Windows Server 2019 – SMB Direct RDMA driver mismatch causes connection fallback – Fix and Prevention

📖 ~1 min read

Table of contents

1. Symptom & Impact
2. Environment & Reproduction
3. Root Cause Analysis
4. Quick Triage
5. Step-by-Step Diagnosis
6. Solution — Primary Fix
7. Solution — Alternative Approaches
8. Verification & Acceptance Criteria
9. Rollback Plan
10. Prevention & Hardening
11. Related Errors & Cross-Refs
12. References & Further Reading

Symptom & Impact

SMB traffic falls back to TCP, increasing CPU load and reducing throughput.

Environment & Reproduction

Occurs after asymmetric NIC firmware updates across file and client nodes.

Get-NetAdapterRdma
Get-SmbClientNetworkInterface
Get-SmbServerNetworkInterface

Root Cause Analysis

RDMA capability negotiation fails due to firmware-driver capability mismatch.

Quick Triage

Check whether RDMA is enabled and currently used for active SMB sessions.

Step-by-Step Diagnosis

Compare RoCE/iWARP mode settings and driver versions on both endpoints.

Solution — Primary Fix

Align NIC firmware and RDMA driver versions, then reset SMB sessions.

Disable-NetAdapterRdma -Name 'RDMA1'
Enable-NetAdapterRdma -Name 'RDMA1'
Close-SmbSession -Force

Solution — Alternative Approaches

Temporarily increase SMB multichannel links and tune TCP stack while RDMA remediation is scheduled.

Verification & Acceptance Criteria

SMB sessions use RDMA again and throughput returns to expected baseline.

Rollback Plan

Revert firmware pair to previously matched set used in production.

Prevention & Hardening

Track RDMA compatibility matrix and enforce paired upgrades.

Related Errors & Cross-Refs

Related to NIC RSS/VMQ issues and cluster CSV latency alarms.

Related tutorial: View the step-by-step tutorial for Windows Server 2019.

View all Windows Server 2019 tutorials on the Tutorials Hub →

Browse all common problems & solutions on the Tutorials Hub.

References & Further Reading

Microsoft SMB Direct and RDMA deployment guidance for Windows Server.