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Suppose I have a device that supports 10/100/1000/10GBase-T ("Firewall") and another device that supports 10/100/1000/NBase-T ("Modem").

Now, I could connect these two devices using 1000Base-T. However, I would prefer the higher speed of NBase-T. So there needs to be a device in between that "translates" NBase-T to 10GBase-T. The classic idea here would be a switch that supports both NBase-T and 10GBase-T. But due to other constraints, such a switch would be quite expensive (i.e. me needing 8+ ports with at least one more 10GBase-T port).

Because of this I consulted a networking-experienced friend and he suggested using a PoE-injector that supports NBase-T and 10GBase-T. My question is now:
Do PoE-injectors typically require the negotiated input and output speed to match or can they do the kind of "translation" layed out above?

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  • Looking through the tags seems to indicate that this stack might not be the right one for this question (10G feels more enterprise-y while 2.5G feels more consumer to me). Please flag for migration to SU / SF if the question might be more fitting there than here.
    – SEJPM
    Commented Apr 3, 2023 at 16:28
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    A PoE injector does not adapt speeds, it only adds power for devices that request power. If your modem is for WAN, your WAN interface is likely less than 1 Gbps anyway, so it would be no advantage to connect to its LAN at a higher speed.
    – Ron Maupin
    Commented Apr 3, 2023 at 16:32
  • @RonMaupin "A PoE injector does not adapt speeds" sounds like the answer to the question to me? And as it would happen, the WAN hitting >1Gbps is a realistic possibility in this scenario. If you want a more realistic scenario: Imagine a switch not supporting NBase-T but 10GBase-T (with a corresponding NAS / Firewall) and a consumer-grade computer supporting NBase-T but not 10GBase-T (though such a switch is rarer these days).
    – SEJPM
    Commented Apr 3, 2023 at 16:36
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    If your 10GBASE-T device has SFP+ ports, consider using a transceiver that supports NBASE-T. Apparently there are transceivers that negotiate 10G speed via SFP+ and 2.5G / 5G speed via the RJ45 port. (There are also transceivers that negotiate 2.5G via SFP directly, which would likely be incompatible.) Commented Apr 4, 2023 at 2:18
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    @Zac67 I'm pretty sure there is. Here's a reviewed list: servethehome.com/sfp-to-10gbase-t-adapter-module-buyers-guide In fact, I have personally tested a no-brand SFP+ module in a ConnectX-3, which negotiated 2.5Gbit with a 2.5BASE-T USB adapter just fine. The ConnectX-3 being released in 2012 certainly have no support for NBASE-T. Apparently, these adapters simply drop any packets not fitting into the 2.5Gbit pipe on the floor and let flow control handle the rest. Commented Apr 5, 2023 at 10:39

2 Answers 2

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"NBASE-T" or "MGBASE-T" is any speed from 2.5GBASE-T or 5GBASE-T (sometimes including 10GBASE-T).

The best speed in common between a 10/100/1000/10GBase-T and a 10/100/1000/NBase-T port is 1000BASE-T. A device that translates speeds e.g. between 5GBASE-T and 10GBASE-T is a switch, nothing else. In theory, you could use a switching media converter, but that's a switch, too (and not much cheaper than one with a few ports more).

Using a PoE injector doesn't change a thing. Or in other words: "Do PoE-injectors typically require the negotiated input and output speed to match?" - yes, they do, generally. A PoE injector does not (auto) negotiate any link speed, it just passes through the data part of the link, including fast link pulses for Auto Negotiation. The speed is negotiated between the data endpoints, through the injector.

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  • ah, good to know about the meaning of "NBASE-T", I actually thought it meant the same as "2.5GBASE-T" :)
    – SEJPM
    Commented Apr 3, 2023 at 16:44
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The answer to the question definitely is that poe injectors don't renegotiate speeds, so you can't use it to adapt between devices. Data lines pass straight through injectors and the power lines on the input side dead end, while the power lines on the output end come from whatever power device is creating the 48v for the poe.

But I think we need more clarification on the speeds and formats your two devices support here. 10/100/1000 is not really an Nbase-t speed, that is standard gigabit and GBase-T. While Nbase does refer to anything above 1000, typically you see it applied just to 2.5g and 5g. So is your modem 1000 capable only? Or is it one of the newer 2.5g devices a lot of ISPs are putting out into the field now?

Is your firewall using an RJ45 port or an SFP+ port?

Or if your modem only supports gigabit then this whole question doesn't matter since that isn't even Nbase-t and both your devices already support gigabit between them.

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  • Removed the off-topic product recommendation that is also off-topic as a consumer-grade device.
    – Ron Maupin
    Commented Apr 4, 2023 at 15:05

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