Operation Manual – MPLSH3C S9500 Series Routing Switches Chapter 1 MPLS Architecture1-8When the value exceeds the threshold value, it is considered that a loop presents, andthe process for establishing LSP is terminated.The path vector method refers to that the path information is recorded in the messagebound with the forwarding label, and, for every hop, the corresponding router checks ifits ID is contained in this record. If not, the router adds its ID into the record; and if yes,it indicates that a loop presents and the process for establishing LSP is terminated.1.3.4 LSP Tunnel and HierarchyI. LSP tunnelMPLS supports LSP tunnel technology. On an LSP path, LSR Ru and LSR Rd are boththe upstream and the downstream for each other. However, the path between LSR Ruand LSR Rd may not be part of the path provided by routing protocol. MPLS allowsestablishing a new LSP path between LSR Ru and LSR Rd, and LSRRu and LSR Rd are respectively the starting point and ending point of this LSP. TheLSP between LSR Ru and LSR Rd is referred to as the LSP tunnel, which avoids thetraditional encapsulated tunnel on the network layer. If the route along which the tunnelpasses and the route obtained hop by hop from routing protocol is consistent, thistunnel is called hop-by-hop routing tunnel. And if the two routes are not consistent, thenthe tunnel of this type is called explicit routing tunnel.R1 R2 R3 R4R21 R22Layer 1Layer 2R1 R2 R3 R4R21 R22R1 R2 R3 R4R21 R22Layer 1Layer 2Figure 1-5 LSP tunnelAs shown in Figure 1-5, LSP is a tunnel between R2 and R3.II. Multi-layer label stackIn MPLS, a packet may carry multiple labels which are in the form of stack. Operationsto the stack follow the “last in first out” principle and it is always the labels at the top ofthe stack that decide how to forward packets. Pushing label indicates to add a label intoa outgoing packet, then the depth of the label stack is the former one plus 1, and thecurrent label of the packet changes to the newly added one; popping a label indicates toremove a label form a packet, then the depth of the packet is the former one minus 1,and the current label of the packet changes to the label of its underlayer.Multiple-layer label stack is used in LSP tunnel. When a packet travels in LSP tunnel,there will be multiple layers for the label of the packet. Then, at the ingress and egressof each tunnel, it is necessary to implement pushing and popping operation for the labelstack. For each pushing operation, the label will be added with one layer. And there isno depth limitation for the label stack from MPLS.
