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With the advent of WDM technology, IP backbone carriers are now connecting core routers directly over point-to-point WDM links (IP over WDM). The advances and standardization in optical control plane technologies like GMPLS have substantially increased the intelligence of the optical layer and shown promise toward making dynamic provisioning and restoration of optical layer circuits a basic capability to be leveraged by upper network layers. In light of this, an architecture where a reconfigurable optical backbone (IP over OTN) consisting of SONET/SDH crossconnects/switches interconnected via DWDM links provides connectivity among IP routers is an emerging alternative. As carriers evolve their networks to meet the continued growth of data traffic in the Internet, they have to make a fundamental choice between the above architectural alternatives. In the current business environment, this decision is likely to be guided by network cost and scalability concerns. A reconfigurable optical backbone provides a flexible transport infrastructure that eases many operational hurdles, such as fast provisioning, robust restoration, and disaster recovery. It can also be shared with other service networks such as ATM, frame relay, and SONET/SDH. From that perspective, an agile transport infrastructure is definitely the architecture of choice. The IP-over-OTN solution is also more scalable since the core of the network in this architecture is based on more scalable optical switches rather than IP routers. But what about cost? Since the IP-over-OTN solution introduces a new network element, the optical switch, is it more expensive? We address that question by comparing IP-over-WDM and IP-over-OTN architectures from an economic standpoint using real-life network data. We show that contrary to common wisdom, IP over OTN can lead to substantial reduction in capital expenditure through reduction of expensive transit IP router ports. The savings increases rapidly with the number of nodes in the network and traffic demand between nodes. The economies of scale for the IP-over-OTN backbone increase substantially when we move traffic restoration from the IP layer to the optical layer. We also compare the two architectures from the perspective of scalability, flexibility, and robustne- ss. Our observations make a strong case in favor of a switched optical backbone for building scalable IP networks.