千兆以太网如何硬件强制降级成百兆以太网？

找到答案了， 虽然是比较ugly的方法。
Since 1000BASE-T uses two more pairs of conductors than 10 or 100 Mbit
Ethernet, I bypassed each of those extra pairs with a 220 pF capacitor.
(Disregard the erroneous 22 pF marking in the photos.) This filters out the
high frequency signals of 1000BASE-T, forcing the target devices to revert to
100BASE-TX which can then be monitored. The capacitors don't adversely affect
lower frequency RS-232 signals, so all eight conductors function when
monitoring serial connections. Sure, it's an ugly hack, but it's an ugly hack
that fits in your pocket.

对呀，八根线只接四根不就欧啦？