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[risk_reversal]

I was reading this page MTU Problems

http://www.kitz.co.uk/adsl/MTU.htm

At the bottom there is this paragraph

~ Tweak your router?

Many routers have a setting in which you can alter its MTU size. You can change this, but you must make sure that the MTU setting on your router is the same as on your PC. It does not matter too much if the MTU on your router is higher than that on your PC, but the setting in your router must not be less than the MTU on your PC.

What would be the effects of having the MTU set on a pc lower than the MTU set in the router? By that I mean any actual visible effects seem by the user of such a pc eg longer webpage load time, other delays, etc....

Cheers

PS. Hope that I have posted in the correct section....

[roseway]

MTU defines the largest packet of data which can be handled without fragmentation (i.e. breaking the packet into two or more smaller packets). If an incoming packet is (say) 1460 bytes and the router MTU is 1450, then the incoming packet will be fragmented into two packets, the larger one being 1450 bytes. Now, if the PC MTU is 1440 then the already fragmented 1450-byte portion of the incoming packet will be fragmented again. This fragmentation slows down data transmission.

The largest packet which is used is 1500 bytes, so if the router MTU is set to 1500 it will never fragment packets travelling in either direction, and this is the best technical option.

[risk_reversal]

Many thanks for your reply roseway

So basically the only perceivable symptom of having o/s MTU set at a greater value than the router's MTU would only be this fragmentation of packets with consequence of a slow down in data transmission.

Example:

Incoming packet is 1460. Router MTU set at 1460. o/s MTU, XP, left at default (which I understand is 1500).
This scenario would lead to packet fragmentation and slow down and with no other side effects.

Have I understood this correctly?

Cheers

[roseway]

That's just about right, although there are ISPs who insist on certain MTU values and won't transmit the data if you exceed these values.

[risk_reversal]

Most kind.

Let me ask you a final question please.

The ISP in question is Newnet (and whose MTU is 1460).

when you say

although there are ISPs who insist on certain MTU values and won't transmit the data if you exceed these values

Does that mean you get nothing at all or get a service but intermittent.

Cheers

[roseway]

I think that this only applies to one LLU ISP, and I think (again) that you get no service at all if the value is too high. I've no personal knowledge of this, but I've seen it reported by others.

[risk_reversal]

Cheers

[jeffbb]

Hi
The method I used to set MTU is to be found under  Working out your maximum MTU   here 

Regards Jeff

[Azzaka]

The easiest way to overcome 'most' MTU issues is to enable PING on the modem and set it to 1500. This allows ICMP echo to respond to the MTU tag size and adjust accordingly.

~azz