Speed and Duplex Setting

Speed determination issues can result in no connectivity. However, issues with autonegotiation of duplex generally do not result in link establishment issues. Instead, autonegotiation issues mainly result in performance-related issues. The most common problems with NIC issues deal with speed and duplex configuration. The following table show all possible settings of speed and duplex for FastEthernet/Gigabit Ethernet NICs and switch ports. This is only applicable for 10/100/1000 Mbps (1000BASE-T) NICs only

NIC (Spd/Dplx)	Switch (Spd/Dplx)	Resulting NIC Spd/Dpx	Resulting Catalyst Spd/Dplx	Comments
AUTO	AUTO	1000 Mbps Full-duplex	1000 Mbps Full-duplex	Assuming maximum capability of Catalyst switch, and NIC is 1000 Mbps, full-duplex.
1000 Mbps Full-duplex	AUTO	1000 Mbps Full-duplex	1000 Mbps Full-duplex	Link is established, but the switch does not see any autonegotiation information from NIC. Since Catalyst switches support only full-duplex operation with 1000 Mbps, they default to full-duplex, and this happens only when operating at 1000 Mbps.
1000 Mbps Full-duplex	1000 Mbps Full-duplex	1000 Mbps Full-duplex	1000 Mbps Full-duplex	Correct Manual Configuration
100 Mbps Full-duplex	1000 Mbps Full-duplex	No Link	No Link	Neither side establishes link, due to speed mismatch
100 Mbps Full-duplex	AUTO	100 Mbps Full-duplex	100 Mbps Half-duplex	Duplex Mismatch
AUTO	100 Mbps Full-duplex	100 Mbps Half-duplex	100 Mbps Full-duplex	Duplex Mismatch
100 Mbps Full-duplex	100 Mbps Full-duplex	100 Mbps Full-duplex	100 Mbps Full-duplex	Correct Manual Configuration
100 Mbps Half-duplex	AUTO	100 Mbps Half-duplex	100 Mbps Half-duplex	Link is established, but switch does not see any autonegotiation information from NIC and defaults to half-duplex when operating at 10/100 Mbps.
10 Mbps Half-duplex	AUTO	10 Mbps Half-duplex	10 Mbps Half-duplex	Link is established, but switch does not see Fast Link Pulse (FLP) and defaults to 10 Mbps half-duplex.
10 Mbps Half-duplex	100 Mbps Half-duplex	No Link	No Link	Neither side establishes link, due to speed mismatch.
AUTO	100 Mbps Half-duplex	100 Mbps Half-duplex	100 Mbps Half-duplex	Link is established, but NIC does not see any autonegotiation information and defaults to 100 Mbps, half-duplex.
AUTO	10 Mbps Half-duplex	10 Mbps Half-duplex	10 Mbps Half-duplex	Link is established, but NIC does not see FLP and defaults to 10 Mbps, half-duplex.