Ethernet frame header size of TAP device and apropriate MTU calculation

[Donald Pearson]

My understanding is that PMTUDiscovery determines what mtu to use for
tunneled frames in switch mode so that they will fit without fragmentation.


On Sat, Jan 19, 2013 at 5:30 AM, SVM <svm7 at mail15.com> wrote:

> Hello, all! I have many questions about tap device architecture.
>
> What is a right way to calc mtu on TAP device to avoid fragmentation on
> real eth device?
>
> I suppose TAP MTU = 1500-8(UDP)-20(IP)-18(**Ethernet) = 1454.
> So I'd set 1454 for tap device: "ip link set mtu 1454 dev eth0"
>
> I'm not shure about what is the exact size of ethernet frame header, which
> tap device use in switch mode?
> Is there FCS field? - Sure it should be.
> But is there InterFrame Space and Preamble in the "switch environment"?
>
> And how this correlate with PMTU and PMTUDiscovery options?
> Should I really manually change MTU for tap device?
>
> I very appreciate your help.
> Thanks!
>
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