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 1 |
| AUTO | 100 Mbps, Full-duplex | 100 Mbps, Half-duplex | 100 Mbps, Full-duplex | Duplex Mismatch 1 |
| 100 Mbps, Full-duplex | 100 Mbps, Full-duplex | 100 Mbps, Full-duplex | 100 Mbps, Full-duplex | Correct Manual Configuration2 |
| 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. |
1 A duplex mismatch can result in performance issues, intermittent connectivity, and loss of communication. When you troubleshoot NIC issues, verify that the NIC and switch use a valid configuration. 2 Some third-party NIC cards can fall back to half-duplex operation mode, even though both the switchport and NIC configuration are manually configured for 100 Mbps, full-duplex. This is because NIC autonegotiation link detection still operates when the NIC is manually configured. This causes duplex inconsistency between the switchport and the NIC. Symptoms include poor port performance and frame check sequence (FCS) errors that increment on the switchport. In order to troubleshoot this issue, try to manually configure the switchport to 100 Mbps, half-duplex. If this action resolves the connectivity problems, this NIC issue is the possible cause. Try to update to the latest drivers for your NIC, or contact your NIC card vendor for additional support. A manual setup of the speed and duplex for full-duplex on one link partner results in a duplex mismatch. This happens when you disable autonegotiation on one link partner while the other link partner defaults to a half-duplex configuration. A duplex mismatch results in slow performance, intermittent connectivity, data link errors, and other issues. If the intent is not to use autonegotiation, both link partners must be manually configured for speed and duplex for full-duplex settings.