Thank you for contacting Intel Embedded Community.
The information that may help you is the answers to the questions 2.25 and 2.23, on page 9 of the Intel(R) Ethernet Controller I210/I211 Frequently Asked Questions (FAQs) document # 335346.
We hope that this information may help you.
Thanks a lot for your hint, while I'm still confused.
I use I210 with an external flash memory. And after I update the MAC address, I'll power down and power on the product.
For the pin12 the document mentioned, I tried the pull-up or pull-down scheme.
I can't figure out why the MAC address can only be changed once. Certainly, after reprograming the flash offline, the MAC address can be changed once online again.