# Check the InfiniBand placement boundary before troubleshooting MPI reachability

> Are the HPC VMs positioned to use Azure's RDMA fabric rather than only sharing ordinary Ethernet connectivity?

- Canonical URL: https://update.dsesecurity.com/updates/dse-20260909-082-check-the-infiniband-placement-boundary-before-troubleshooting-mpi-reachability/
- Publisher: Detection Systems & Engineering (DSE Security)
- Author: DSE Security Editorial Team
- Published: 2026-09-10T00:30:34+00:00
- Modified: 2026-09-10T00:35:08+00:00
- Last reviewed by DSE: 2026-09-09
- Resource type: Checklist
- DSE priority: Information
- Topics: IT, Networks & Infrastructure
- Reading time: 2 minutes

## What you need to know

Are the HPC VMs positioned to use Azure's RDMA fabric rather than only sharing ordinary Ethernet connectivity?

## Potentially affected

Owners of RDMA-capable Azure HPC deployments checking InfiniBand placement and network addressing.

## DSE recommendation

Map each communicating node to its actual scale set or availability set before changing MPI settings.

## Article

## Source facts

Azure’s documented HPC RDMA path uses InfiniBand, not the ordinary Ethernet interface. VMs needing RDMA communication must share a scale set or availability set; a scale-set deployment requires one placement group for its InfiniBand communication. Microsoft reserves 172.16.0.0/16 for the RDMA network and warns against overlapping the virtual network with it. Appropriate InfiniBand drivers are also required. [Microsoft Learn](https://learn.microsoft.com/en-us/azure/virtual-machines/setup-infiniband).

## Applicability

Confirm the exact RDMA-capable VM size, supported operating system, driver stack, and deployment model. The source contains older platform examples; use its current linked size and image guidance when qualifying a new build. Do not infer fabric eligibility solely from successful ordinary network tests.

## DSE recommendation

Map each communicating node to its actual scale set or availability set before changing MPI settings. Have the compute owner confirm the scale-set placement configuration and the network owner compare address ranges with the reserved fabric space. Resolve a placement or address-design mismatch through an approved deployment plan, preserving the application’s existing recovery path.

## Verification

On designated test nodes, inspect the actual fabric interface and driver state, then exercise the intended multi-node workload. Capture placement identifiers, selected interface, and observed communication separately from an Ethernet reachability result. If the job still fails within a valid fabric boundary, pass that evidence to the MPI owner for library-specific investigation. Avoid changing several placement, driver, and runtime variables together without a way to attribute the result.

## Official references

[Microsoft Learn: Set up InfiniBand](https://learn.microsoft.com/en-us/azure/virtual-machines/setup-infiniband). Source reviewed September 9, 2026.

## Primary reference

- Name: Set up InfiniBand on HPC VMs - Azure Virtual Machines - Azure Virtual Machines | Microsoft Learn
- Authority: Microsoft Learn
- URL: https://learn.microsoft.com/en-us/azure/virtual-machines/setup-infiniband
- Source publication date: Not stated by the source

## Citation and use

Preferred citation: “Check the InfiniBand placement boundary before troubleshooting MPI reachability,” DSE Security, https://update.dsesecurity.com/updates/dse-20260909-082-check-the-infiniband-placement-boundary-before-troubleshooting-mpi-reachability/
Publishing principles: https://update.dsesecurity.com/updates/dse-updates-editorial-methodology/
Usage and citation policy: https://update.dsesecurity.com/usage/
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