e from any kind of Layer 2 link to any other kind of Layer 2 link, without depending on any particular control protocol. Compare this to ATM, for example: ATM can switch only to and from ATM and can use only ATM signaling protocols, such as PNNI or Interim Interface Signaling Protocol. MPLS supports several types of label formats. On ATM hardware, it uses the well- defined virtual channel identifier (VCI) and virtual path identifier (VPI) labels. On Frame Relay hardware, it uses a data link connection identifier (DLCI) label. Else- where, MPLS uses a new generic label, known as a shim, which sits between Layers 2 and 3. Because MPLS allows the creation of new label formats without requiring changes in routing protocols, extending technology to new optical transport and switching could be relatively straightforward. MPLS Stacks Label stacking is a powerful feature of MPLS that enables LSRs to insert an addi- tional label at the front of each labeled packet, creating an encapsulated tunnel that can be shared by multiple LSPs. At the end of the tunnel, another LSR pops the label stack, revealing the inner label. An optimization in which the next-to-last LSR peels off the outer label is known in IETF documents as penultimate hop popping. Whereas ATM has only one level of stacking (virtual channels inside virtual paths), MPLS supports unlimited stacking (see Figure 10.16). An enterprise could The provider edge routers map ports to MPLS labels to create virtual leased lines (VLLs) ATM/Frame ATM/Frame Relay Relay MPLS Backbone ATM/FR VLL Ethernet VLL Tunnel LSP Ethernet IP VLL Ethernet Label Edge Label Edge Router Router IP/PPP IP/PPP Figure 10.16 MPLS stacks The Next-Generation Network Infrastructure use label stacking to aggregate multiple flows of its own traffic before passing the traffic on to the access provider. The access provider could then aggregate traffic from multiple enterprises before handing it off to the backbone provider, and backbone provider could aggregate the traffic yet again before passing it off