4A0-103 practice question 4 of 30

Which of the following statements is true of an established LDP session?

Full set: 249 questions

Answer options

A: It uses a UDP connection with an arbitrary port number.

B: It uses a UDP connection with well known port number 646.

C: It uses a TCP connection with an arbitrary port number.

D: It uses a TCP connection with well known port number 646. Correct answer

Correct Answer:

D. It uses a TCP connection with well known port number 646.

Explanation

The Label Distribution Protocol (LDP) operates in two main stages. First, LDP routers discover each other using Hello messages sent over UDP to multicast address 224.0.0.2 on port 646. Once discovery is complete, they establish a reliable session to exchange label mapping information. This session is built using the Transmission Control Protocol (TCP) and connects to the well-known TCP port 646. The question specifically asks about the "established LDP session," which is the TCP connection.

Why Incorrect

A. LDP uses TCP for its session to ensure reliable delivery of label mappings; UDP is only used for discovery hellos.
B. LDP uses TCP, not UDP, for the established session. UDP port 646 is used for discovery messages.
C. The LDP session uses the well-known port 646 for the listening side of the connection, not an arbitrary port.

References

1. Rosen, E., et al. "LDP Specification." RFC 5036, IETF, October 2007, Section 2.5.1, "LDP Session Establishment". "An LDP session is a TCP session... The LDP well-known TCP port is 646."

2. Nokia 7750 SR OS MPLS Guide, Release 21.10.R1, Section: LDP Operation. "LDP uses TCP for session communication... LDP Hellos are sent over UDP."

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