) NH = A, PUSH 51, LL = xyz NH = B, SWAP 57, LL = fgh NH = this, POP S-bit 1 Forwarding Unlabeled IP Packet Fig. 13.8 Tables in an LSR that define how packets are processed. manner will be by process switching, which involves passing the packet to a software process rather than directly switching the packet in hardware. Consequently, this type of forwarding is generally limited to network operation and maintenance traffic where possible. When a labeled packet arrives at an LSR, the label is compared against an Incoming Label Map (ILM), as shown in Figure 13.8. The ILM matches each incoming label value to a Next Hop Label Forwarding Entry (NHLFE), which defines what must be done to the packet and then allows the packet to be passed out of the LSR. The NHLFE contains the following information: Next hop for the packet One operation from PUSH, SWAP, or POP Optionally, link layer encoding for output onto media toward the next hop The label operations (PUSH, SWAP, POP) have been explained previously. However, it should be noted that in some cases the PUSH operation might put on a number of labels (multiple PUSHes) to create a label stack. This is useful in the ingress LSR for VPN applica- tions. The next hop is needed SO that the correct link layer address can be put on the packet. In practice the forwarding process in high-speed LSRs is carried out in an ASIC. In this case it is useful to have the optional link layer encoding (e.g., Ethernet header) available in an (almost) complete form SO that it can be taken directly from the NHLFE. In the case of Ethernet the only additional processing required is to fill in the length field and calculate the Cyclic Redundancy Check (CRC). It is possible that the next hop for Chapter 13 Multiprotocol Label Switching the packet might be the same LSR. In some cases this will be because the POP operation has resulted in an unlabeled packet, and the LSR must then forward the packet using conven- tional IP forwarding. This can be determined from the S-bit of the shim label, which is 1 