Maximum MTU for PPPOE

[#1]

I'm getting 1472 to the router, that makes it 1472+28=1500 ?
ping -D -s 1472 192.168.0.254
PING 192.168.0.254 (192.168.0.254): 1472 data bytes
1480 bytes from 192.168.0.254: icmp_seq=0 ttl=64 time=6.031 ms
^C
--- 192.168.0.254 ping statistics ---
1 packets transmitted, 1 packets received, 0.0% packet loss
round-trip min/avg/max/stddev = 6.031/6.031/6.031/0.000 ms

The RPi gives me the same
ping -c 3  -M do -s 1472 192.168.0.254
PING 192.168.0.254 (192.168.0.254) 1472(1500) bytes of data.
1480 bytes from 192.168.0.254: icmp_req=1 ttl=64 time=1.78 ms
1480 bytes from 192.168.0.254: icmp_req=2 ttl=64 time=1.68 ms
1480 bytes from 192.168.0.254: icmp_req=3 ttl=64 time=1.71 ms
--- 192.168.0.254 ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2003ms
rtt min/avg/max/mdev = 1.688/1.730/1.783/0.039 ms

~ $ ping -c 3  -M do -s 1474 192.168.0.254
PING 192.168.0.254 (192.168.0.254) 1474(1502) bytes of data.
ping: local error: Message too long, mtu=1500
ping: local error: Message too long, mtu=1500
ping: local error: Message too long, mtu=1500
--- 192.168.0.254 ping statistics ---
3 packets transmitted, 0 received, +3 errors, 100% packet loss, time 2005ms

[#2]

For those techies that may want to have a play with the 2704n, see http://community.plus.net/forum/index.php/topic,137629.0.html
Also, in ADSL mode it definitely defaults to 1500 MTU (when supplied in ADSL configuration). Other variations yet to be tested.

[#3]

I found a website http://www.letmecheck.it/mtu-test.php that runs a test to determine MTU.
I get 1492. Using a TG582n and HG612 here.

[#4]

For wan ping try this....
——=====WARNING.. BACKUP YOUR SETTINGS CONFIG BEFORE YOU START=====—-
By going to 192.168.1.254/expert_user.html   Advanced Setup/Management/Settings/Backup then click the Backup Settings button and save to your computer. Before you start on your quest for external pings, be aware that every upload of your config will restart the router automatically, very frustrating.
This tutorial works for me on a stock 2704n router with an FTTC connection, information was collated in this thread from Darsh,MATTY123123 and peteB

I take no responsibility what so ever if anything should go wrong.

Copy the text from here  http://192.168.1.254/dumpcfgdynamic.cmd (keep the original text safe somewhere first) then save a working copy as wanping.conf
Now open wanping.conf back up and check this thread from here and adjust the config with the info from reply 99 taking note of comparisons between posts 100/101 and 105, this may take a few goes.
Before you save and close wanping.conf remove any space before the first line in the file, i.e. before here >>> <?xml version="1.0"?>
Then go to the end of the file and remove the spurious few lines at the end of the file back to…
</DslCpeConfig>  <<<<<here, then save changes.
You should now be able to upload wanping.conf back to your router using the following url http://192.168.1.254/updatesettings.html
If the config doesn’t upload, you have done something wrong, if it uploads… happy days.

[#5]

Where was I, oh yes.. I'm getting 1478 from here
My ping tests on the other side of the Wan are giving me 1450 which is a little confusing as the test above adds the +28 for overheads into the test results by default.

[#6]

npr said the site was not to be trusted Jafreer, so digging a little deeper brings me to the conclusion that the test results from here are correct.

Quote from: jafreer

I found a website http://www.letmecheck.it/mtu-test.php that runs a test to determine MTU. I get 1492. Using a TG582n and HG612 here.

[#7]

speedguide is better as its way faster.
but the letmecheck site did get my correct mtu, just way slower than speedguide does, plus speedguide gives extra useful info.

[#8]

with Speedguide, I get 1300, which seems a big difference from the 1492 on the other site.

[#9]

Quote from: 11110_110

I'm getting 1472 to the router, that makes it 1472+28=1500 ?

Don't know about the 2704n but the Technicolor routers can have a different MTU for the LAN interface and for the WAN interface. IIRC the LAN MTU is 1500.
Just tested my ZyXEL and that has a LAN MTU of 1500
C:\Users\npr>ping -f -l 1472 192.168.1.1
Pinging 192.168.1.1 with 1472 bytes of data:
Reply from 192.168.1.1: bytes=1472 time=3ms TTL=64
Reply from 192.168.1.1: bytes=1472 time=3ms TTL=64
Reply from 192.168.1.1: bytes=1472 time=3ms TTL=64
Reply from 192.168.1.1: bytes=1472 time=2ms TTL=64
And a WAN MTU of 1492
>ping -f -l 1464 bbc.co.uk
Pinging bbc.co.uk [212.58.244.18] with 1464 bytes of data:
Reply from 212.58.244.18: bytes=1464 time=20ms TTL=56
Reply from 212.58.244.18: bytes=1464 time=21ms TTL=56
Reply from 212.58.244.18: bytes=1464 time=20ms TTL=56
Reply from 212.58.244.18: bytes=1464 time=21ms TTL=56
Ping statistics for 212.58.244.18:
    Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
    Minimum = 20ms, Maximum = 21ms, Average = 20ms
>ping -f -l 1465 bbc.co.uk
Pinging bbc.co.uk [212.58.244.18] with 1465 bytes of data:
Reply from 192.168.1.1: 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.

[#10]

It gets even more fun with IPv6 as some routers can have different MTUs for IPv4 and IPv6.

[#11]

Quote from: 11110_110

npr said the site was not to be trusted Jafreer, so digging a little deeper brings me to the conclusion that the test results from here are correct.

I think the results may all be accurate, it's just that they are testing different things. Speedguide.net uses the TCP MSS announcement it receives which comes from your computer after a stage of routing LAN to WAN (your result MTU 1478). Letmecheckit.net tests ICMP fragmentation purely on the WAN (your result MTU 1492). Pinging the router's local IP tests ICMP fragmentation purely on the LAN (your result MTU 1500). Pinging bbc.co.uk tests ICMP fragmentation after routing between LAN and WAN (your result MTU 1478).
It looks like you have 1500 purely on the LAN, 1492 purely on the WAN, and 1478 when traffic is traversing the two. I can't pretend to understand it all, but it seems to me the 1478 results are down to your router. If I change LocalNetwork MTU on my TG582n I get something similar with TCP traffic.

[#12]

Just to sling this into the melting pot, remember that ICMP packets may be handled differently from tcp syn packets by a particular router anywhere between the user and a given website. I was only reminded of this when reading an old thread and g1000 has effectively said this in the previous post.

[#13]

Quote from: jafreer

with Speedguide, I get 1300, which seems a big difference from the 1492 on the other site.

FWIW, if you've got a Cisco VPN client installed on a Windows machine, it'll set the MTU to 1300 on all interfaces even when its not running.

[#14]

You are a star if2k - that is exactly what I have (Cisco VPN for work).
I suppose the next question is whether this will be affecting the performance of the specific machine with the VPN installed?
Thanks again.

[#15]

Good shout lf2k. 
Do you know if the Cisco VPN is running as a service, and if so when you disable the service do you get a sensible MTU?