This message came out of funny discussion on testers telegram channel. Auto-negotiation in Ethernet world was developed back to 1995 (as part of 802.3u). Still, surprisingly, a lot of people confused (primeraly out of ignorance, I suppose).
Why it is important in our case? Simply, because if you do it wrong, then you may screw it and will experience funny stuff (like "famous" cracklings, same famous "radio disconnection", etc).
Golden rule 1: do not
f**king touch it (Auto), just leave it. Repeat: leave it Auto at every ehternet port - PC, switch (if in use), AP (if in use), router (if in use).
Rationale for the above rule (under spoiler)
in 99.99% of common networking equipment the Ehternet (as well as FastEthernet and GigabitEthernet) ports are set by default in Auto (means Auto-negotioation). And it is done for purpose. SunSDR FastEthernet port is "100BaseT Auto" (means 100 Mbps with Auto-negotiation). Hence, the corresponding port (ether direct connection to PC, or switch/router port) is recommended to be Auto.
Golden rule 2: you have a right to experiment and find yourself a fun. Just remember to read and understand clause 28 of the IEEE 802.3 prior.
Remark for rule 2 (under spoiler):
If you chose "fun-way", then make sure you divert it to Golden rule 1, prior opening new thread of Github case with "cracklings/connection loss/etc".
If you disagree with Golden rule 1, you better drop SunSDR and buy yourself classic TX without Ethernet - this will make your life much easier.
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For those who are unsure, but lazy to read clause 28 of the IEEE 802.3 - here below (under spoiler) the brief explanation how Auto-negotiation works and why you better do not touch it unless it is absolutely nesessary (0.01% of the chance it is).
Each communicating interface from the Ethernet pair shares its parameters via FLPs (Fast Link Pulses), exchanging information about duplex mode (full-duplex or half-duplex) and speed (10, 100, 1000 Mbps). The final duplex and speed selection on the interfaces is set the way, which allows both interfaces to achieve the highest performance.
For auto-negotiation to occur, both interfaces must be configured in Auto mode.
What will happen if one of the interfaces is auto-negotiating and the other is manual? Simple: one interface is sending FLPs (containing info with its capabilities) and the other interface has already hard-set a speed rate and a duplex mode, making it impossible to negotiate with its uplink. Then, the device in auto-negotiating mode must determine itself what are the appropriate speed and duplex mode to connect - sometimes this is possible (because of each - 10, 100, 100 Mbps speeds has different signalling methods).
However, the determination is very dependent on the vendor implementation (drivers. priorities, defaults); hence there is very high chance that devices of two different vendors may fail to decipher is its link partners’ duplex mode and speed, which (99.99% chahce) will lead to a mismatch. And mismatch (in our particular case) will (99.99% chahce) lead to link performance degradation, and, as the result, packets dropouts (cracling sounds, SunSDR disconnects, delays, etc.)
What Should I Do If A Mismatch Occurs?
Obviously: use the Golden rule 1