To solve a specific server clustering problem I am opting to split the patching of the servers to both "in the same room" and "in another room". We have multimode cabling between all rooms in a full mesh, so there are enough fibres. I plan to use regular 25G SFP28 SR controllers and transceivers to patch the servers into the switches "in the same room", as I will not exceed 100m. However, the distance to patch the servers "in another room" will be more than 100m and about 200m in the longest path. That means that regular 25g SFP28 SR transceivers are out of the question. There do exist eSR and cSR multimode transceivers, that will increase the distance to 300m and 500m respectively.

My question is: Do I have to take care that the SFP28 controllers I order to be fitted into the servers support eSR and/or cSR transceivers? Or is that entirely up to the transceivers?

I was planning on ordering SuperMicro's AOC-A25G-b2 controllers that are based on Broadcom BCM57414.

1 Answer 1


There are different grades of multi-mode fiber. 25GBASE-SR's reach depends on that grade. OM5 (lime) or OM4 (violet) reach 100m, OM3 (aqua) 70m nominally. Variants like "eSR" or "cSR" are not standardized, so the reach is stated in the vendor specs. If you use those make sure you can reorder replacements at a later time or buy spares right away.

Whatever you do, make sure that the transceivers you plan are compatible with the equipment you plug them into. Many device manufacturers limit your choice to 'original' transceivers that they list in their specifications and 'generic' transceivers are rejected.

There are many 3rd-party vendors offering 'compatible' transceivers that the switch/NIC in question take for the original. These usually work fine and may even be identical to the 'original' part, but quality may vary. However, what you can really use depends on your support requirements and company policies.

If you are not limited in any way (support contracts, policies, vendor limitations), pretty much any SFP28 transceiver should run in any SFP28 slot.

Additionally, many users begin to change their cabling to single-mode fiber for speeds beyond 10 Gbit/s. SMF termination used to be much more expensive than MMF but the price gap has shrunk quite a bit in the past years (depending on vendor, of course), so SMF is very often the medium of choice for inter-datacenter connects, with MMF becoming limited to intra-rack or possibly only inter-rack connects.

  • Thanks, you confirmed what I had thought would be the case. You are right, I forgot to mention our multimode fibre grades, which are OM4 (with some OM2/3 that I do not plan to use).
    – alex
    Aug 5, 2022 at 7:41
  • @Zac67 this is true for 10Gb but for 25G over OM4, isn't the reach 100M ?
    – JFL
    Aug 5, 2022 at 7:52
  • Oh sorry, wrong table... Yes standard 25GBASE-SR over OM4 is limited to 100m.
    – Zac67
    Aug 5, 2022 at 8:00
  • These flexoptix 3rd party transceivers are stated to be up to 300m via OM4: flexoptix.net/en/p-8525g-03.html?co10887=102882 They are marked to be "eSR", which I gathered to be something like "extended Short Range" but couldn't find technical descriptions (only marketing). Hence my question.
    – alex
    Aug 5, 2022 at 8:22
  • 1
    Yes, eSR and cSR variants are available be quite a few vendors. Still, you might need to compare the exact specs as there's no IEEE standard - interoperability is highly likely but not guaranteed.
    – Zac67
    Aug 5, 2022 at 8:26

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