 ASIC (De Ghein, 2006). It turns out that CEF is also ideally suited to perform the FEC-to-NHLFE mapping, and hence this latter term is replaced by CEF in most Cisco documentation. This discussion SO far mainly concerns LSRs that use Ethernet for the link layer. It is possible to use the native ATM cell format as the link layer but to use MPLS to control the ATM switch table. It should be noted that some features of ATM require some changes to the way MPLS is implemented compared to Ethernet. For example, ATM only supports a native "label stack" of depth 2 through its VCI and VPI fields SO if greater depth is required a shim label has to be carried with the data (the IP datagram). The main MPLS architecture document (RFC 3031) specifies how most of the adaptation to use ATM can be performed; however, RFC 3035 provides more detail. When ATM was introduced it was mainly designed to create circuits between two end- points. However, there soon developed an interest in providing one-to-many and many-to- one circuits that in ATM are termed point-to-multipoint and multipoint-to-point, respectively. MPLS was designed with IP in mind, which is not circuit oriented and effectively allows flows from multiple sources to be treated similarly because they are destined for a common IP prefix. In MPLS this can be achieved through label merging. This is where two incoming labels map to one NHLFE (a single outgoing label). However, this can cause problems with link layers, such as ATM (see RFC 3031 for more detail). The forwarding architecture presented here shows how unlabeled traffic can enter MPLS network and be forwarded through it until it leaves unlabeled. To achieve this, the FEC-to-NHLFE map and the ILM-to-NHLFE map have to be populated with coordinated entries. In MPLS it was decided that the main (standardized) mechanism to carry out this task would be signaling. This is the subject of the next section. Signaling An MPLS implementation needs a signaling protocol to set up the label switching in the LSRs and thus configur