[Soekris] changing mtu size on openbsd problems

[Chris Cappuccio]

peter [peter at deeptrauma.net] wrote:
> 
> 
> Chris Cappuccio wrote:
> 
> >Why would you set the MTU on your physical ethernet interface below 1500?
> >
> >That makes no sense at all.
> >  
> >
> Wrong.
> 

You mis-read me.

I did not say 'changing the MTU on the pppoe interface'

I said 'changing the MTU on the physical ethernet interface'

> Setting the MTU below 1500 is the correct solution for this problem,
> 1436 is a good value which will allow for 1436 of data with a 64 byte
> PPPoE header to equal a total of 1500 bytes of data.
> 

Exactly, you want the PHYSICAL ETHERNET INTERFACE to be able to pass 1500
byte packets, not 1440 or 1436 or some other value.

> Increasing the MTU to be above 1500 would just make this problem much
> worse as you'd end up with a packet with more data and less room on the
> wire for headers. Even if your able to increase the MTU of your ethernet
> chipset are you sure that the switch or router is able to handle packets
> which are of the same size.
> 

If you could increase the MTU past 1500 bytes, and if your DSL modem and
provider supported it, you could pass full 1500 byte packets through the
pppoe interface.

And yes, the maximum frame size is different than the MTU, by a fixed
number of bytes for ethernet headers, etc.  That's irrelevant to the
conversation.

-- 
"Attacks always get better; they never get worse."
  -- "Old NSA saying"