MPLS LSP Tunnel Characteristics
Short answer
D An MPLS LSP tunnel is a unidirectional tunnel established for traffic flow in a single direction.
Question
Key Takeaways
The question tests the directional nature of LDP-derived LSPs versus TE tunnels, where the common trap is assuming symmetry due to IP routing behavior.
This page explains that an MPLS Label Switched Path (LSP) tunnel is fundamentally unidirectional, distinguishing it from bidirectional protocols like RSVP-TE.
Common mistake: Candidates often select 'bidirectional' because they confuse standard LDP LSPs with Bidirectional P2MP or BFD sessions, not realizing individual LSPs are one-way.
Expert Analysis
Why the Answer Is Correct
An MPLS Label Switched Path (LSP) created via LDP (Label Distribution Protocol) is inherently a unidirectional tunnel. The label binding and forwarding state are established for traffic flowing in a single direction from the ingress to the egress router. While IP routing is bi-directional, the MPLS forwarding plane treats each LSP as a separate entity for one direction of flow.
Why the Other Options Are Wrong
Option A is incorrect because a basic LDP LSP does not support bidirectional signaling by default; separate LSPs are needed for return traffic. Option B is incorrect because standard LDP LSPs do not provide security features like encryption; they are logical tunnels within a provider network. Option C describes the forwarding mechanism (hop-by-hop label swapping) rather than the tunnel's directional characteristic, making D the more precise answer regarding the tunnel's topology definition.
Community Comment Notes
Community consensus strongly supports option D. One user noted that while traditional LDP involves hop-by-hop forwarding, the tunnel itself is defined by its unidirectional nature for traffic optimization. Another comment confirmed that explicit TE tunnels are set up as unidirectional for resource reservation, reinforcing the concept that MPLS paths are directional constructs.
Exam Strategy
Remember that MPLS LSPs are analogous to point-to-point links in the data plane. Just as a serial link connects two points in one direction per interface instance, an LSP carries labels in one direction. Always check if the protocol mentioned (like RSVP-TE vs LDP) changes this baseline assumption.
Frequently Asked Questions
Are all MPLS tunnels unidirectional?
How does return traffic work if LSPs are unidirectional?
Community Votes
100% of anonymous learners picked answer D. Votes are pick records left by other test-takers — they are not the verified answer.
Discussion 3 contributions
Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.
From public community threads 3
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kldoyle97 👍 3 Answered DTraditional MPLS LDP - Hop by Hop forwarding. No tunnels involved MPLS LDP TE (Traffic Engineering) involves tunnels unidirectional MPLS Traffic Engineering (TE), where explicit, unidirectional tunnels are set up for traffic flow optimization, resource reservation, or meeting specific SLA requirements.
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brownleaf 👍 2 Answered DD is correct! https://networklessons.com/mpls/introduction-to-mpls-traffic-engineering-te
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cloud29 👍 2 Answered DThe answer is D
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