[40803] in North American Network Operators' Group
Re: Ethernet NAPs (was Re: Miami ...)
daemon@ATHENA.MIT.EDU (Marc Slemko)
Wed Aug 22 20:28:34 2001
Date: Wed, 22 Aug 2001 17:24:12 -0700 (PDT)
From: Marc Slemko <marcs@znep.com>
To: Nanog <nanog@merit.edu>
In-Reply-To: <20010822194343.A93930@ussenterprise.ufp.org>
Message-ID: <Pine.BSF.4.20.0108221717430.8992-100000@alive.znep.com>
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Errors-To: owner-nanog-outgoing@merit.edu
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"...
