MTU, ADSL and Routers

Hi all

AIUI BT recommends an MTU of 1492

However, it occurred to me that my ADSL connection is via a 3COM router
where I have set the MTU to 1492.

But I understood that across a LAN an MTU of 1500 could be better.

Sooo, am I better setting my desktop PCs up for max performance across the
LAN and leaving the Router to sort things out to the WAN?

I currently have 3 machines connected via a switch which itself connects to
the ADSL Router.

And what would be the optimum settings, MTU, TTL, etc for a LAN as opposed
to an ADSL modem?

Or what?

--
Regards
Barry

On Thu, 7 Dec 2006 15:39:44 +0000, BJH <(E-Mail Removed)>
wrote:

>Hi all
>AIUI BT recommends an MTU of 1492

bad start :-) They don't.

>However, it occurred to me that my ADSL connection is via a 3COM router
>where I have set the MTU to 1492.
>
>But I understood that across a LAN an MTU of 1500 could be better.
>
>Sooo, am I better setting my desktop PCs up for max performance across the
>LAN and leaving the Router to sort things out to the WAN?

in theory yes, in practice often not. The router should handle 1500 on
its LAN interface and a lower number on its WAN interface without
having to fragmeent packets but in practice they seem to struggle.

If you feel a lower WAN MTU is a benefit I would set the PCs to the
same value.

>I currently have 3 machines connected via a switch which itself connects to
>the ADSL Router.
>
>And what would be the optimum settings, MTU, TTL, etc for a LAN as opposed
>to an ADSL modem?

1500 is the ethernet and Windows default, smaller MTUs means more
packets so a bit more overhead.

Phil

On Thu, 07 Dec 2006 16:02:18 +0000, Phil Thompson wrote:

>>AIUI BT recommends an MTU of 1492

> bad start :-) They don't.

Ooops, sorry, that's the trouble trying to do two jobs at once, I meant
1458, of course!

--
Regards
Barry

"BJH" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> Hi all
>
> AIUI BT recommends an MTU of 1492
>
> However, it occurred to me that my ADSL connection is via a 3COM router
> where I have set the MTU to 1492.

I think 1492 is useful for PPPoE connections, but know of no relevance for
PPPoA (which is typical for ADSL-based Internet access in the UK).

> But I understood that across a LAN an MTU of 1500 could be better.

That's the maximum - and default - for encapsulation within normal Ethernet
frames. In general, the bigger the MTU, the greater efficiency in terms of
bandwidth and processing time. The latter is completely insignificant on
modern hardware until you get into thousands of packets per second.

> Sooo, am I better setting my desktop PCs up for max performance across
> the LAN and leaving the Router to sort things out to the WAN?

I would leave the MTU on the PCs alone. The chances are good that Path MTU
Discovery will be used, so if the router is configured with a lower MTU, you
effectively end up using that lower MTU for Internet data without
sacrificing any LAN performance.

On a PPPoA connection, you can tweak the MTU to improve upload/download
speeds slightly, by making it so that MTU-sized packets completely fill a
number of ATM cells. The greatest number less than or equal to 1500 for
which this applies is 1478. In theory this should be the optimal MTU, and in
practice, I found it was. However, if memory serves, the improvement over
1500 was something like 1.5% (as expected).

I suspect this matters less on typical "up to 8Mbit/s" ADSL services. If you
have such a service, on balance, I think the best thing would be to set the
MTU to 1500 on the router.

Alex

On Thu, 7 Dec 2006 17:46:05 -0000, Alex Fraser wrote:

> I suspect this matters less on typical "up to 8Mbit/s" ADSL services. If
> you have such a service, on balance, I think the best thing would be to
> set the MTU to 1500 on the router.

Hi Alex

Thanks for that, very intersting

--
Regards
Barry

On Thu, 7 Dec 2006 16:40:53 +0000, BJH <(E-Mail Removed)>
wrote:

>Ooops, sorry, that's the trouble trying to do two jobs at once, I meant
>1458, of course!

they don't recommend that, either. The SINs say BT's ADSL is fully
compatible with 1500 standard MTU.

There was a time a couple of years ago when something was broke and a
1458 MTU was a workaround, but that was then. There are arguments for
minor optimisation using 1472 or 1430 to minimise the ATM tax /
overheads but TBH 1500 is the safest bet.

Phil

Phil Thompson wrote in message
(E-Mail Removed):

> On Thu, 7 Dec 2006 16:40:53 +0000, BJH <(E-Mail Removed)>
> wrote:
>
>> Ooops, sorry, that's the trouble trying to do two jobs at once, I
>> meant 1458, of course!
>
> they don't recommend that, either. The SINs say BT's ADSL is fully
> compatible with 1500 standard MTU.
>
> There was a time a couple of years ago when something was broke and a
> 1458 MTU was a workaround, but that was then. There are arguments for
> minor optimisation using 1472 or 1430 to minimise the ATM tax /
> overheads but TBH 1500 is the safest bet.

What about AOL? Is the advice still to reduce a router's MTU to 1400, or is
this not necessary these days?

On Fri, 8 Dec 2006 18:53:20 -0000, "Martin Underwood" <a@b> wrote:

>What about AOL?

frankly, WGAS.

> Is the advice still to reduce a router's MTU to 1400, or is
>this not necessary these days?

I think their latest off-the-wall non-standard idea is to use 1450
MTU.

I can't remember if they're using PPPoE or PPPoA this week, or if it
depends on that device (modem or router - WTF !).

1400 would be safe.

Phil

In message <4579b4a5$0$8756$(E-Mail Removed)>, Martin
Underwood <a@b.?.invalid> writes
>Phil Thompson wrote in message
>(E-Mail Removed):
>
>> On Thu, 7 Dec 2006 16:40:53 +0000, BJH <(E-Mail Removed)>
>> wrote:
>>
>>> Ooops, sorry, that's the trouble trying to do two jobs at once, I
>>> meant 1458, of course!
>>
>> they don't recommend that, either. The SINs say BT's ADSL is fully
>> compatible with 1500 standard MTU.
>>
>> There was a time a couple of years ago when something was broke and a
>> 1458 MTU was a workaround, but that was then. There are arguments for
>> minor optimisation using 1472 or 1430 to minimise the ATM tax /
>> overheads but TBH 1500 is the safest bet.
>
>What about AOL? Is the advice still to reduce a router's MTU to 1400, or is
>this not necessary these days?
>
>
It is still necessary to change the MTU to 1400 for AOL.
--
Devs
"Punchdown Pete the old Kroner"

Devs wrote in message
vfklpPBw$+(E-Mail Removed):

> In message <4579b4a5$0$8756$(E-Mail Removed)>,
> Martin Underwood <a@b.?.invalid> writes
>> Phil Thompson wrote in message
>> (E-Mail Removed):
>>
>>
>> What about AOL? Is the advice still to reduce a router's MTU to
>> 1400, or is this not necessary these days?
>>
>>
> It is still necessary to change the MTU to 1400 for AOL.

Thanks. I always do change the MTU for a router on AOL, but I wanted to
confirm that it was still necessary to do. You've done that.

What are the symptoms of using too large an MTU on AOL? Will the router fail
to authenticate, or will the connection drop and/or run slowly at times of
very heavy usage such as a large download?