SMB multichannel constraints with mixed NIC speeds

Question

SMB multichannel constraints with mixed NIC speeds

Hakim Ismail 20

On our file server, SMB Multichannel transfers aren’t spreading traffic across all available adapters. The issue seems tied to mismatched interface speeds one NIC runs at 10GbE while another operates at 25GbE. Because of this imbalance, the multichannel logic fails to fully utilize both paths, leaving performance uneven and underwhelming. We’re considering whether configuring SMB Multichannel constraints through Set‑SmbServerConfiguration could help align adapter usage and enforce more predictable channel distribution. What’s the way to configure SMB Multichannel constraints so that mixed‑speed NICs (10GbE and 25GbE) can be handled properly without breaking performance expectations ?

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Answer 1

Set-SmbServerConfiguration is not the cmdlet used to configure SMB Multichannel constraints.

For mixed-speed NICs, the documented behavior is:

SMB Multichannel automatically selects network adapters of the same type and same speed for simultaneous use.
If adapters have different speeds, SMB Multichannel automatically selects the fastest adapter, and the slower adapters remain idle.
If SMB prefers an adapter that is not desired, use SMB Multichannel constraints to restrict SMB communication to specific network interfaces.

A practical way to handle a 10GbE + 25GbE mix is:

Verify which interfaces SMB currently sees on both client and server:

   Get-SmbClientNetworkInterface
   Get-SmbServerNetworkInterface

Confirm adapter capabilities and status:

   Get-NetAdapter
   Get-NetAdapterRSS
   Get-NetAdapterRDMA
   Get-NetAdapterHardwareInfo

During a long-running file copy, verify which multichannel connections are selected:

   Get-SmbConnection
   Get-SmbMultichannelConnection
   Get-SmbMultichannelConnection -IncludeNotSelected

If the 10GbE adapter should not participate in SMB traffic, apply SMB Multichannel constraints so SMB uses only the intended interfaces, such as the 25GbE adapters.

This approach matches the documented guidance for mixed adapter behavior: SMB Multichannel does not balance across unequal-speed adapters as if they were equivalent. It prefers the faster eligible path. Constraints are the supported way to limit SMB to defined interfaces when adapter selection must be controlled.

Also check for conditions that prevent an adapter from being used even when it is visible:

incorrect client routing
existing multichannel constraints
blocked network interface request/response packets
failed communication over the extra interface, such as firewall or TCP handshake issues

If only link-local addressing is present, verify the network profile and ensure the SMB-In firewall rule is enabled for the active profile.

Answer 2

Hello Hakim,

What you are seeing is expected SMB Multichannel behavior. When network adapters have different speeds, such as 10 GbE and 25 GbE, SMB Multichannel does not normally aggregate them together. For adapters of the same type/capability, SMB selects interfaces of the same speed and generally prefers the fastest set, so the slower adapter can remain idle.

Set-SmbServerConfiguration is therefore not the correct command for forcing traffic distribution across the 10 GbE and 25 GbE adapters. If you need to restrict which interfaces a client uses for a particular SMB server, use New-SmbMultichannelConstraint on the SMB client, for example:

New-SmbMultichannelConstraint -ServerName "FileServer01" `

-InterfaceAlias "Ethernet25G1","Ethernet25G2"

The constraint selects the interfaces SMB is allowed to use; it does not provide weighted load balancing or make a 10 GbE + 25 GbE pair behave like a 35 GbE aggregated link.

You can confirm SMB’s interface selection with:

Get-SmbClientNetworkInterface

Get-SmbServerNetworkInterface

Get-SmbMultichannelConnection -IncludeNotSelected

Get-SmbMultichannelConstraint

For predictable throughput, I would use matched-speed NICs with equivalent RSS/RDMA capabilities for the Multichannel paths rather than trying to force the 10 GbE and 25 GbE adapters into the same set.