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Message 1 of 4

New Full Fibre service 900/110Mbps MSS/MTU

So BT is now starting to roll our full fibre to thge premises in a few areas.

Which raises the age old question of MTU size?  Obviously, which is a shame in my opinion BT have stuck with PPPoE as its connection, rather than move to a simple DHCP model, for their network authentication mechanism for their new service.

I was presuming (or hoping) that the full 1500 MTU was now supported without having to use Mini-Jumbo frames?

Does anyone know what the optimal setting is please? I am moving to a Ubiquiti UDM Pro very soon, and wanted to understand if the standard Ethernet MTU is natively supported? I cannot test on my existing Asus AX11000 unfortunetly as it nails the packets at 1492.

But what is the optimal MSS / MTU Settings ?


All the best


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Message 2 of 4

Re: New Full Fibre service 900/110Mbps MSS/MTU

FTTP is still a PPPOE service with the same MTU etc.

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Message 3 of 4

Re: New Full Fibre service 900/110Mbps MSS/MTU

Be aware that while BT supports baby jumbo frames, the UDM/UDM Pro doesn't have the ability to increase the PPPOE MTU above 1492 as it stands in the current firmware. It was possible in the older USG through manual editing of the config file but don't think anyone has managed to do it in the UDM.
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Message 4 of 4

Re: New Full Fibre service 900/110Mbps MSS/MTU

You can actually get baby jumbo frames working pretty reliably on the UDM Pro, if you set the parent interface to 1508 MTU and the PPP interface to 1500 both in config and on interface!

$ cat

WAN_IFACE=$(cat /etc/ppp/peers/$PPP_IFACE | grep "plugin" | cut -d " " -f 3)

echo "Updating $PPP_IFACE config for 1500 MTU/MRU..."
sed -i 's/1492/1500/g' /etc/ppp/peers/$PPP_IFACE
echo "Setting $PPP_IFACE MTU to 1500..."
ifconfig $PPP_IFACE mtu 1500
echo "Setting $WAN_IFACE MTU to 1508..."
ifconfig $WAN_IFACE mtu 1508