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Symptoms Unable to bring up links in PX3-400GQ-T2 card using QSFP28 transceiver  Environment STC 4.99 PX3-400GQ-T2 PX3 2-PORT 400/200/100/50GBE QSFP-DD QSFP 28 transceiver QSFP DD transceiver Explanation/Resolution On 4.99 PX3-400GQ-T2 does not support NRZ modulation, only PAM4 modulation, so that is the reason why QSFP28 modules won’t work on this particular card, even at a slower rate (50G) on this release Starting in the 5.16 release (November 2020) PX3-400GQ-T2 supports either NRZ / PAM4 for 100 and 50 GB Refer to the following KB to get the complete list of releases where NRZ is supported on each speed: FAQ19784 - Spirent TestCenter: Can a QSFP28 interconnect be used in a PX3-400G port in 100G NRZ mode? If using an older release you need to use QSFP-DD transceiver. Root Cause Not supported in current release Bad transceiver / interconnect used in older release Need to upgrade to latest releases.