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Load adaptive energy saving schemes for backbone IP networks use dynamic transport circuit services to adapt the active network resources to the current traffic demand in order to reduce the network's energy consumption. Recently, several approaches, categorized as Switch-off schemes, have been proposed which attempt to reduce the energy consumption of already existing networks by switching off IP ports and links during periods of low traffic. Although it has been shown that these schemes can notably decrease the network's energy consumption, they are prone to instabilities in the IP routing service and decreased resilience due to reduced connectivity, and they may induce monitoring reconfigurations. To address these challenges, we propose the Switch-On scheme in an IP-over-WDM network, where the network is designed so that the essential IP connectivity is maintained during low traffic periods while dynamic circuits are switched on in the optical layer to boost network capacity during periods of high traffic demand. Switching on the optical links during peak network loads can address some of the challenges associated with switching off IP ports and links during the low traffic periods. In this paper, we provide a comparative analysis of load adaptive energy saving schemes and present a discussion of the trade-off between energy efficiency and routing stability. The performance results and analytical study show that the multilayer approaches in IP-over-WDM networks carry significant potential for improvement in energy efficiency.