Ethernet NAPs (was Re: Miami ...)

Wed Aug 22 23:43:43 UTC 2001

I just have to speek up that this is all very well and good, but
it's also a good way to make a NAP that doesn't work.

_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.

A larger MTU is a fine plan, but make sure if you try that anywhere
that the switch is set for the larger sizes and all devices are
capable of that larger frame size, or you're in trouble.

-- 
Leo Bicknell - bicknell at ufp.org
Systems Engineer - Internetworking Engineer - CCIE 3440
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