ADSL FTTC switching out the Operator Router random webpages not loading

After swapping out my PlusNet router that was chained into a BT VDSL modem, I started to experience loading failures in random webpages. Although the internet kinda worked, HTTPS was behaving exceedingly poorly and would just hang there.

 
These symptoms frequently point to an MTU issue, as information at the end of each packet is getting chopped off. It normally isn't so noticeable on unencrypted data, since there is a higher chance of the last portion of a packet being blank and most data formats themselves are quite tolerant to partial corruption, whereas SSL has to be strict in order to provide the necessary guarantees.

As is common, my MTU for the PPPoE interface defaulted to 1492, exactly 8 bytes less then the 1500 set on the ethernet interface -which is the largest frame size for conventional equipment.

I then started experimenting with different MTU values. I tried loads of random ping tests with different packet sizes, anything I googled that was MTU and ping I tried with no effect.

I got the Plusnet router back up and running, however its web interface is next to useless as it didn't provide any information.

So I went on trying setting smaller and smaller MTU values on the PPPoE interface, however nothing worked.

Then, just out of desperation I started investigating "Baby Jumbo Frames RFC4638". Unfortunately i couldn't find any options for enabling it on the router, however a post on the Ubiquiti forums showed a configuration with a higher than 1500 MTU.

So I set the Ethernet interface with an MTU of 1508, which it accepted with no complaint, then set pppoe0 to MTU 1500 and this is when everything started working.





Ubiquiti forum post on similar issue relating to MTU - http://community.ubnt.com/t5/EdgeMAX/BUG-PPPoE-client-pppoe-dialing-before-MTU-set-on-eth0/td-p/917204