Current core networks are typically a mix of several layers of technologies on top of each other, such as IP-over-ATM-over-SDH (Internet Protocol; Asynchronous Transfer Mode; Synchronous Digital Hierarchy), as illustrated in Fig. 2(a). However, there is a trend to move to more homogenous architectures where IP is routed directly over WDM links (Fig.2 (b)). Given this trend, we will focus on the latter architecture only. (Fig. 2(c) will be discussed in section IV)