This Cisco module QSFP-40G-SR-BD is Bi-Directional (BiDi) 40Gbps QSFP, which means a single link used for RX & TX, each link 20Gbps, two LC links in total gives 40Gbps connection. If we will use only a single link instead of two, will that QSFP work on 10/20Gbps speed on a single link or it will not work at all?

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    Ouh dear. How could I miss the fact that such a module exists, even from 3rd party suppliers? that would've made life easier in the last few weeks... Dec 12, 2018 at 22:46
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can provide and accept your own answer.
    – Ron Maupin
    Dec 25, 2018 at 10:13
  • Dear Ron, as you might notice, there is unfortunately no proper answer to my question. We have "one guess" with no proves for it.
    – BBK
    Jan 9, 2019 at 19:23

1 Answer 1


Hi and welcome to Network Engineering.

Probably, you won't be able to specify nor auto-detect "speed 20000" on the QSFP+ switchport, and as long as not all 4 lanes of the PHY return an "up" signal back into the port, line protocol for a 40G port will stay down.

The transceiver might support 4x10G breakout mode - in that case, you might be able to define a 2x 10G Etherchannel across one strand of fibre. However, coming to think of it, support for 4x10G on this transceiver variant is unlikely: After all, how would such a 4x10G breakout be implemented physically? One would need an optical demultiplexer somewhere in the fibre path...

  • Marc, thank you for your response. My initial guess was precisely the same. But I want to be sure and would appreciate any documentation or links which will prove that guess.
    – BBK
    Dec 12, 2018 at 23:27

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