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Today I can't change my MTU?


Today I can't change my MTU?

Noticed slightly slower download speeds today so I went through the usual procedure of changing pipes.
For some reason whenever I have been on the Junipers in the past my MTU always defaulted to 1458 but when connected to the redbacks it reverted to my favoured 1478 - this has always given me the best results.
so I would reboot the router until I connected to one of the Redbacks.
Today though, whichever I connect to I'm getting 1458?
I use TCP Optimizer which is still showing 1478 and have rebooted many times, changed PMTU on and off, router is till showing 1478 and I've rebooted and completely turned that off as well.
All registry interfaces are 1478.

I've used the site for the MTU discovery and also the Dslreports site, the latter site reports:
Max packet sent (MTU): 1458
Max packet recd (MTU): 1478

From that it looks like my end?
Called support and they say that nothing has changed overnight.

I'm at a loss, has anyone got any ideas, please?
Community Veteran
Posts: 3,181
Thanks: 19
Fixes: 2
Registered: 31-07-2007

Today I can't change my MTU?

any Rwin/MTU utility only changes the settings for you LAN cards not your router, unless its a PCI ADSL modem, so you have to use the Web admin or Telnet admin for your router to change the MTU.
Unvalued customer since 2001 funding cheap internet for others / DSL/Fibre house move 24 month regrade from 8th May 2017

Today I can't change my MTU?

Now tried 1458 settings and my readings on MTU detection sites show as 1438, so I'm losing 20 somewhere...

Had a good look around the router's config and found MRU.
This was set on 1478, I changed it to 1498 and all tests now show as 1478.
Bit strange as I hadn't changed anything in the router for a couple of months.


Today I can't change my MTU?

Take a look at the MTU Tweak sticky, set your MTU, reboot and test using Plus Net Speed Checks. Remember to adjust your router to match the same MTU.

You can't just lose MTU, it's changed by software that you have run.

Today I can't change my MTU?

Thanks for the reply.

I've been rebooting after every change.

I've changed the MRU on the router to 1498, MTU to 1478 and MSS to 1438.
I'd never touched this MRU setting before so why this problem has just suddenly happened is beyond me.
All seems ok now with all test sites detecting 1478, although a little more packet loss than normal.
Community Veteran
Posts: 14,469
Registered: 30-07-2007

Today I can't change my MTU?

What router are you using as you problem rings a bell, I saw something similar on ADSLguide?

Found it: ADSLguide Thread. Are you using a solwise router?

Today I can't change my MTU?


I'm using a Dynamode R-ADSL-C4.

Since changing the MRU to 1498 all appears ok.
However, I have noticed a little more packet loss and now, for some reason I cannot do a tracert - each reply is timing out?