MTU 1508 possible with RT-AC86U?

[crapinon]

Hi, my RT-AC86U is up and happily running Firmware Version 384.6.
I am connected with BT (UK) via PPPOE which supports baby jumbo frames (MTU 1508) via a bridged Zyxel Modem with MTU 1508 support.
In the WAN settings I have set it to the max allowed MTU and MRU of 1500.

However when testing, packets get fragmented.
with the result:

crapinon@box:~$ ping -M do -c 4 -s 1472 9.9.9.9
PING 9.9.9.9 (9.9.9.9) 1472(1500) bytes of data.
From 192.168.100.1 icmp_seq=1 Frag needed and DF set (mtu = 1492)
ping: local error: Message too long, mtu=1492
ping: local error: Message too long, mtu=1492
ping: local error: Message too long, mtu=1492

Is the feature supported?
Do I need to change something?

Thanks,

 

[crapinon]

PS. This thread does suggest a solution but it is very old:
http://forums.thinkbroadband.com/ds...fc-4638-asus-routers.html?fpart=2#Post4308576

Thanks!

 

[agilani]

Increasing the packet mtu to the limit is never a good idea and could leave the network degradation and packets that will need to be re transmitted into smaller fragments. You don't know what all of the networks that the traffic will have to traverse and all of them will need to support that mtu.

Jumbo frames are pretty much for where you can control the network from client to switch/router to client. Every single device will need to support jumbo frames. Even then, the results are mixed. I did a test with 9k jumbo frame and even though i didn't see any packet fragmentation through my ac88, the throughput went down significantly. Not sure why. I switched my backups back to default mtu.

You can enable jumbo frames under lan switch control in the asus router. I'm not sure that it would work over wan due to what i shared above.

 

[crapinon]

I was wondering if this feature is available in the new generation current branch (382.xx and newer), RMerlin?

 

[ColinTaylor]

PS. This thread does suggest a solution but it is very old:
http://forums.thinkbroadband.com/ds...fc-4638-asus-routers.html?fpart=2#Post4308576

It looks like that commit is still present in the current code. So as far as I can tell it takes the MTU/MRU from the GUI and adds 8 to it.

If you SSH into the router and issue the an "ifconfig -a" what MTU's do you see (it might be a limitation of the network interface)?

 

[crapinon]

Thanks Colin, the ifconfig output is:

ppp0      Link encap:Point-to-Point Protocol
          inet addr:109.153.87.107  P-t-P:172.16.17.103  Mask:255.255.255.255
          UP POINTOPOINT RUNNING MULTICAST  MTU:1492  Metric:1
          RX packets:4860316 errors:0 dropped:0 overruns:0 frame:0
          TX packets:1443019 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:3
          RX bytes:6772348663 (6.3 GiB)  TX bytes:603331782 (575.3 MiB)

 

[AndreiV]

BT Infinity MTU is 1492 , as agilani says increasing size will cause fragmentation and many other problems.

You may well be better off REDUCING MTU to 1452 :


https://www.sonicwall.com/en-us/support/knowledge-base/170505851231244

https://www.aaisp.net.uk/kb-broadband-mtu.html

If you are using Windows 8 or higher , your OS automatically sets the network adapter MTU rate to suit the connection so your "tweaks" become somewhat pointless.

 

[crapinon]

That second link to aasip actually finishes with this though:

The upshot of all this? It is possible to get BT FTTC (Fibre to the Cabinet) circuits (which use PPPoE) working on full 1,500 byte PPP by using modified pppd on the customer end, a suitable network card that will handle 1,508 byte frames at 10/100Mb/s. We have done this! (Thanks to TonyHoyle and Simon, customers on irc, for tweaking pppd and testing this for us). The new FireBrick does, of course, support PPPoE with baby jumbo frames to handle 1,500 byte MTU and even bonds multiple lines. Using the right modem (and the DLINK 320B in bridge mode do this) you can negotiate and use 1508 bytes over ADSL as well.

