Re: exchange point media (was: Re: MAE-EAST Moving? ...)

[Martin Cooper <mjc () cooper org uk>]

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.