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Re: Ethernet NAPs (was Re: Miami ...)
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. Now, sure, you certainly have to have agreements between devices in various contexts, but what is and isn't a "working" configuration and why is a bit more complex. A can't-go-wrong simplification, of course, is "always make sure all devices on the same L2 have the same MTU"...