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Spirent KB Article
Doc ID: FAQ19331
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Spirent TestCenter: Why is STC sending 1526 bytes long packets, if the MTU is set to 1500
It's important to mention that Spirent does not fragment packets.
Normally the TCP session starts with an MSS of 1460. The MSS + TCP Header + IP Hdr + Eth Hdr + FCS = Total Frame Size. You also have to consider that the MPLS header is 4 bytes long.
If the MSS (Maximum Segment Size), and then we add all the headers, the packet will be 1526 bytes long.
If the packet size is causing any issues with your DUT, such as packet dropping, you can:
Reduce the MSS in the DUT, if possible.
Increase the MTU value in the DUT.
Product : Spirent TestCenter