1.5Gbps Internet Plan but only getting 800Mbps after fiber line change

[hys17]

I would take a guess and say it's MAC binding on the ONT Port. You only have a single IP Address assigned, so when it grabs it will bind the MAC address of whatever is connected to it, and thereafter only talk to that MAC address. Sometimes just flapping the connection will reset it, if you give the ONT a minute to catch up, but often enough you need to actually reboot it to clear it and allow a different MAC to bind instead

That wouldn't in itself explain the observed symptom, since @hys17 didn't mention replacing his router with something else, only rebooting it. But I bet you're close to the truth. I'm wondering if the router itself changes its WAN-side MAC address during reboot for some reason. If so, it should be possible to turn off that behavior.

A variant of this idea is that the MAC address is stable, but the ONT is grabbing a new WAN IP address via DHCP from the ISP whenever it sees its local connection flap, and then somehow that change isn't reflected to the user's router. The trouble with this theory is that it doesn't explain why a subsequent ONT reboot would fix it.

Anyway, to dig deeper, try checking the router's WAN-side MAC and IP addresses at each step during a reboot cycle, and see if anything changes.

Thanks for the thoughts.

The WAN MAC address is persistent throughout reboots. The WAN IP does not change if I don't ever reboot the ONT.

After a bunch of reboots and even factory resets, the issue persists.
I should mention that the PPPOE requires VLAN ID=40 for some reason (maybe due to CGNAT?). Not sure if that got to do with anything.

But this is a minor thing that I can totally live with for now. Was only curious and would like to learn the logic/theory behind it.

Thanks again guys!