Autonegotiation 

Autonegotiation is a signaling mechanism and procedure used by Ethernet over twisted pair by which two connected devices choose common transmission parameters, such as speed, duplex mode, and flow control.
Both sides having auto neg enabled is recommended.


Is auto-negotiation non configurable? 
Yes, you can un-configure by configuring force-mode. Which is not recommended.

Why does auto-neg need to be turned off to connect to the legacy 100M device?

• • On legacy devices, no auto-neg is supported on these connected clients/devices. Because that reason link will not be established until force mode is enabled on switch side.
  • If both sides have auto neg enabled, then it port will come up after neg speed, duplex and flow control (say 10M/100M/1G ports)
  • But problem arise when one side is set to auto-neg, while the other side legacy client set to 100M/full duplex (because end device/client don't support auto-neg). In this case, the auto-neg side will negotiate to 100M/half duplex, means one side will be full duplex, and one side half, that is the reason we will have problematic connection. The half-duplex side will see a lot of errors, and will keep backing off because it sees traffics on its RX!

Before deploying and connecting legacy clients or devices read data sheet of switches carefully and understand does the switch support auto-neg and force-mode (auto-neg off). 

Good day!

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Yash NN
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