North American Network Operators Group Date Prev | Date Next | Date Index | Thread Index | Author Index | Historical Re: Ethernet NAPs (was Re: Miami ...)
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. 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
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