If you try to connect the onboard Ethernet ports of various dual CPU socket X11 based systems to a 10/100Mb port, or via a 10/100Mb cable, it will NOT negotiate and no link lights will turn on.
According to Supermicro, the chipset specification changed for dual socket X11 boards and they no longer support negotiating to 10/100Mb link speeds.
All X11 dual socket motherboard LAN port only support 1Gb (or 10Gb) network switch.
It is the specification of LAN chipset which did not support 100MB/10MB.
IPMI is out-of-band and uses a different chipset, so it still supports 10/100Mb link speeds.
X11 systems with a single CPU socket use the older chipset specification and still support 10/100Mb link speeds.
You can confirm it from the manual. The way they list it is unusual, for example, on X11DDW-L, the manual shows that the LED will be OFF if no connection or 10/100Mb. According to Supermicro, this means it is not supported.
Our engineering team recommends to either install a PCIe network card or, if Gigabit speed is not required throughout the network, add a 1Gb switch to a 10/00Mb network and let the link between the 1Gb switch and the 10/100Mb network facilitate the link speed downgrade.