Spirent's signature is 20 bytes long. If 20 bytes is added to a UDP packet the minimum length would be 66 bytes.
Spirent uses an algorithm that combines its signature with the UDP checksum to save the two overflow bytes and allow a 64 byte packet.
This will work in a normal environment but when performing NAT overload with port translation, the DUT is recalculating the UDP checksum and causes an incorrect signature field.
Spirent TestCenter will still receive these packets but not as signature packets.