3Com Router 3000 Ethernet FamilyConfiguration Guide Chapter 1 MPLS Architecture3Com Corporation1-51.3.3 LSP Tunnel and HierarchyI. LSP tunnelMPLS supports LSP tunnel technology. On an LSP path, LSR Ru and LSR Rd areupstream and downstream for each other. However, the path between LSR Ru andLSR 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 in consistent, thistunnel is called hop-by-hop routing tunnel. And if the two routes are not in consistent,then the tunnel of this type is called explicit routing tunnel.R1 R2 R3 R4R21 R22Layer 1Layer 2Figure 1-4 LSP tunnelAs shown in Figure 1-4, LSP is a tunnel between R2 and R3.II. Multi-layer label stackWhen the packet is sent in LSP tunnel, there will be multiple layers for the label of thepacket. Then, on the ingress and egress of each tunnel, it is necessary to implementincoming and outgoing operation for the label stack. For each incoming operation, thelabel will be added with one layer. And there is no depth limitation for the label stackfrom MPLS.The labels are organized according to the principle “last in first out” in the label stack,and MPLS processes the labels beginning from the top of the stack.Suppose that a packet has the label stack depth of m, then the label at the bottom of thestack is the label of first level, and the label at the top of the stack is the label of level m.The packet with no label can be regarded as the packet of blank label stack (namely,the label stack depth is zero).1.3.4 Forwarding Labeled PacketsIn Ingress, the packets entering the network are classified into Forwarding EquivalenceClass (FEC) according to their characteristics. Usually, FEC is classified according tothe IP address prefix or host address. The packets with the same FEC will pass throughthe same path (i.e., LSP) in MPLS area. LSR assigns a short label of fixed length for theincoming FEC packet, and then forwards it through the corresponding interface.
