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I'm performing a set of tests on an embedded device, one of which is "To verify that the control system and instrument supports auto-negotiation". I need to verify that "Expected auto-negotiation is achieved based on ability flags".

From what I understand the auto-negation messages and the ability flags don't go beyond the NIC, so I wont be able to capture them using wireshark.

Is this assumption correct, and if so, how can I inspect these messages?

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Auto negotiation is performed on the physical layer - the NIC driver may report what has been negotiated or it may not (only speed). If you want to look deeper into this I see two variants:

  1. Use switch ports that can be configured to "Auto-10", "Auto-100" and so on. This setting doesn't deactivate autoneg on the switch but restricts the port to only negotiate a single speed. This variant doesn't really tell what's going on but it can tell you if the device is capable of correctly negotiating different speeds.
  2. Build an analyzer. Given the low cost and versatility of tiny single-board computers, it should be possible to set one up for tapping into autoneg. For detailed analysis, the fast link pulses (FLP) require a 10 MHz resolution. Wikipedia has a short summary. Details can be found in IEEE 802.3 Clauses 28 and 73.

Both variants are passive. If you need a lab simulation to check whether the device works in all possible scenarios you'd have to expand variant 2 so that you generate your own FLP bursts.

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