP-FCS = 14+4+46+4
            • Although the default Ethernet size shows as 64 bytes, due to the VLAN, the actual length will be 68 bytes. 
 
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Product : Spirent TestCenter,L2-3,Adaptive Streaming,API,Windows GUI,PGA