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10/100/1000 speed/duplex auto negotiation

Interesting info from Cisco (I’m ommiting obvious info about manual/auto settings for speed/duplex which result mismatch as this is a quite well known problem)

100Mbps glitchy NICs

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.


Gigabit Ethernet has an autonegotiation procedure that is more extensive than what is used for 10/100 Mbps Ethernet (Gigabit autonegotiation specification IEEE 802.3z-1998). The Gigabit autonegotiation negotiates flow control, duplex mode, and remote fault information. You must either enable or disable link negotiation on both ends of the link. Both ends of the link must be set to the same value or the link does not connect.

If either device does not support Gigabit autonegotiation, disable Gigabit autonegotiation in order to force the link up. The default configuration of all Cisco switches is autonegotiation-enabled. If you disable autonegotiation, it hides link drops and other physical layer problems. Only disable autonegotiation to end-devices, such as older Gigabit NICs that do not support Gigabit autonegotiation. Do not disable autonegotiation between switches unless absolutely required, as physical layer problems can go undetected and result in spanning tree loops. Rather than disable autonegotiation, you can contact the vendor for a software or hardware upgrade for IEEE 802.3z Gigabit autonegotiation support.

Categories: Cisco, LAN
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