40g DAC between Juniper switches not working

I tried to connect Juniper EX4300 switches with Juniper QFX5100 switches with 40gbps DAC cables (QSFP+-40G-CU3M). Connection didn’t come up. First I thought I had to take them out of the VC (standard those QSFP slots on EX4300 are VC ports), but even after changing that it didn’t work.

Google search later I found out you need to disable autonegotation on the EX4300:

This issue occurs because EX4300 has auto-negotiation enabled on the 40G DAC interface by default. Any other Juniper switches have auto-negotiation disabled by default.

Juniper KB


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