MTU 1500 for BT ADSL with OpenWrt

Just a handy tip if you’re using BT ADSL with an OpenWrt router. I didn’t know you could get an MTU of 1500 on ADSL. I have that on my Infinity connection at home, but it wasn’t until I played with the router at my Father’s that I found it was possible on ADSL too.

Why do you want an MTU of 1500? Without going into all the technical details of Ethernet, here is the simple version: Your home network (wired or wireless) is Ethernet based and as standard Ethernet uses packets of 1500 bytes but traditionally ADSL routers in the UK were configured to use 1492. This meant any time you sent a full size packet to the router, to forward on to the internet, it had to break it down in to two packets. This adds overhead and is inefficient, resulting in reduced throughput (slower speeds).

If you are using OpenWrt connected to a modem that supports an MTU of >1500 you can fix this. I suggest the OpenReach white modems originally provided for Infinity, which can also be configured as PPPoE modems to connect an Ethernet OpenWrt router to ADSL.

To get the MTU up to 1500 for the PPP connection you need to increase the MTU of network interface to 1508. That’s easy, simply edit the WAN interface in the OpenWrt Luci GUI (no need to edit WAN6 as well). Or edit /etc/config/network (example from Netgear WNDR3700, interface names will vary by device):


config interface 'wan'
  option ifname 'eth1'
  option _orig_ifname 'eth1'
  option _orig_bridge 'false'
  option proto 'pppoe'
  option username 'user@isp.net'
  option password 'password'
  option mtu '1508'

This will only set the MTU for the physical interface. The PPPoE connection will still use 1492 and there doesn’t appear to be any way to fix this in the GUI. So add the following to a new file called /etc/hotplug.d/iface/99-mtufix (you’ll need to connect via SSH to do this):


#!/bin/sh

[ "$ACTION" = "ifup" ] || exit 0
[ "$DEVICE" = "pppoe-wan" ] || exit 0

logger -t mtufix "Setting MTU of $DEVICE to 1500."
/sbin/ifconfig $DEVICE mtu 1500

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