Grant <> wrote:

> >> A while back I was having networking issues.  I eventually tried
> >> drastically lowering the MTU of all the systems onsite and the
> >> issues disappeared.  I always thought the issue was due to the MTU
> >> on our modem/router.  Today I read that AT&T DSL requires a 1492
> >> MTU so I increased the MTU of our systems up to 1492 and haven't
> >> had any issues.  Do certain ISPs require you to change the MTU of
> >> your entire network, or is this likely due to our AT&T
> >> modem/router itself?  
> >
> > AFAIK the MTU is defined for every network interface separately.
> > For an ADSL connection it is common that a lower MTU is needed
> > because of the PPPoE header information that is encapsulated in the
> > ethernet frames. But in that case it is sufficient to lower the MTU
> > just for the WAN interface that is connected to the DSL modem.
> > If you don't use protocol encapsulation in your LAN then there
> > should be IMHO no reason for lowering the MTU of your internal
> > interfaces.  
> So I should be OK with 1492 MTU on the modem/router and 1500 inside
> that LAN?    That hasn't been my experience but I haven't tried in a
> while.  Wouldn't that lead to fragmentation issues?  Admittedly, my
> understanding of this is weak.

FWIR it is sufficient when all interfaces that are connected to a 
layer 2 network are using the same MTU for the respective layer 3 
protocols. So it should be ok when the MTU of the (logical) ppp 
interface is set to 1492 even when the MTU of the (physical) Ethernet 
interface is set to 1500. This is the case for my router that is 
connected to my DSL modem. I don't have any network problems and 
always maximum internet speed.

I'm not a network expert and don't understand all the details. Also 
my English is not good enough to explain it in a better way. 
But to be honest, I'm not sure that I could explain it better in my 
native language. ;-)

Probably there are other members on this ML that can give your more
useful information about this topic.


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