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With the advent of the generalized multiprotocol label switching (GMPLS) architecture for the unified control of IP/MPLS over WDM networks, the task to simultaneously optimize both the IP and the optical layer has gained much interest. In this paper we review the most important challenges raised by the integrated routing and wavelength assignment problem in GMPLS-based IP over WDM networks. More closely, we try to answer the question whether to try to set up a new lightpath for each and every connection request, or instead, groom into some already existing lightpaths is more beneficial. We identify the fundamental trade-offs of both approaches and point out their intrinsic weaknesses. Our findings will be supported by illustrative simulation results.