Forced 2.5Gbps negotiation with 1.0Gbps = successful, why?

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OpenSource Ghost
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Joined: 14 Feb 2022
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PostPosted: Sun Feb 12, 2023 16:38    Post subject: Forced 2.5Gbps negotiation with 1.0Gbps = successful, why? Reply with quote
Auto-negotiation is disabled everywhere. Ethernet NIC is set to forced 2.5Gbps full duplex negotiation and router port for that Ethernet NIC is set to 1.0Gbps full duplex negotiation. There are no devices between Ethernet NIC and router port. This configuration works, but Windows Network settings show that Ethernet NIC (forced to 2.5Gbps full duplex negotiation) connects at 1.0Gbps. This is most likely because router port negotiation is forced to 1.0Gbps and 1.0Gbps the highest possible speed on this router.

If 2 ends (Ethernet NIC and router port) have are forced to use non-matching full duplex negotiation speeds, then why does such a configuration work? Isn't it supposed to simply not connect due to non-matching forced negotiation speeds?
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blkt
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Joined: 20 Jan 2019
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PostPosted: Sun Feb 12, 2023 22:02    Post subject: Reply with quote
Entirely up to unspecified Ethernet NIC & driver, nothing to do with DD-WRT, probably to counter blatant user error.
OpenSource Ghost
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Joined: 14 Feb 2022
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PostPosted: Mon Feb 13, 2023 14:17    Post subject: Reply with quote
Intel i226-V 2.5Gbps is the NIC. Using either forced 1.0Gbps negotiation or forced 2.5Gbps negotiation in that NIC driver settings results in 1.0Gbps forced negotiation when router is also set to forced 1.0Gbps negotiation. What's the user error? I am asking whether it is normal for forced 1.0Gbps negotiation on one end and 2.5Gbps forced negotiation on another end to work at all. Wouldn't it normally not negotiate at all due to different speeds being forced on each end?
blkt
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Joined: 20 Jan 2019
Posts: 5660

PostPosted: Mon Feb 13, 2023 15:50    Post subject: Reply with quote
User error is expecting ability to force link speed, a misconfiguration, 2.5 Gbps link to a 1 Gbps router switch port.
Intel i226 is at least a third silicon revision to i225 also RealTek RTL8156B or RTL8156BG are in a similar situation.

You can research, plenty of information out there but TLDR use current hardware, drivers, quality Ethernet cables.
Known "workarounds" to force 1 Gbps link rate, or disable power management for no drop stability on these NICs.

But these are for 2.5 Gbps links, on both ends. It is not unusual for a NIC to automatically use what is compatible.
Anyway most problems are from cheap Ethernet wire, defective connection or something simple dust or oxidation.
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