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

  • From: Leo Bicknell
  • Date: Wed Aug 22 20:39:00 2001

On Wed, Aug 22, 2001 at 05:24:12PM -0700, Marc Slemko wrote:
> > 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.

I was assuming the TCP connection was between two end stations, not
to the device itself.  Consider:

host---GE---exchange router--GE--<fabric>--FE--exchange router---GE---host

The two hosts talk, get MSS of 9k (ish), SYN, SYN+ACK, ACK are all
small and pass, the first data packet tries to be 9k, and gets lost
between the fabric and the exit (FE) router.

Yes, if the host talked to the FE device itself MSS should prevent 
any issues (eg, for BGP sessions across the exchange).

Leo Bicknell - [email protected]
Systems Engineer - Internetworking Engineer - CCIE 3440
Read TMBG List - [email protected],