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Date:      Thu, 19 Oct 2000 15:14:39 -0500
From:      "C. Stephen Gunn" <csg@waterspout.com>
To:        MIHIRA Sanpei Yoshiro <sanpei@sanpei.org>
Cc:        freebsd-net@FreeBSD.ORG
Subject:   Re: [Patch] VLAN MTU1500 patch for FreeBSD 4.1-RELEASE and later
Message-ID:  <20001019151439.A17464@waterspout.com>
In-Reply-To: <200010191411.e9JEBZ817011@lavender.sanpei.org>; from sanpei@sanpei.org on Thu, Oct 19, 2000 at 11:11:35PM %2B0900
References:  <200010191411.e9JEBZ817011@lavender.sanpei.org>

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On Thu, Oct 19, 2000 at 11:11:35PM +0900, MIHIRA Sanpei Yoshiro wrote:

>   This is VLAN MTU1500 patch for FreeBSD 4.1-RELEASE and
> later(sorry, I did not test under 5-current). This patch was created
> by HOSOKAWA Tatsumi-san(hosokawa@FreeBSD.org), but he was too busy
> to announce and commit.  So I post this patch by proxy.
> 
>   This patch was tested:
> 	- with Intel PRO/100+ Management Adapter(fxp)
> 	- with Netgear Gigabit Ethernet (MM SC) GA-620

In my experience with VLAN interfaces, most of this code should
not be necessary.  The problem lies in the difference in the MTU
used by IP, and the maximum frame size a card/driver will receive.

There isn't currently a mechanism in FreeBSD to either allow the
physical device to report what its maximum receive framesize is, or
to allow the vlan driver to request a larger one.

While inelegant, I've simply configured my ethernet cards (ti/xl) to
allow larger frames to be passed to ether input.  With this patch
installed, you can simply:

  ifconfig xl0 up
  ifconfig vlan0 vlan 123 vlandev xl0 inet 1.2.3.4 netmask 0xffffff00 mtu 1500

You have to set the MTU on the vlan device because the the default
is ETHERMTU - EVL_ENCAPLEN (1496).  If your hardware _actually_
delivers the oversize frames you're fine.

My patch for the xl driver (works on 3c905B-TX cards) is available 
online.  It simply allows for larger frames.

   http://www.physics.purdue.edu/~csg/FreeBSD/xl_vlan.patch

 - Steve


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