I see 10G SFP+ modules (RJ45 port) popping up claiming to be able to also run 1G and foremost 2.5G/5G (NBASE-T).

What if the switch itself only refers to the following standards?

  • IEEE 802.3ab 1000BASE-T
  • IEEE 802.3z 1000BASE-X
  • IEEE 802.3ae 10GBASE-X
  • IEEE 802.3ba 40GBASE-X

Will it be able to use 2.5G with such an SFP+ module?

What does this depend on? Does it depend on the PHY?

On a copper port-only switch the PHY is probably located inside the switch. In an SFP-based switch the PHY is located inside the SFP module I guess.

So why does it depend on the switch? (Especially in the above example where the maximum port speed is 40Gbps, why wouldn't it support 2.5G?)

I found a guy who claims that using a third party 10/5/2.5/1GBASE SFP+ module, his Netgear switch which is not specified for 802.3bz now does exactly that.



Generally, the switch needs to be able to generate the data rate at the MAC - so no SFP+ module will run at 2.5 or 5 Gbit/s if the switch doesn't support those rates. That may or may not be officially documented by the vendor.

In any case, the switch has to support the PHY line code (PCS) as well. Most 10GBASE-T modules include a transcoder (from -R to -T) but that is not fully transparent to the switch. There also seem to be (cheaper) 10GBASE-T modules with less transcoding that require the SFP+ slot to feed the required precoded signal.

Since 5/2.5GBASE-T use the same line code as 10GBASE-T, they might just work in special cases when the MAC rate is supported.

Most fiber modules use -R PCS code (64b66b) that is already provided by the switch port, so there's no need for transcoding and mostly just 'analog' circuitry and a wide range of potential compatibility. Still, the module needs to be accepted by the switch: many vendors use a lock-in scheme where only original modules are accepted - as per I²C inquiry which can be (more or less easily) forged, so there's a large market of 3rd party, 'compatible' modules.

  • Okay so what you're saying is 1) the switch must support the line code 2) the MAC must support the data rate 3) the module must not get locked out by the switch software. Concerning #1: if the switch can only provide -R then it will not work since the line code would be different. However since the manufacturer himself provides a 10GABSE-T module (but doesn't mention 2.5G or 5G) we can assume that either a) the module contains a transcoder or b) the module instructs the switch to send -T encoding as you described. Whatever is the case, 3rd party modules will do the same. (...) – Marki Mar 28 '20 at 19:04
  • (...) In that case the only unknown factor remains the MAC. It might work but is totally unsupported and we should get an official 802.bz compatible switch, which means: 1) MAC layer must support data rate 2) PHY layer operates like 10GBASE-T but at a lower signaling rate (but which is independent of the switch if it is an SFP-slot-only device where the PHY resides at the module anyway). So in the end it might just be marketing that prevents you from officially using 2.5 or 5G. – Marki Mar 28 '20 at 19:05

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