On 09/20/16 10:35, wabe wrote: > Grant <[email protected]> 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
