Tom's Hardware > Forum > General Networking > General Gateways, Routers and Firewalls > What's with my MTU??

What's with my MTU??

Forum General Networking : General Gateways, Routers and Firewalls - What's with my MTU??

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mrOne   02-21-2007 at 11:31:59 AM

Hi. Suppose to have a 24mbit dsl line but this only reports up to eg. 6-8mbit average.(yes, it s**)Have a Thompson Speedtouch 585 v6 wireless modem (with latest FW) which also acts as a router. Now, my ISP. use a ppoe protocol so i had to configure my NIC to MTU1492, this by using Both DrTcp And manually correctly adding a string to the WinXP registry. The modem is suppose to be set at 1492 but the technic staff can't verify that it really is(though they claim it's set to 1492 def.) since there's no documentation inside the modem, or at Thompson's website. The most strange thing is when i'm doing a ping with the -f -l parameters, the conn reports an MTU at 1452(??!) The same thing at the tweaktest here and at www.dslreports.com. Here's the string from the tweaktest at this site:
----------------------------------------------
MTU = 1452
"MTU is not fully optimized for broadband. Consider increasing your MTU to 1500(1492 in my case) for better throughput. If you are using a router, it could be limiting your MTU regardless of Registry settings."
-------------------------------------------------------
What's Going ON??

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blue68f100   02-23-2007 at 08:45:43 PM
- 0 +

If you use VPN it will drop the MTU down to 1452 range.

Try using DrTCP to check/change you settings.

Reply to blue68f100
mrOne   02-23-2007 at 09:02:42 PM
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LOL. Thx for your reply but haven't you read my post? Where i mention i already tested drTCP. It doesn't work out, at least not visible And VPN? The only VPN is in that case, PPoE..the protocol my ISP is using. Or please define VPN?

Thx

Reply to mrOne
Zakkas   02-24-2007 at 04:20:14 PM
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An MTU of 1452 shouldn't affect your connection. Did you verify with your DSL provider that they have you set for the speeds your paying for? Or they can send a tech out to verify this?

Also here is a link to Thompson's website and if you scroll towards the bottom and find your model gateway (585) there is a user guide you can download.

Thompsons support page

Also that gateway might not support that high of throughput on its WAN port.

Reply to Zakkas
mrOne   02-24-2007 at 04:38:11 PM
- 0 +

The most weird thing is that the distortion that causes this problem is not to be found. I've already consulted a tech and he tested my line from the station to my house -no problem, cables inside my house -no problem, cable of the modem plugged into his own measure device -he measured 17/10 mbits(!!)
NO other sources for distortion either, eg. telephones, microwave ovens, etc.
I observated this though: at this command: (to my modem) "ping 192.xxx.xxx.xxx -f -l 1464"(+28=1492) had no packetfragmentation, eg. worked out as it should. However, when running the same command but to a WAN-address it defaults to MTU 1452(??!) Conclusion: when running behind Internet -the proper default MTU according to my ISP 1452. When surfing, it drops to 1452. This is weird...or?

Reply to mrOne
Zakkas   02-24-2007 at 06:26:56 PM
- 0 +

That might be where your problem is. Internally your PC/LAN side of the gateway is trying to push data out the ADSL port at 1492...but the WAN is set to 1452 by default and therefore might affect your throughput outside. Try using dr.tcp again and setting your PC to transmit data with mtu of say 1400.


The only other thing I can think of is the throughput from the LAN to WAN of your Speedtouch 585 gateway...it might not support speeds over 10mbps.

Reply to Zakkas
mrOne   02-24-2007 at 07:02:49 PM
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Ok, great. But how can the Wan be set to 1452 and why??

Reply to mrOne
Zakkas   02-25-2007 at 08:57:17 PM
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Try this, open your web browser and go to http://192.168.1.254.

From there click the Broadband Connections link.

Then click the DSL Connection link. View the information provided. See what it says for your Bandwidth.

Reply to Zakkas
mrOne   02-25-2007 at 10:36:36 PM
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Right now it says 961/13.320

Reply to mrOne
Zakkas   02-27-2007 at 08:59:53 PM
- 0 +

Did you try using Dr. TCP to set your PC to mtu of 1400 and then run a speed test?

Reply to Zakkas
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