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[Weaver]

Has anyone who uses MTU 1492 (or even smaller MTUs such as in VPN tunnels) had any problem experiences in practice? Particular websites, banks and so on?

[burakkucat]

It has been a very long while since I last bothered about the MTU, let alone modified to what it was set, in my modem/router . . .

Thinking about it, neither the current device (TP-Link Archer VR900) nor the previous device (Huawei HG622) had any means of inspecting and changing the value.

[roseway]

Ditto here. Linux systems always seems to set the PC's MTU/MRU to 1500, and although I've tried playing with this setting, and the setting on routers, I've never noticed any significant difference, or any websites which require particular values.

[Weaver]

RevK has given out reports of problem websites that don't like MTU 1492, because they don't pick up ICMP packets because of bad firewalling, or because they simply insist on MTU 1500. However he hasn't named and shamed anyone in particular. So I wondered if there are known examples of breakage or if time has made administrators do the right thing and accept 1492 byte packets from all the many PPPoE users who are stuck with MTU 1492. (I'm not. Even though I have PPPoE modems, they speak RFC4638 and so I can handle MTU of 1500+8=1508 over the Ethernet link to the Firebrick, and MRU/MTU 1500 over the Internet, as can BTW FTTC users.)

[Weaver]

See http://aa.net.uk/kb-broadband-mtu.html

[aesmith]

We have lots of customers using VPN.  In some cases we've had to tweak mss (rather than MTU) to get certain things to work.  Normally Microsoft things, for example on one installation everything worked except RDP until we added the mss over-ride.

[Weaver]

Andrews and Arnold and the Firebrick routers can tweak MSS in TCP packets ('MSS-clamping') which, I suspect, avoids a host of problems. I had it MSS-clamping set to on both at Andrews and Arnold's end and in my own Firebrick when I had to use MTU 1492 because I was using PPPoE and a separate modem that was not a Draytek Vigor 120/130, and I was on 20CN.

The way out is to use either
* an integrated modem-router, or
* a Draytek Vigor 120 or 130 modem, or
* a modem and router that speaks RFC4638 and also to be on BTW 21CN - not 20CN - or
FTTC.

[Weaver]

Perhaps lots of SOHO routers do MSS-clamping automatically without the user noticing, so could be this avoids a lot of problem cases.

[kitz]

I did a serious bout of MTU tweaking a couple of weeks ago.    I couldnt access the Vodafone website to access my account.   Although I could view their subdomains such as .form & .blog    With it being https and requiring a log in, my obvious first thought was MTU.     After weeks of messing around and vodafone resetting and refreshing my account I finally discovered that at the end of Nov, vodafone made some changes to their servers and routing..  and it was my IP address which was blocked

Anyhow your post reminded me that my MTU on the PC is still 1400 and I forgot to put it back, because I dont have any issues with that either.. although it may affect my speeds.  I can still access my online banking account and credit card co etc fine using 1400.

The Zyxel VMG8324 automatically sets your MTU to 1492 if you set up a PPPoE 

[burakkucat]

[slightly off topic]
Can anybody recall the command format for discovering the optimal MTU setting, for a Linux kernel based system, please?
[/slightly off topic]

[Chrysalis]

I cannot remember specific issues but I recall from time to time I would come across problematic sites.
Also a mtu below 1500 can lead to laggyness in establishing connections whilst mtu negotiates.

Very glad I dont have to deal with that crap now on sky because they dont use PPPoE.

[NewtronStar]

[slightly off topic]
Can anybody recall the command format for discovering the optimal MTU setting, for a Linux kernel based system, please?
[/slightly off topic]

Will this link help BC http://www.cyberciti.biz/faq/how-can-i-setup-the-mtu-for-my-network-interface/

Chysalis gave me a good tip moons ago and it was to have the MTU on the PC side set the same as the router so for me it is set at 1492 & 1492

[burakkucat]

Will this link help BC http://www.cyberciti.biz/faq/how-can-i-setup-the-mtu-for-my-network-interface/

Thank you for that link. Unfortunately it does not tell me anything relevant that I do not already know.

Chysalis gave me a good tip moons ago and it was to have the MTU on the PC side set the same as the router so for me it is set at 1492 & 1492

In my particular case, I do not see reported as to what the value is set in my modem/router.

[NewtronStar]

This router the MTU settings are under the Broadband settings so have look around here

[burakkucat]

Thank you for the suggestion. So I went and looked . . . and there is was!