Ethernet - speed and duplex mismatches

When using Ethernet to connect network devices, there are a couple of configurations that can be applied to Ethernet ports on both switches and routers that can affect the operation of the link at Layer 2. These are the speed and duplex settings. The following describes these parameters as well as their behavior.

1. Speed: The speed of an Ethernet connection refers to the maximum data rate that it can handle, typically measured in bits per second (bps). In the early days of Ethernet, speeds were often 10 Mbps. As technology advanced, Fast Ethernet (100 Mbps), Gigabit Ethernet (1 Gbps), 10 Gigabit Ethernet (10 Gbps), and even faster standards were introduced. The speed setting on a modern Ethernet device defines the maximum rate at which the device can send and receive data. The speed setting can have values that correspond to these specific
2. Duplex: The duplex setting of an Ethernet connection refers to how data transmission takes place between devices. There are two types of duplex settings:
   • Half-Duplex: In half-duplex mode, data can be transmitted in both directions, but not at the same time. When one device is transmitting, the other must wait and vice versa. This method was common in older Ethernet systems but is rarely used in modern networks because it's not as efficient.
   • Full-Duplex: In full-duplex mode, data can be sent and received at the same time, effectively doubling the potential throughput. Almost all modern Ethernet connections are full-duplex.

By default, today's network devices have both the duplex and speed settings set to auto. This means that autonegotiation is enabled and the connected devices will determine the best speed and duplex settings for their connection.

If a device with autonegotiation enabled is connected to a device that has its speed and duplex manually configured, the autonegotiation cannot correctly determine the speed and duplex settings of the other device. As a result, it will default to half-duplex mode and to the lowest supported speed (10Mbps) to avoid potential collisions and to ensure connectivity is established.

If there is a speed or duplex mismatch, meaning that each end of a link is manually configured but the settings on each end are not the same, then the link will remain down.

Links:

https://networklessons.com/switching/troubleshooting-interfaces#Duplex_speed_issues

https://forum.networklessons.com/t/troubleshooting-interfaces/1183/26?u=lagapides

https://forum.networklessons.com/t/troubleshooting-interfaces/1183/28?u=lagapides