SRs from the MPLS network. These LSRs perform the same lookup as the intermediate LSRs, but the {outgoing interface, label value} pair marks the packet as exiting the LSP. The egress LSRs strip the labels from the packets and forward them using Layer 3 routing. So, if LSR A identifies all packets for Host Z with the upper LSP and labels them with the value 21, they will be successfully forwarded through the network. Note that the exact format of a label and how it is added to the packet depends on the Layer 2 link technology used in the MPLS network. For example, a label could correspond to an ATM Virtual Path Identifier/Virtual MPLS routing - uses same IP routing mechanisms control protocol to distribute labels between neighboring LSRs labels placed in LIB - contains input I/F, input label and output I/F, Routing output label Routing Protocol Protocol Messages Messages Database Database Database labels MPLS forwarding - user plane labels labels employs label swapping forwarding process at MPLS ingress and egress packet-based LSR may send packet to L3 (FIB) for processing Labeled Labeled Packets Packets 0123456789012345678901234567890 Label Stack Entry Label: Label Value, 20 bits Exp: Experimental Use, 3 bits Bottom of stack, 1 bit Time to Live, 8 bits LSR operations are swap, pop or push a label Header defined in RFC3032 ATM-based MPLS may use VPI/VCI for labels GMPLS defines label values for other media Figure 7-15 MPLS operation. Chapter 7 Channel Identifier (VPI/VCI) or a Frame Relay Data Link Connection Identifier (DLCI). For other Layer 2 types (such as Ethernet and PPP), the label is added to the data packet in an MPLS shim header, which is placed between the Layer 2 and Layer 3 headers. In a similar way, a label could correspond to a fiber, a DWDM wavelength, or a TDM time slot. In order for LSPs to be used, the forwarding tables at each LSR must be populated with the mappings from {incoming interface, label value} to {out- going interface, label value}. This process is called LSP setup, or label dis- tri