Ran into this same issue with pfsense+ 25.07
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Re: WAN Interface flapping on autoselect
I have a Netgate 4200, and recently updated to 25.07. The WAN uplink is to a Spectrum modem with 2.5Gb ethernet port.
Even before the update, I had been noticing that the WAN interface would sometimes revert back to 1000Base-T with the Speed and Duplex set to "Default (no preference, typically autoselect)."
So I thought I would change it to "autoselect" to see if this would help keep the link on 2500Base-T. This appeared to work for a while, but then the WAN link would drop and go into a disconnect/connect cycle about every 90-120 seconds or so.
I found I had to set it back to "default" which says "typically" autoconnect". Apparently default and autoconnect are not quite the same setting, as the latter was unstable so I reset it back to "default".
I thought I'd share this in case someone else had tried to change this selection. It's not clear why the "default" works and explicitly setting it to "autoconnect" causes the link to flap.
Curious if anyone else has ran into this issue.
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Yes I have seen that on the igc driver. It really only supports auto-select. For some reason when the driver sets anything it can cause link issues.
Also worth noting is that if you have set autoselect and then go back to default it may not reset that since 'default' sets nothing. You may need to reboot to get the NIC and driver back to the actual default state.
