[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).