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Date: Sat, 2 Aug 2003 11:34:38 -0700 (PDT)
From: Tom Samplonius <tom@sdf.com>
To: Terry Lambert <tlambert2@mindspring.com>
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cc: Boris Kovalenko <boris@tagnet.ru>
Subject: Re: bge & vlan stranges
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On Sat, 2 Aug 2003, Terry Lambert wrote:

...
> I suppose you want to do this because you are trunking a channel
> that goes to a border device, and for some reason you have disabled
> receipt of all ICMP, instead of only abusable ICMP, and thus you
> have broken end-to-end path MTU discovery.
> 
> It would be best if you were to simply fix your ICMP.

  Probably wouldn't be affective anyhow.  L2 switches assume that they can
encapsulate 1500 byte ethernet frames into 802.1q properly.  It is part of
the 802.1q standard.  If the NIC can't understand the frame because it is
now 1504 bytes, it will be a layer 2 discard.  There will be no ICMP
message sent in this case.  You could argue that the switch should also be
configured with a 1456 byte MTU to allow for the addition of the 802.1q
encapsulation.  But a L2 switch is not going to send a L3 message like a
ICMP "unable to fragment" fragment.  So MTU detection buys you nothing.

  The fact of the matter is, if you use 802.1q encapsulation, the total
frame size can be 1504.  That is the standard.

> -- Terry


Tom