I am seeing what appears to be a 2.5GBASE-T interoperability issue between a MikroTik CRS310-8G+2S+ and a UniFi U6 Enterprise access point.
Setup
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MikroTik CRS310-8G+2S+
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UniFi U6 Enterprise
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Ubiquiti UACC-PoE+-2.5G injector
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Known-good cabling
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RouterOS version: 7.23.2
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RouterBOOT firmware: 7.23.2
The AP, injector and cables all work correctly at 2.5G when connected through a UniFi switch. The CRS310 ports also negotiate and run at 2.5G correctly with my servers, so the ports themselves appear to be functional.
When the U6 Enterprise is connected to the CRS310, the port initially negotiates at 2.5G full duplex, but the link drops again after a few seconds. This repeats several times before it eventually falls back to 1G, which remains stable.
Example log
ether8 link up (speed 2.5G, full duplex)
ether8 link down
ether8 link up (speed 2.5G, full duplex)
ether8 link down
ether8 link up (speed 2.5G, full duplex)
ether8 link down
...
ether8 link up (speed 1G, full duplex)
The logs show that the 2.5G link is successfully negotiated, but it cannot remain established. The switch and AP then renegotiate until they settle at 1G.
Tests completed
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Same AP, injector and cables connected to a UniFi 2.5G switch: stable at 2.5G
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CRS310 2.5G ports connected to servers: stable at 2.5G
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Auto-negotiation enabled on the CRS310
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Attempting to force 2.5G is not supported, which appears to be expected for NBASE-T
This suggests a possible PHY/autonegotiation compatibility issue between the CRS310 and the U6 Enterprise, or possibly a RouterOS Ethernet driver issue.
Has anyone else tested this specific combination or seen similar 2.5G link flapping on the CRS310?
I would be interested to know whether any of the following have helped:
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A particular RouterOS version
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Updating or downgrading RouterBOOT
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Testing under SwOS
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Restricting advertised rates
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Any MikroTik support response or confirmed PHY compatibility issue
For now, I have restricted the port to 1G to keep the AP stable.