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. |