C. Devices' duplex settings.
When upgrading a network to gigabit speeds, it's essential to ensure that all devices, including endpoint NICs (Network Interface Cards), are configured to operate at the same speed and duplex mode. If the duplex settings are inconsistent, it can cause performance issues, such as slow speeds, packet loss, and network congestion.
Duplex settings refer to whether a NIC operates in half-duplex or full-duplex mode. Half-duplex mode allows a device to either transmit or receive data at a time, while full-duplex mode allows simultaneous transmission and reception of data. If some devices are set to half-duplex and others are set to full-duplex, it can cause communication issues and slow down the network.