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Re: Ethernet NAPs (was Re: Miami ...)

  • From: Miquel van Smoorenburg
  • Date: Thu Aug 23 07:29:46 2001
  • Distribution: cistron
  • Newsgroups: lists.nanog

In article <[email protected]>,
Marc Slemko  <[email protected]> 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.

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