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From: Theo <theom+news@chiark.greenend.org.uk>
Newsgroups: comp.misc
Subject: Re: Learned Something New About Ethernet Today
Date: 19 Jul 2025 22:55:12 +0100 (BST)
Organization: University of Cambridge, England
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Originator: theom@chiark.greenend.org.uk ([93.93.131.173])

Computer Nerd Kev <not@telling.you.invalid> wrote:
> Lawrence D'Oliveiro <ldo@nz.invalid> wrote:
> > Today I discovered that there are Ethernet interfaces that are not 
> > backward-compatible with older, slower speeds. Specifically, anything 
> > using the Intel X722 controller chip can do 1-gigabit and 10-gigabit, but 
> > not 100 megabits or 10 megabits per second. Connect it to a 10/100 switch 
> > port and ... silence.

Yes, it simplifies the silicon design to throw out the 10 and 100Mbps parts
which are likely never going to be used.  The simplest design is single rate
(10G) but 1G/10G dual rate is better than quad rate 10M/100M/1G/10G.

> A quick search suggests that controller is targeted at servers.
> Does it connect to any physical ports with connectors that are
> used for 10/100 Ethernet? I'd expect them only to be used with
> the higher speed Ethernet connectors used on servers, which I
> don't know of being used for 10/100 interfaces. Such servers
> might use a different controller chipset for any RJ45 Ethernet
> sockets.

X722 appears to be targeting SFP+, so it's designed for optical transceivers
or direct attach cables.  It seems there are some versions that support
BASE-T copper (RJ45 connectors) but almost mobody wants to hook those up to
a 10/100 network.

Theo