Subject: Re: CVS commit: syssrc
To: Jonathan Stone <jonathan@DSG.Stanford.EDU>
From: Andrew Gillham <>
List: current-users
Date: 10/25/2000 15:05:17
Jonathan Stone writes:
> In message <>Manuel Bouyer writes
> >On Tue, Oct 24, 2000 at 10:09:40AM +0100, David Brownlee wrote:
> >
> >Well, you know it when the vlan interface is attached: if mtu is 1500,
> >the interface is vlan-capable; if it's 1496 it's not.
> 1496? Say what? You mean somebody *broke* Ethernet on non-VLAN-capable
> interfaces?  I really hope not.

No, the MTU in question is on the vlan interface after attaching it
to the parent.  If the parent doesn't support the larger vlan mtu then
the vlan interface ends up 4 bytes less than the parent MTU. (or is
it hard coded to 1496?)

Anyway, I do think it is a bit lame that ifconfig can't tell whether
the parent supports the vlan mtu or not until after it is attached.

Seems sort of like having to do the following:
	# ifconfig stf0 create
	ifconfig: SIOCIFCREATE: Invalid argument
	# ifconfig tun0 create
	ifconfig: SIOCIFCREATE: Invalid argument
	# ifconfig gif0 create

Aha!  There is a "gif" cloner interface available.  Oh, but wait, I can
just do "ifconfig -C" to get a list. :-)  Why not a "ifconfig -V" or
a "ifconfig -c" (show capabilities) option?

I would imagine there are other capabilities that it might be worth
displaying without configuring anything.  Maybe hardware checksum,
jumbo frame support, vlan tagging (hardware), etc.