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"...