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Hey Jamison,
The consensus view is this: 1500 Bytes is the MTU of the internet, and MTU1500 has so much inertia that you or me or all this mailing list together could not change it.
The sages also tell us that PMTUd--path MTU discovery--does not work with any reliability and thus does not work in reality. Things like NAT, firewalls and deep packet inspectors routinely break ICMP of all kinds.
Most crushingly for an IX: there's no such thing as layer-2 MTU
discovery. In the same L2 domain, all routers need to use the
same MTU, or stuff will break (Jeremy pointed to one example of
failing BGP message exchange). If I set my router to MTU1500, and
you set yours to MTU9000, how would we ever know? This is a
misconfiguration, so what could coordinate those values on an IX?
Thus, we should all use MTU 1500.
We could have a separate VLAN that's MTU 9000 where all the routers connected would set their interface to MTU 9000. The routers wouldn't have any MTU breakage over that L2 domain. SIX does this, and it seems the best and most rational way to allow MTU 9000 for those who want it.
Finally, views do differ, i'm sure others have opinions that do not match mine.
Cheers,
Jonathan
Jonathan Stewart Network Engineer LES.NET - AS18451 Desk: 1-204-666-6191 Mobile: 1-204-990-2120 130 Portage Avenue E Winnipeg, MB R3C 0A1 CANADA