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
>> 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.
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