How does an Exchange Server 5.5 site relate to an Exchange 2000 routing group?
In a mixed Exchange 2000/Exchange 5.5 topology, a site is represented as an administrative group and a routing group. An Exchange 2000 routing group, contained within the administrative group, is more analogous to a sub-site than to a site because it is for purely topological groupings. However, it is advisable that servers belonging to sites in Exchange 5.5 also be applied in this case. Members belonging to the same routing group are expected to have very high bandwidth and availability.