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Ethernet Speed and Duplex

Hi All,

I have a few queries on ethernet speed and duplex settings and auto negotiation function.

  • The document in the link below says that even when the speed on the far end device is hard coded and the fast ethernet port can auto sense the correct speed this is how its said to be acheived ". In order to detect the speed, the link partner senses the type of electrical signal that arrives and sees if it is 10 Mb or 100 Mb" Can someone explain how the electrical signals help in speed detection ? What is the one end is a Fast ethernet port and hard coded to 10 Mb what will the fast ethernet port at the other end detect how will the electrical signals be different to denote a 10 Mb connection at the far end.

  • It also says in another section that " You might see cyclic redundancy check (CRC) error messages when both the speed and duplex settings are hardcoded on the two devices. This might be because any one of the devices runs an earlier version of Cisco IOS. You can upgrade the Cisco IOS or set the speed and duplex to auto on both devices in order to resolve this." Any guesses why this may happen ?

  • It is always seen the when reconfiguring speed and duplex on any interface if the duplex is configured before the speed the interface bounces. Any guesses why this would happen ? 

Thanks in Advance

Umesh Shetty

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