Very weird issue. I was on the Vodafone NZ Red network and had set my MTU to 1492 as that supposedly is the optimal size for pppoa. I had to switch to the Vodafone wholesale network in order to get ownership of my phone number so I could transfer it to a voip system.
After the transfer I had very weird connection issues, for example https would only work up to a point, i.e. I could make the ssl connection, send the HTTP request, but the response would never arrive. Or ssh to a server, but when I typed ls the reply would never come back.
Ultimately my issues are described here. MTU Path Discovery is broken. I suspect Vodafone's wholesale network might be blocking things, but don't know for sure. Could be my firewall for all I know. When I change the MTU on the clients to 1492 everything works. But that's annoying, so set the MTU on the modem back to 1500.
Another weird thing on Linux is that I can't do MTU Path discovery. For example:
ping -s 10000 www.dslreports.com
works fine. Setting the DF bit:
ping -M do -s 1472 www.dslreports.com
gives me the biggest packet I can sent. Which I don't really understand because as I understood it the biggest packet on ppoa is slightly smaller, i.e. 1478. But I suppose with ATM cells they just carry the MTU and don't care what the real limit is.
Anyway, drove me crazy for hours.