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

[miquels () cistron-office nl (Miquel van Smoorenburg)]

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.
-- 
"Answering above the the original message is called top posting. Sometimes
 also called the Jeopardy style. Usenet is Q & A not A & Q." -- Bob Gootee