Subject: Re: CVS commit: syssrc
To: None <>
From: Jonathan Stone <jonathan@DSG.Stanford.EDU>
List: current-users
Date: 10/25/2000 20:41:56
In message <>Jason R Thorpe writes
>On Wed, Oct 25, 2000 at 03:30:34PM -0700, Jonathan Stone wrote:
> > >No, that's the MTU of the *VLAN* interface, not the underlying Ethernet
> > >interface.
> > 
> > Then it doesn't acutally do what was asked for.
>What on earth are you talking about? 

What was orginally asked for was, "how can I tell if this interface
supports real VLANs?", without having to set up a VLAN and see
whether it worked.

If you think a response which says, in essence, "turn on a VLAN on
that NIC, producing a possibly-broken extended Ethernet, and then
check the MTU of the NetBSD VLAN i/f to see if you shot yourself
in the foot" answers that, then you're ... very confused.

>	If the Ethernet interface supports ETHERCAP_VLAN_MTU, the
>	VLAN interface connected to that Ethernet interface will
>	have an MTU of 1500.

Yes, but that requires you to set up the VLAN, in order to know
whether VLANs are properly supported. And what if you innocently set
up the VLAN inteface, forget to check, and it silently fails, by
creating a 1496-byte VLAN interface?  or is that not what you're
saying happens?  I thought it was :).

>	Otherwise, the VLAN interface connected
>	to the Ethernet interface will have an MTU of 1496.  

Which is broken. Its not a working VLAN. Anyone who tries it is goping
to run into all sorts of problems. the result is an Ethernet with an
MTU of 1496 bytes, for any frames which have to cross that VLAN link.
1460-byte TCP payloads silenlty fail; pings work.  thats really pernicious.

In much the the same way that FDDI-to-Ethernet "bridges" were broken
devices. FOr much the same set of reasons--if anything even more
pernicious, since the network admin may not be aware that NEtBSD has
the misfeature of allowing "Vlans" with 1496-byte MTUs. I think that
allowing such misconfigurations is a really, really *dumb* thing for
us to do.

>	MTU of the Ethernet interface always is 1500.

Thats just dandy for the local host, but anyone trying to send
1497-byte frames that have to go through the 1496-MTU VLAN hop still lose.
"EtherNOT", to quote a private email.

If there's really a need to support "EtherNOT"s, well, fine, add a knob
to allow it. But the knob shouldn't be ON by default.