Which statement is true about LDP?

The Label Distribution Protocol (LDP) is a signaling protocol, not a routing protocol. Its primary function is to advertise and distribute MPLS labels for Forwarding Equivalence Classes (FECs). To determine the path for a Label Switched Path (LSP), LDP consults the IP Routing Information Base (RIB), which is populated by an Interior Gateway Protocol (IGP) like OSPF or IS-IS. LDP binds labels to prefixes learned from the IGP and uses the IGP's next-hop information to forward labeled packets. Therefore, LDP is entirely dependent on the IGP for path calculation and all routing-related decisions.

A. LDP and IGPs use different timers. LDP's default Hello interval is 5 seconds, and IGPs like OSPF do not exchange full databases at a 60-second interval.

B. The timers for LDP and IGP updates are not standardized at 30 seconds. This statement is factually incorrect regarding the default operation of these protocols.

D. LDP does not perform routing functions. It is a signaling protocol that distributes labels for paths that have already been calculated and installed by an IGP.

1. IETF RFC 5036: LDP Specification. Section 1.1

"LDP Overview

" states: "LDP depends on the routing protocols to provide it with routing information. LDP's job is to use the routing information to set up LSPs." This directly supports the correct answer (C) and refutes option (D).

2. IETF RFC 5036: LDP Specification. Section 2.5.1

"Hello Message

" specifies the default value for the Hello interval is 5 seconds

which contradicts the timers mentioned in options (A) and (B).

3. Davie

B.

& Venaas

S. (2004). MPLS-Enabled Applications: Emerging Developments and New Technologies (3rd ed.). Wiley. Chapter 2

"Label Distribution Protocols

" explains that LDP relies on the existence of an IP routing table

populated by an IGP

to make its decisions about label advertisement and next-hop selection.

4. Stanford University

CS244: Advanced Topics in Networking. Course materials on MPLS architecture consistently differentiate the roles of the control plane protocols. They describe how IGPs (e.g.

OSPF

IS-IS) are responsible for routing

while LDP is responsible for signaling based on the routes established by the IGP.