RFC4638 support?

kamicrazy: Hi fe31nz

Thanks for providing your info. I assume you tested this with an Edgerouter, which is my choice of router too.

I have similar reasons for wanting 1500 MTU, it avoids problems like broken path discovery and a lot of stuff assumes a 1500 MTU

If you ever get to test it again, would you please share the results, this sort of info was impossible to find.

Yes, I am running a Ubiquiti EdgeRouter Lite (firmware 1.7.0).

I decided to test again today, and it appears that an MTU of 1500 is now supported! I made these changes to my config:

set system offload ipsec disable
set system offload ipv4 forwarding disable
set system offload ipv4 pppoe disable
set system offload ipv4 vlan disable
set system offload ipv6 forwarding disable
set system offload ipv6 vlan disable
set interfaces ethernet eth0 mtu 1508
set interfaces ethernet eth0 vif 10 mtu 1508
set interfaces ethernet eth0 vif 10 pppoe 0 mtu 1500
delete firewall options mss-clamp
delete interfaces ethernet eth1 ipv6 router-advert link-mtu 1492
delete interfaces ethernet eth2 ipv6 router-advert link-mtu 1492

The disabling of offloading was so that I could make sure that tshark could capture all the packets on the PPP connection, and the rest are for changing to MTU of 1500 and removing the workaround that allowed IPv6 to work with the smaller MTU over PPP.

After committing the changes, I unplugged the ERLite from the ONT and ran tshark on it to capture all the traffic on the eth0.10 interface. Then I plugged it into the ONT again and waited for the PPP connection to come up. Once it was up, the eth0.10 interface had an MTU of 1508 and pppoe0 had 1500.

Then I booted my test Windows 10 PC, and where the "netsh interface ipv6 show subinterfaces" command used to show an MTU of 1492, it now showed 1500. So I tried the command "ping -6 -n 2 -l 1452 google.com" and it worked. Previously, the maximum ping packet size that worked was 1444. Then I tried using Firefox to go to http://www.facebook.com, which used to fail without the "ipv6 router-advert link-mtu 1492" options due to large packets going missing. But now it worked.

It seems that a while back when Snap had some scheduled maintenance outages, what they were doing was replacing the routers that do the VLAN 10 PPPoE connections for them. Or at least they replaced the one that I connect to from Palmerston North. I noticed at the time that its MAC address had changed. It seems from the MAC address it is now a Juniper box, and it handles an MTU of 1500 for PPP connections.

I am not sure if the 1.7.0 firmware is correctly negotiating RFC4638 though - I could not see any PPP-Max-Payload options being negotiated. Instead, it seems to be negotiating for no MTU or MRU options at all, and accepting a default of 1500 that results. But since it works, that is fine by me.

So now I am going to renable offloading and enjoy the larger packet size. My main Windows 7 box will not actually use the new packet size until I reboot it - it is still remembering the old MTU option in the router advertisment packets it was previously receiving and Windows does not forget that option on receiving a new router advertisment without it. But I had planned a reboot for later tonight anyway.