Does the ConnectX5 support far end loopback, if so how do I enable it.

We are debugging our EDR TCA (Stratix 10 and Ultrascale Plus based) and we are seeing 10 or 20 symbol errors per second on the HCA and also on your switch. We are using a 10M Mellanox optical cable. We don’t see any errors when connecting two of our FPGA based TCAs together, we can run overnight without errors. We are using a four second test time during the initialization and training testing cycle and it’s passing the test because the link on the HCA comes up at 25G so the connection must be pretty good. We would like to enable far end loopback on the HCA so that we can compare what we send with what your HCA sees. Is this supported and if so how do I enable it?

Hi Rosen,

Please download the MFT (Mellanox firmware tool) software and use the mlxlink tool to enable the internal loopback in CX-5

For more information please refer to the following page

To download the software

Or connect port to port on the same adapter and check if you encounter any issues .



A couple of questions. There are two loopback modes mentioned in the mlxlink man page, PH and EX

l |–loopback Configure Loopback Mode [NO(No Loopback)/PH(phy loopback)/EX(external loopback)]

What do they do precisely? We want to lookback our signal back to our TCA as seen by your HCA, do either of those modes do that or is the strictly loopback as seen from the HCA’s point of view? FPGAs call this far end loopback as opposed to near end which is where the device loops back to itself. Is either PH or EX what we would call far end loopback?

Second question, the loopback command doesn’t seem to be working. When we give it a loopback command the status that’s returned is always No Loopback.

mlxlink -d /dev/mst/mt4119_pciconf0.1 --loopback PH

Operational Info

State : [32mActive

[0mPhysical state : [32mLinkUp

[0mSpeed : [32mIB-EDR

[0mWidth : [32m4x

[0mFEC : [32mNo FEC

[0mLoopback Mode : [32mNo Loopback

[0mAuto Negotiation : [32mON


Supported Info

Enabled Link Speed : [32m0x0000003f (EDR,FDR,FDR10,QDR,DDR,SDR)

[0mSupported Cable Speed : [32m0x0000003f (EDR,FDR,FDR10,QDR,DDR,SDR)


Troubleshooting Info

Status Opcode : [32m0

[0mGroup Opcode : [32mN/A

[0mRecommendation : [32mNo issue was observed.


Configuring Port Loopback…