On Thu, 6 Dec 2001, Richard Rauch wrote:

> > mouse-pppoe sets it to 1400???  I don't know why its so low, pppoe adds 8
> > bytes of encapsulation, so thats an mtu of 1492 over ethernet (mtu 1500).
>
> prometheus# ifconfig ppp0
> ppp0: flags=8051<UP,POINTOPOINT,RUNNING,MULTICAST> mtu 1400
>         inet 65.68.73.156 -> 65.68.75.254 netmask 0xff000000
>         inet6 fe80::240:f4ff:fe2f:4de8%ppp0 -> :: prefixlen 64 scopeid 0x4

Strange, I don't know why mouse-ppoe uses such a low mtu - its a waste
really.  I guess the lower the MTU the more bugs you might work around.
I'd suggest trying to set it to 1492.  Other than for working around
broken NetBSD machines, 1492 is ideal.

> > I think you should be able to adjust the MTU all you want (although I've
> > heard some drivers don't support it).  I've done it on my -current gateway
> > and client, but there might be a bug in NetBSD-1.5 or in the specific
> > ethernet driver you use.  Try just lowering the mtu on the route instead
> > of changing the actual interface.
>
> On the route?  How would I go about that?  Or would it be enough to
> lower the MTU on the ``leaf node'' and ignore the gateway?

Use the "-mtu" flag when adding or changing the route (see my last
e-mail).  If the interface mtu for two machines on the same network don't
agree, you can get into trouble (possibly causing the crash you saw).
Specifically, I've seen my machine say something like "received oversize
packet, discarding".  I think changing route mtu's is safe because its
assymetrical.

Rick