[VLAN] 802.1Q ARP frame size?

[James Harper]

I have a box with a via_rhine adapter connected to a cisco 2950. The
light doesn't flash, but the 'runt frame' count is almost the same as
the good packet count.

With vlan enabled, I'm not sure that the via_rhine adapter does any
padding at all, so I get some pretty tiny frames.

Doesn't seem to affect functionality in any way though.

James

> -----Original Message-----
> From: vlan-bounces at candelatech.com
[mailto:vlan-bounces at candelatech.com]
> On Behalf Of Sascha Pollok
> Sent: Thursday, 31 August 2006 17:30
> To: Linux 802.1Q VLAN
> Subject: Re: [VLAN] 802.1Q ARP frame size?
> 
> > >>> I noticed that the 802.1Q ARP frame size generated by the Linux
> > > driver
> > >>> (V1.8) is only 64 octets (not 68 octets).
> > >>>
> > >>> It seems the minimum frame size for 802.1Q is 68 octets.
> > >> The 64-octets is an ethernet physical level issue, not a VLAN
issue,
> > > so
> > >> there is no need to pad the frame to 68 bytes.
> > >>
> > >
> > > So what happens when a switch untags the vlan tag and sends the
> untagged
> > > packet out a port? Is it the responsibility of the switch to then
pad
> > > the packet? Or does this just happen automatically anyway?
> >
> > It's the switches duty..and most NIC's hardware will do the
padding..I
> > assume switch hardware can easily do the same.
> 
> When I first was sent a Cisco 2970-24TS switch when it was brand new,
> I noticed lots of "too small but good" frames and orange blinking of
the
> port's LED when receiving minimum-sized dot1q frames. It turned out
that
> these switches require 68 byte tagged frames minimum. If they receive
> 64 Byte frames, they still forward it but complain about it. This is
> unfortunately not fixable in the IOS because the bug is in the
burned-in
> software on the ASICs. There is a patch around for the e1000 that
> pads the packets to 68 bytes instead of 64. I guess newer ASICs do not
> have this bug.
> 
> Just my $0.02
> Sascha
> 
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