North American Network Operators Group
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Re: Ethernet NAPs (was Re: Miami ...)

  • From: Marc Slemko
  • Date: Wed Aug 22 20:30:05 2001 
On Wed, 22 Aug 2001, Leo Bicknell wrote:

> _ALL_ devices on a layer-2 fabric need to have the same MTU.  That
> means if there are any FastEthernet or Ethernet connected members
> 1500 bytes is it.  It also means if you pick a larger value (4470,
> 9k) _ALL_ members must use the same value.
> 
> If you don't, the behavior is simple.  A 9k MTU GigE arps for a
> 1500 byte FastEthernet host.  Life is good.  The TCP handshake
> completes, life is good.  TCP starts to send a packet, putting a
> 9k frame on the wire.  Depending the switch, the switch either
> drops it as over MTU for the FastEthernet, or the FastEthernet card
> cuts it off at 1500 bytes, and counts it as an errored frame
> (typically with a jabber or two afterwards) and no data flows.

Well, the reasoning "why" is a bit more complex than that...  The
TCP handshake will result in the FE host saying "hey, I can do a
max 1460 byte mss".  The other host with a larger MTU won't send
larger packets than remote MSS + 40 bytes header over that TCP
connection, end of story.

Now, sure, you certainly have to have agreements between devices
in various contexts, but what is and isn't a "working" configuration
and why is a bit more complex.  A can't-go-wrong simplification,
of course, is "always make sure all devices on the same L2 have
the same MTU"...