We have been using Draytek Vigor 120 ADSL modems with various RouterBoards on many of our sites, each time we just add a PPoE client on to the interface the modem is plugged in to, supply it the ADSL user name and Password and it works fine.
It always create 2 dynamic Mangle Rules to lower the MSS.

On our latest install we found we could ping out but not view any web pages, we then found there was only a mangle rule created for outgoing traffic not incoming.

In the PPoE client it shows under general Max MTU 1480 and Max MRU 1480, however on the status page MTU is 1480 and MRU is 1500.
Tried a reboot incase this was a fluke but still only 1 rule created.

I then added the second mangle rule staticly and web traffic started working fine.

Where do these settings come from? are they supplied by the ISP when the modem connects?
Why would this not be creating both rules?