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On 09/20/16 10:35, wabe wrote: |
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> Grant <emailgrant@×××××.com> wrote: |
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> |
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>>>> A while back I was having networking issues. I eventually tried |
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>>>> drastically lowering the MTU of all the systems onsite and the |
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>>>> issues disappeared. I always thought the issue was due to the MTU |
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>>>> on our modem/router. Today I read that AT&T DSL requires a 1492 |
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>>>> MTU so I increased the MTU of our systems up to 1492 and haven't |
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>>>> had any issues. Do certain ISPs require you to change the MTU of |
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>>>> your entire network, or is this likely due to our AT&T |
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>>>> modem/router itself? |
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>>> AFAIK the MTU is defined for every network interface separately. |
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>>> For an ADSL connection it is common that a lower MTU is needed |
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>>> because of the PPPoE header information that is encapsulated in the |
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>>> ethernet frames. But in that case it is sufficient to lower the MTU |
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>>> just for the WAN interface that is connected to the DSL modem. |
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>>> If you don't use protocol encapsulation in your LAN then there |
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>>> should be IMHO no reason for lowering the MTU of your internal |
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>>> interfaces. |
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>> |
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>> So I should be OK with 1492 MTU on the modem/router and 1500 inside |
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>> that LAN? That hasn't been my experience but I haven't tried in a |
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>> while. Wouldn't that lead to fragmentation issues? Admittedly, my |
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>> understanding of this is weak. |
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> FWIR it is sufficient when all interfaces that are connected to a |
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> layer 2 network are using the same MTU for the respective layer 3 |
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> protocols. So it should be ok when the MTU of the (logical) ppp |
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> interface is set to 1492 even when the MTU of the (physical) Ethernet |
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> interface is set to 1500. This is the case for my router that is |
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> connected to my DSL modem. I don't have any network problems and |
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> always maximum internet speed. |
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> |
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> I'm not a network expert and don't understand all the details. Also |
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> my English is not good enough to explain it in a better way. |
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> But to be honest, I'm not sure that I could explain it better in my |
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> native language. ;-) |
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> |
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> Probably there are other members on this ML that can give your more |
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> useful information about this topic. |
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> |
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> -- |
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> Regards |
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> wabe |
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> |
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Rather than guess and take random values read on the net - measure it. |
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|
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Google calculate mtu - netgear and others show ways to test upstream to |
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get the ideal size using ping |
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|
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You are looking for the largest MTU value before fragmentation starts to |
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occur. |
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|
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BillK |
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