[t0mas88]

This is quite a common issue with PPPoE connections like the one OP seems to use with his own router. You need to increase the MTU of the physical underlying ethernet connection to 1508 to allow a 1500 MTU for the encapsulated packets inside the PPPoE tunnel. Otherwise you'll run into weird issues and unreachable websites.

[toast0]

You also need to make changes on the PPPoE server, which is hard because if a provider is running PPPoE in 2023, they probably don't care about doing things well (but maybe I'm just bitter about CenturyLink)

I have a browser based mtu test http://pmtud.enslaves.us/

Currently IPv4 only, requires a somewhat recent browser, and client to server testing is iffy, but if you start the test and get OK in the notes field for both directions, your MTU settings are probably fine (or something is doing proper mss clamping between your client and my server, my server is limited to 1500 MTU so problems with jumbograms can't be detected)

[ta1243]

I think openreach requires all their customers to use pppoe - certainly I've had it with both BT and A&A

[lxgr]

Increasing the MTU at the sender side to something >1500 is not a great idea. It’s unlikely that the path will support 1508 byte end-to-end.

A better idea would be to reduce the MSS inside the tunnel.

[Nextgrid]

The MTU is only increased between the router and DSL modem to account for PPPoE overhead, so that the MTU inside the PPPoE tunnel (and thus to the internet) can be a standard 1500 (otherwise it would be 1492).