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Hello Smith Oliver,
Event ID 21502 during RDMA-based migrations usually points to Priority Flow Control (PFC) mismatches between the host NICs and the upstream switches. When the NIC detects that PFC is disabled or misaligned, it will drop the RDMA path and cause the migration to fail instantly.
The proper way to re-align QoS and PFC is to configure consistent settings across both the host adapters and the switches. On the host side, you can use PowerShell to verify and set QoS policies with commands like Get-NetQosPolicy and New-NetQosPolicy. Ensure that SMB Direct traffic (port 445) is tagged with the correct DSCP value and that PFC is enabled for that traffic class. On the switch side, configure the same DSCP/PFC mappings so that the NIC and switch agree on which traffic classes require lossless transport.
After applying these changes, restart the NICs or reboot the host to ensure the new QoS policies take effect. It’s also a good idea to run Get-SmbClientConfiguration and Get-SmbServerConfiguration to confirm that RDMA and SMB Direct are enabled and aligned with your QoS settings.
I hope the response provided some helpful insight. If you find this answer useful, please hit “accept answer” so I know it addressed your concern.
Jason