 prefix in the routing table Packets with a certain destination IP address and transport layer port (i.e., a single application flow) Packets with the same IP Differentiated Services (DiffServ) code point Packets that all leave a core network through the same egress The first FEC in the preceding list is the most obvious when MPLS is simply used to support conventional IP routing and is often the default FEC if no other is explicitly defined. The last FEC in the list allows specific transit pipes to be constructed in a service provider's network and can be automatically configured by taking information from the BGP protocol. Processing of Packets in a Label Switch Router An LSR forwards labeled packets solely based on the information in the top label of the label stack. Part of the design of MPLS is that the LSR does not inspect data after the label. The effect of this is that a labeled packet is transported unchanged through an LSP. This also applies to the lower layers of the stack, which are processed only after the topmost label is removed. An LSR performs one or more of the following actions on the top label of the label stack: SWAP: Accept a labeled packet and swap the label for a new label. PUSH (one of): Accept an unlabeled packet and push a label onto the packet. Accept a labeled packet, SWAP the top-level label, push another level of label onto the label stack. POP: Remove a label from top of the label stack. In addition to these operations, the LSR has to be able to forward IP packets in the normal manner. One reason for this is that the LSR needs to process signaling messages before any LSP has been set up. Another reason is that if the LSR is acting as an egress router, it has to able to act as a conventional IP router SO that it can forward the packets to the correct destination using IP forwarding. It is likely that IP packet forwarding in the conventional The MPLS Architecture Link Layer Header Taken from NHFLE with Minimal Postprocessing Unlabeled IP Packet NHFLE Table Classifier (e.g., IP prefix