BT Infinty, PPPoE and 1500 MTU

[bmb]

In the UK BT Infinity supports an MTU of 1500, but I can only input up to 1492 on the PPPoE WAN set up with Merlin's 374_32 (and probably all previous versions). Is there a way to set it to 1500 by CLI?

I was reading about it here, PPPoE and RFC 4638. I know 1492 was always the max MTU for a PPPoE connection, but the BT network supports 1500 and it seems to make sense to use it, if only to match the local network.

 

[jlake]

In the UK BT Infinity supports an MTU of 1500, but I can only input up to 1492 on the PPPoE WAN set up with Merlin's 374_32 (and probably all previous versions). Is there a way to set it to 1500 by CLI?

I was reading about it here, PPPoE and RFC 4638. I know 1492 was always the max MTU for a PPPoE connection, but the BT network supports 1500 and it seems to make sense to use it, if only to match the local network.

This says use 1492 if you have a third party router. You have a third party router, so use 1492.

http://btbusiness.custhelp.com/app/...ng-web-pages-when-using-a-third-party-router?

 

[bmb]

The BT infrastructure supports RFC4638, the router they supply supports it, and the RT-N66U hardware supports it.

It was quite easy to implement with a little research. If anyone in the UK using the BT network wants to use the RT-N66U with MTU 1500 here's how.

Telnet in and then

ifconfig eth0 mtu 1508
ifconfig vlan2 mtu 1508
ifconfig ppp0 mtu 1500

The WAN config still shows 1492, but before running the above commands we see

C:\Windows\system32>ping google.com -f -l 1472

Pinging google.com [173.194.34.131] with 1472 bytes of data:
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.
Packet needs to be fragmented but DF set.

Ping statistics for 173.194.34.131:
Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),

MTU is 1492 and fragmenting packets, then after,

C:\Windows\system32>ping google.com -f -l 1472

Pinging google.com [173.194.34.136] with 1472 bytes of data:
Reply from 173.194.34.136: bytes=1472 time=28ms TTL=52
Reply from 173.194.34.136: bytes=1472 time=28ms TTL=52
Reply from 173.194.34.136: bytes=1472 time=31ms TTL=52
Reply from 173.194.34.136: bytes=1472 time=28ms TTL=52

Ping statistics for 173.194.34.136:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 28ms, Maximum = 31ms, Average = 28ms

The PPPoE WAN 1500 MTU now matches the local network and packets are not fragmented..

 

[shadow4dog]

I've tried to increase the mtu to 1500 on my connection on the new beta 1, but on my router I don't have vlan2 only vlan1!

I can change the mtu for eth0 and ppp0 to 1500 but it causes some websites not to load.

Does anyone know how I find the missing vlan?

Thanks
Tim

 

[KAD]

there's not a lot of vlan control in the gui

log in to telnet/ssh

robocfg show

will show you the status of all ports
in vlan1 and vlan2

 

[shadow4dog]

Yes. I found out about the missing VLAN using robocfg and there's my vlan2 but I can't access it through ifconfig and so can't set that interfaces MTU.

The change works kind of but I'm assuming the missing VLAN MTU setting means that it doesn't work on some websites. I had this working on this router with Tomato 1.28 with no issues so I know it should be possible.

I'm stuck!

 

[KAD]

what's the output of ifconfig


assuming your vlans are working correctly mtu should be applied to an interface, which would display when running ifconfig

 

[shadow4dog]

Output from robocfg

admin@RT-N66U:/tmp/home/root# robocfg show
Switch: enabled gigabit
Port 0: 100FD enabled stp: none vlan: 2 jumbo: on mac: xx:xx:xx:xx:xx:xx
Port 1: 100FD enabled stp: none vlan: 1 jumbo: on mac: xx:xx:xx:xx:xx:xx
Port 2: DOWN enabled stp: none vlan: 1 jumbo: on mac: xx:xx:xx:xx:xx:xx
Port 3: 100FD enabled stp: none vlan: 1 jumbo: on mac: xx:xx:xx:xx:xx:xx
Port 4: 100FD enabled stp: none vlan: 1 jumbo: on mac: xx:xx:xx:xx:xx:xx
Port 8: 1000FD enabled stp: none vlan: 1 jumbo: off mac: xx:xx:xx:xx:xx:xx
VLANs: BCM53115 enabled mac_check mac_hash
1: vlan1: 1 2 3 4 8t
2: vlan2: 0 8u

Output from ifconfig

br0 Link encap:Ethernet HWaddr xx:xx:xx:xx:xx:xx
eth0 Link encap:Ethernet HWaddr xx:xx:xx:xx:xx:xx
eth1 Link encap:Ethernet HWaddr xx:xx:xx:xx:xx:xx
eth2 Link encap:Ethernet HWaddr xx:xx:xx:xx:xx:xx
lo Link encap:Local Loopback
ppp0 Link encapoint-to-Point Protocol
ppp10 Link encapoint-to-Point Protocol
v6in4 Link encap:IPv6-in-IPv4
vlan1 Link encap:Ethernet HWaddr xx:xx:xx:xx:xx:xx
wl1.1 Link encap:Ethernet HWaddr xx:xx:xx:xx:xx:xx

 

[KAD]

ifconfig should have given you quite a bit more info including the mtu for each interface

my though was which interface is not at 1500 ??
it's probably the bridge br0 that you need to set to 1500

I'm also not sure what will happen if you call up vlan2

but that can be done with

ifconfig vlan2 up
ifconfig vlan2 mtu 1500

but again, with your current set up, not sure what bringing vlan2 up will do

 

[shadow4dog]

Yeah sorry there was loads of output from the ifconfig command including all my IP addresses etc... didn't understand how much you needed. It's the same config I had working with Tomato.

When setting MTU for eth0 1508 and ppp0 at 1500. The WAN connection kind of works. I can load almost all websites but some have weird behaviour... twitch.tv will let me load the main page, including video, but I can't logon...

Doing an: ifconfig vlan2 up - gives me the following:

admin@RT-N66U:/tmp/home/root# ifconfig vlan2 up
ifconfig: SIOCGIFFLAGS: No such device

It doesn't really matter I guess.