Some background: PPPoE connections (as used on BT FTTC connections) have an 8 byte overhead that isn’t present in PPPoA (BT ADSL) connections. Now, an overhead of 8 bytes every 1500 is on the order of 0.5% which is unfortunate but not a disaster. Just set the MTU to 1492 and away you go right? Sadly, some client / server combinations just don’t work if they can’t send 1500 byte packets.

Fortunately a fix was designed in the form of RFC4638 (commonly known as baby jumbo frames which I find much easier to remember). Technicolor routers support this, but it isn’t enabled by default and in fact needs to be enabled using Telnet.

At any point, you can check what the MTU is set to using the following:

:ppp iflist


The commands you need differ depending on if you are using the integrated modem or not. If the Technicolor is connected directly to a phone line:

:eth ifconfig intf=eth_Internet mtu=1508
:eth ifconfig intf=eth_Internet_VLAN mtu=1508
:ppp ifdetach intf=Internet
:ppp ifconfig intf=Internet ppp-max-payload=enabled mru=1500
:ppp ifattach intf=Internet
:saveall

If using an external modem:

:eth ifconfig intf=eth_Internet mtu=1508
:ppp ifdetach intf=Internet
:ppp ifconfig intf=Internet ppp-max-payload=enabled mru=1500
:ppp ifattach intf=Internet
:saveall

You can test it with the command:

:ping proto=ip addr=8.8.8.8 size=1500 DF-bit=enabled