This is what I am trying to set up. RMerlin says in the changelog it worked previously:
3.0.0.4.374.39 (31-Jan-2014) CHANGED: Allow PPPoE MTU up to 1500, for ISPs that support RFC 4638.
374.40 Beta 1 (1-March-2014) FIXED: PPPoE with an MTU of 1500 requires the WAN interface to have its MTU set at 1508 (patch by pinwing)

BT Infinity max MTU is 1500.

 

[ColinTaylor]

Thanks Colin, the ifconfig output is:

ppp0      Link encap:Point-to-Point Protocol
          inet addr:109.153.87.107  P-t-P:172.16.17.103  Mask:255.255.255.255
          UP POINTOPOINT RUNNING MULTICAST  MTU:1492  Metric:1
          RX packets:4860316 errors:0 dropped:0 overruns:0 frame:0
          TX packets:1443019 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:3
          RX bytes:6772348663 (6.3 GiB)  TX bytes:603331782 (575.3 MiB)

What happens if you try to change it manually? I'm not sure whether you'd need to change ppp0 or eth0 or both.

ifconfig ppp0 mtu 1508 up

ifconfig eth0 mtu 1508 up

 

[crapinon]

Hi Colin,

that has worked from the terminal whilst the system is up. I will now try and get the setting to persist in wan-start

admin@RT-AC86U:/tmp/home/root# ifconfig ppp0 mtu 1500 up
admin@RT-AC86U:/tmp/home/root# ping -c1 -M do -s 1472  9.9.9.9
PING 9.9.9.9 (9.9.9.9): 1472 data bytes
1480 bytes from 9.9.9.9: seq=0 ttl=55 time=12.205 ms

--- 9.9.9.9 ping statistics ---
1 packets transmitted, 1 packets received, 0% packet loss
round-trip min/avg/max = 12.205/12.205/12.205 ms

THANKS!

 

[crapinon]

The problem now is that I can't set eth0 to mtu 1508, I get:

ifconfig eth0 mtu 1508 up
ifconfig: SIOCSIFMTU: Invalid argument

I notice that even though the firmware GUI has jumbo frames enabled the switch shows:

 ifconfig bcmsw
bcmsw     Link encap:Ethernet  HWaddr 0C:9D:92:46:4B:B8
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1

which must need fixing.
What I need to set is this (which unfortunately isn't allowed):

#!/bin/sh
ifconfig bcmsw mtu 1508 up
ifconfig br0 mtu 1500 up
ifconfig eth0 mtu 1508 up
ifconfig eth1 mtu 1500 up
ifconfig ppp0 mtu 1500 up

 

[ColinTaylor]

Here's a bit of a random idea, but why don't you try logging into the router and going to the WAN page. Change the Connection Type to Automatic IP, scroll down and change the MTU then apply it. Wait for it to be applied then switch the connection back to PPPoE and apply that.

 

[crapinon]

I couldn't get it to work so I have installed standard ASUS firmware to test further.
With plain ASUS firmware within the GUI the max allowed MTU is 1492 - which works as expected.
In RMerlins Firmware Version 384.6, 1500 is selectable in the GUI - but doesn't work as expected...

It must be a bug.
'ifconfig ppp0', shows that MTU remains stuck at 1492

 

[agilani]

If you are using Windows 8 or higher , your OS automatically sets the network adapter MTU rate to suit the connection so your "tweaks" become somewhat pointless.

interesting, do you have any references for this? I thought the driver settings determined it.

 

[proc999]

It is not possible on the RT-AC86U because the linux kernel is compiled without the CONFIG_BCM_JUMBO_FRAME option, even if the the BCM HND ethernet supports 2044 byte length mini jumbo frames.
Unfortunately the ethernet driver source is not open in the ASUS GPL (probably because of the TrendMicro security driver).
The source code of this ethernet driver is available in the TP-LINK AC2300 GPL,
and compiling this source in the ASUS GPL code I can reach this mini jumbo MTU, but it needs some work to make the Trend Mirco driver to work with this custom driver.

I don't know if we could ask ASUS to turn on the CONFIG_BCM_JUMBO_FRAME option when compiling this kernel and the ethernet driver.