On Friday, July 08, 2011 07:06:06 PM Les Mikesell wrote: >And the > gigabit spec requires auto-negotiation. According to clause 37.1.4.4 "User Configuration with Auto-Negotiation" of IEEE 802.3-2008, auto-negotiation with 1000Base-X is optional, but encouraged. In contrast, for 1000Base-T, we find the following information in clause 40.5.1: "40.5.1 Support for Auto-Negotiation All 1000BASE-T PHYs shall provide support for Auto-Negotiation (Clause 28) and shall be capable of operating as MASTER or SLAVE." 'Shall' being legal language for 'absolutely without a doubt must' in this case. Overwise the PHY wouldn't be able to determine which PHY is the clock MASTER.