Snap ADSL Belkin N600 DB and MTU / WMM issues

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muncedog

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#137903 12-Dec-2013 20:48

So I am with snap, on a Belkin N600 DB latest firmware router. When I first got snap (today) only google worked. I then changed the MTU in windows down from 1500 to 1492 and everything worked. This is not ideal as I don't want all guests to the house to have to faff about with MTU settings, not to mention its a right pain on android/iphones etc (if possible). Tried altering them on the Router, no effect.

In the end I disabled the 802.11n and just enabled g on its own, that allows me to disable WMM on the belkin, which in turn appears to fix the issue.

Is this just a bad router, or snap, or my lack of understanding of something here? Any explination of the link between WMM and MTU size would interest me, or was that just coincidence?

Tore up the drive today over 5 hours to fix the cable!

DonGould

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#951476 13-Dec-2013 10:05

Interesting.

I would love to properly understand how MTU discovery is meant to work in both v4 and v6.

MTU has caused problems for networks for decades and still seems to catch people all the time (myself included).

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muncedog

35 posts

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#951480 13-Dec-2013 10:17

Indeed, it should be handled automatically and if windows setting is to large, then it's the routers job to split right?
Also, running tests using ping with a set sized packet, I note that when approaching the MTU or slightly larger, it gets timeout... Increase it futher and then it splits properly. I think that is the mismatch between windows and the router. I.e command asks packet to not be split and its under windows MTU , butrouter mMTU is smaller so doesn't know what to do? Hence timeout ?

Problem seems to be actual MTU is less than 1500 and windows default it 1500...

What a headache !

sbiddle

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#951501 13-Dec-2013 10:45

You should only ever need to change the MTU on a Windows PC in a few unique circumstances. This is not one of them.

You've probably got the MTU configured wrong in your router. What is your connection type and how is it connected?

sbiddle

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#951503 13-Dec-2013 10:49

muncedog:
Problem seems to be actual MTU is less than 1500 and windows default it 1500...

What a headache !

Not a headache - it's how things work.

Without knowing more about your setup it's impossible to offer any advice but what I can tell you is that 1492 is pretty much the standard for PPPoE or PPPoA connections.

muncedog

35 posts

Geek

#951552 13-Dec-2013 12:28

I had tried 1492 its snap broadband pppoa.

By lowering it in windows seemed to be the only way to make it function.that effect was repeatable and fully reproducible. However, wondering how reliable the router is as a conduit works then reboot it and it doesn't!

I don't understand enough how it and the relation between PC and router MTU works. Why would packets just over the MTU become lost or at least ping timeout?

I have disabled all additional features firewalls etc.

Wireshark confirmed losses with streams missing packets...

Noise levels and margins. Look acceptable....

Anyway currently working, will see how stable it is..

DonGould

3892 posts

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#951560 13-Dec-2013 12:40

You set the MTU, did you set the MTR?

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Ragnor

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#951654 13-Dec-2013 14:30

You should not need to change windows imo, this sounds like a router firmware (not lowering MTU when traffic passes through the router) bug to me.

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sbiddle

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#951713 13-Dec-2013 15:28

Ragnor: You should not need to change windows imo, this sounds like a router firmware (not lowering MTU when traffic passes through the router) bug to me.

This device is only a router, not a modem/router combo. As the OP hasn't given full details of the setup we can only speculate but clearly there is a modem in place and possibly double NAT. If this is the setup there are lots of places such a problem could occur.

muncedog

35 posts

Geek

#951963 13-Dec-2013 23:30

Hi,

So the device is a router/modem (Belkin N600 DB) with adsl modem built in. I do also wonder if it is a bug in the firmware (using latest) or some 'feature' breaking things. A while back I swear I remember someone saying snap used a NAT at some point which would effect me...

Anyway, router seems to have settled down now, I tried the auto-setup feature again after restoring defaults. Seems to be running smooth now but it has set the MTU to 1400. I wonder if this is from some sort of negotiation or the default (no idea about MTU negotiation?)

Anyway, all seems well now, fingers crossed. Its syncing at 10M, so not too shabby!

Thanks for the help everyone!

Ragnor

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#951996 14-Dec-2013 02:20

sbiddle:

This device is only a router, not a modem/router combo.

There is a modem/router and router version... here's the modem/router N600

http://www.belkin.com/au/F9J1102/p/P-F9J1102/