On 09/20/16 10:35, wabe wrote:
> Grant <emailgr...@gmail.com> 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.
>
> --
> Regards
> wabe
>
Rather than guess and take random values read on the net - measure it.

Google calculate mtu - netgear and others show ways to test upstream to
get the ideal size using ping

You are looking for the largest MTU value before fragmentation starts to
occur.

BillK


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