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Re: exchange point media (was: Re: MAE-EAST Moving? ...)
From: Martin Cooper <mjc () cooper org uk>
Date: Fri, 23 Jun 2000 17:49:28 +0100


Alex Bligh <amb () gxn net> writes:

LINX also runs mixed media (sadly) - FDDI and more sane things (GigE
and Fast Ethernet). The switches here fragment when switching toward
Fast Ethernet. They would certainly move >1500 byte pings between
FDDI ports, but I have no FDDI ports now to test whether this works
with DF set (i.e. it doesn't fragment FDDI to FDDI), nor whether
ICMP is *ever* generated.

Maybe I'm being ultra-thick here, but what business do layer-2
devices have looking at the contents of layer-3 packet headers
and acting on what they find there? I thought the whole point
of building IXPs at layer-2 was to leave layer-3 policies to
the participants - if not then one might as well start building
IXPs with routers surely?

However, I believe best practice at LINX is, and perhaps should
be elsewhere, to set MTU on your FDDI interfaces to that supported
by ethernet. Given most exchanges started with ethernet and worked
upwards, this should have been best practice everywhere from teh
start.

This seems to me to be exactly the right thing to do
in the context of exchange point fabrics composed of
mixed MTU media, irrespective of the presence or
absence of such schizoid layer-2 devices as those
that have been discussed here.

M.



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