When MPLS is deployed in the network and the network layer protocol is IP, the route corresponding to the FEC must exist in the IP routing table of the LSR, otherwise the label forwarding entry of the FEC will not take effect.

The statement is correct. In an MPLS network that uses IP as the network layer protocol, the Label Distribution Protocol (LDP) works in conjunction with an underlying Interior Gateway Protocol (IGP) like OSPF or IS-IS. The IGP is responsible for building the IP routing table (and the corresponding Forwarding Information Base, or FIB). LDP then uses these routes to establish label bindings for Forwarding Equivalence Classes (FECs), which are typically the destination IP prefixes found in the routing table. An LSR creates a label forwarding entry in its Label Forwarding Information Base (LFIB) by associating an incoming label with an outgoing label and a next-hop. This next-hop information is derived directly from the IP routing table. If the route corresponding to a particular FEC is removed from the IP routing table (for example, due to a network topology change), the LSR no longer has a valid path to that destination. Consequently, the associated label forwarding entry in the LFIB becomes invalid and cannot be used to forward packets, even if a label for that FEC was previously learned. The integrity and availability of the IP route are prerequisites for the activation and operation of the MPLS label forwarding path.

This is a "TRUE OR FALSE" question. The "False" option is incorrect because it would imply that MPLS forwarding for an IP-based FEC can function independently of the IP routing table. This contradicts the fundamental architecture of MPLS, where the control plane (LDP) relies on the routing information provided by an IGP to build the Label Switched Paths (LSPs). The IP routing table provides the essential next-hop and path information that underpins the entire label forwarding process.

1. Huawei HedEx Documentation: In the Huawei NE40E Universal Service Router MPLS Configuration Guide, the section on "MPLS Fundamentals" explains the relationship between the FIB and LFIB. It details that LDP generates label mappings for IP routes that are active in the routing table. The guide states, "LDP allocates labels only to the active routes in the IP routing table... The establishment of the LFIB depends on the FIB." This confirms that the IP route must exist for the label forwarding entry to be effective.

Source: Huawei NE40E Universal Service Router MPLS Configuration Guide (V800R012C10), Section: "MPLS Fundamentals > Basic Concepts > FIB and LFIB".

2. Official Standards (IETF RFC): RFC 3031, "Multiprotocol Label Switching Architecture," is the foundational document for MPLS. It implicitly supports this statement by describing how the forwarding decision is made. The "Next Hop Label Forwarding Entry" (NHLFE) contains the next hop address. This next hop is determined by the network layer routing algorithm.

Source: Rosen, E., et al. "RFC 3031: Multiprotocol Label Switching Architecture." IETF, January 2001, Section 3.10, "The Next Hop Label Forwarding Entry (NHLFE)". The RFC explains that the NHLFE contains the address of the next hop, which is information derived from the network layer's routing table.

3. University Courseware: Lecture materials on MPLS from reputable computer networking courses consistently teach that LDP is a "slave" to the IGP. The IGP determines the path, and LDP simply provides labels for the path that the IGP has chosen.

Source: Stanford University, CS244: Advanced Topics in Networking. Lecture notes on "MPLS" typically explain that the control plane consists of a routing protocol (like OSPF) to build the routing table and a label distribution protocol (like LDP) to distribute labels for the routes discovered by the IGP. This establishes the direct dependency of the LFIB on the routing table.