at speed. Parallel detection determines only the link speed, not the supported duplex modes. This is an important consideration because the common modes of Ethernet have dif- fering levels of duplex support: 10Base-T 10Base-T was originally designed without full-duplex support. Some implemen- tations of 10Base-T support full duplex, but many do not. 100Base-T 100Base-T has long supported full duplex, which has been the preferred method for connecting 100 Mbps links for as long as the technology has existed. However, the default behavior of 100Base-T is usually half duplex, and full-duplex support must be set manually. 1000Base-T Gigabit Ethernet has a much more robust autonegotiation protocol than 10M or 100M Ethernet. Gigabit interfaces should be left to autonegotiate in most situations. 10 Gigabit 10 Gigabit (10G) connections are generally dependent on fiber transceivers or spe- cial copper connections that differ from the RJ-45 connections seen on other Ethernet types. The hardware usually dictates how 10G connects. On a 6500, 10G interfaces usually require XENPAKs, which only run at 10G. On a Nexus 5000 switch, some of the ports are 1G/10G and can be changed with the speed command. Chapter 3: Autonegotiation Because of the lack of widespread full-duplex support on 10Base-T and the typical default behavior of 100Base-T, when autonegotiation falls through to the parallel de- tection phase (which only detects speed), the safest thing for the driver to do is to choose half-duplex mode for the link. As networks and networking hardware evolve, higher-speed links with more robust negotiation protocols will likely make negotiation problems a thing of the past. That being said, I still see 20-year-old routers in service, SO knowing how autonegotiation works will be a valuable skill for years to come. When Autonegotiation Fails When autonegotiation fails on 10/100 links, the most likely cause is that one side of the link has been set to 100/full and the other side has been set to autonegotiation. This results in one si