Question on the topology of Internet Exchange Points

["Kai Chen" <kch670 () eecs northwestern edu>]

A typical Internet Exchange Point (IXP) consists of one or more network
switches <http://en.wikipedia.org/wiki/Network_switch>, to which each of the
participating ISPs connect. We call it the exchange-based topology. My
question is if some current IXPs use directly-connected topology, in
which ISPs just connect to each other by direct link, not through a network
switch?? If so, what's the percentage of this directly-connected case?

Kai