H12-891_V1-0 practice question 34 of 35

Regarding the mpLS handling mode of TTL, is the following description correct?

Answer options

A: Pipe mode. When an IP message passes through the MPLS network, the IPTTL minus 1 is mapped to the WLSTTL field at the incoming node.

B: The TTL in the Correct answer

C: MPLS label has the same meaning as the TTL field in the head. TTL prevents mpLS networks from developing bad routing.

D: Unifore style, when an IP packet passes through the MPLS network, the IPTTL of the incoming node is minus l. The MPLS TTL field is a fixed value.

E: In MPLSVPI, if you need to hide the structure of the MPLS backbone network, you can use Uniform mode on Ingress for private network messages .

Correct Answer:

B. The TTL in the

Explanation

The Time-to-Live (TTL) field in an MPLS label header serves the same fundamental purpose as the TTL field in an IP header: to prevent packets from looping indefinitely in the network. Each Label Switched Router (LSR) that forwards the packet decrements the MPLS TTL value by one. If the TTL reaches zero, the packet is discarded. This mechanism is crucial for network stability, as it contains the impact of transient or persistent routing loops by ensuring no packet can circulate forever.

Why Incorrect

A. In Pipe mode, the IP TTL is not copied to the MPLS TTL. The MPLS TTL is set to a large value (e.g., 255) independently at the ingress LER.
C. In Uniform mode, the ingress LER decrements the IP TTL and copies the result to the MPLS TTL. The MPLS TTL is then decremented at each hop; it is not a fixed value.
D. To hide the MPLS backbone structure from the customer network (e.g., from a traceroute), Pipe mode should be used, not Uniform mode.

References

1. RFC 3032, MPLS Label Stack Encoding, Section 2.1 "Label Stack Entry". This RFC defines the 8-bit TTL field in the MPLS header and its function as a hop count.

2. Huawei HedEx Documentation, NE40E V800R011C10 Configuration Guide - MPLS, Section "MPLS Basics > MPLS TTL Processing". This document details the Uniform and Pipe models, confirming that Pipe mode hides the core and Uniform mode exposes it. It also confirms the loop prevention function of MPLS TTL.

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