[40818] in North American Network Operators' Group
Re: Ethernet NAPs (was Re: Miami ...)
daemon@ATHENA.MIT.EDU (Miquel van Smoorenburg)
Thu Aug 23 07:27:39 2001
To: nanog@merit.edu
From: miquels@cistron-office.nl (Miquel van Smoorenburg)
Date: Thu, 23 Aug 2001 11:27:05 +0000 (UTC)
Message-ID: <9m2pa9$jed$1@ncc1701.cistron.net>
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Errors-To: owner-nanog-outgoing@merit.edu
In article <Pine.BSF.4.20.0108221717430.8992-100000@alive.znep.com>,
Marc Slemko <marcs@znep.com> wrote:
>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.
So it's simply waiting for a routing vendor that sets the MTU
per endpoint based on the MSS in the TCP handshake for the
BGP session.
Mike.
--
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also called the Jeopardy style. Usenet is Q & A not A & Q." -- Bob Gootee
