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Date:      Thu, 6 Dec 2012 16:12:38 +0700 (ICT)
From:      Olivier Nicole <Olivier.Nicole@cs.ait.ac.th>
To:        matthew@freebsd.org
Cc:        freebsd-questions@freebsd.org
Subject:   Re: netstat -i
Message-ID:  <201212060912.qB69CcTG018111@banyan.cs.ait.ac.th>
In-Reply-To: <50C05FD8.1040609@freebsd.org> (message from Matthew Seaman on Thu, 06 Dec 2012 09:05:28 %2B0000)
References:  <201212060551.qB65phdO016130@banyan.cs.ait.ac.th> <50C05FD8.1040609@freebsd.org>

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Matthew,

> > Name    Mtu Network       Address              Ipkts Opkts
> > em1    9000 <Link#2>      00:0e:0c:5c:32:29      92M  129M
> > em1    9000 10.41.170/24  ufo2000               924K  926K
> > 
> > I understand that the line reporting MAc address means the traffic
> > seen at layer2, while the line reporting IP address means the traffic
> > seen at layer3.
> > 
> > How would that be possible to have suh a difference (on a switched
> > network)?
> 
> It's certainly possible -- arp (and dhcp to some extent) involve sending
> broadcast packets at layer 2.  There can be a lot of arp traffic on a
> well-populated network, or if you're going things like running multiple
> layer 3 networks over the same physical infrastructure.  There can be
> other forms of Ethernet-only (rather than IP traffic) -- switches often
> speak to each other like that.  Generally it is not a problem unless it
> is affecting performance, at which point the answer is to segment the
> network into smaller broadcast domains by sub-netting and/or using VLANs.

There is only one layer 3 network on that physical infrastructure (at
least in that VLAN). And there are only 8 machines in that VLAN, no
routing, as the VLAN is primarily designed for NFS.

I did not sjow the most disturbing figure where at output bytes is 3.7
TB at MAC level but only 156 GB at IP level (2000 times less). The
large amount of output bytes is understandable for the machine is an
NFS server.

3TB is not big, but is at IP level, not at MAC level.

Best regards,

Olivier



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