What Breaks an LSP Between Two PE Routers?

Describe MPLS operations (LSR, LDP, label switching, LSP) Describe MPLS Layer 3 VPN

Answer Correct answer: A, C — An LSP between two PE routers breaks when the LDP adjacency is lost or when label bindings no longer match the prefixes in the routing table.

Which two reasons would cause an LSP to break between two PE routers? (Choose two.)

lost LDP adjacency Correct Answer
matching labels
prefix mismatch Correct Answer
IGP hello adjacency
MPLS not enabled

Community Votes

67%

33%

67% of anonymous learners picked answer AC. Votes are pick records left by other test-takers — they are not the verified answer.

Community Insight

The question tests the difference between a prerequisite for an LSP (MPLS/LDP enabled) and a genuine LSP-break cause (lost LDP adjacency, prefix/label mismatch) — the trap is picking a condition that prevents the LSP from ever forming.

An MPLS LSP between two PE routers is torn down when the LDP adjacency used to exchange label bindings is lost, or when the label bindings no longer match the prefixes and masks found in the routing table. This page confirms why the answer is A and C, and why 'MPLS not enabled' cannot be a cause of a broken LSP.

Many learners pick E ('MPLS not enabled') because an LSP obviously cannot exist without MPLS; but that describes an LSP that was never built, not one that breaks between two PE routers, so it fails the wording of the question.

Community Discussion (8 comments)

Kojy 👍 5 Selected: AC

cause LSP break !!! i think if mpls is not enabled there's no LSP in first place

Sammy3637 👍 2 Selected: AE

A. Lost LDP adjacency: The Label Distribution Protocol (LDP) is used to distribute labels in MPLS networks. If the LDP adjacency between two PE routers is lost, the routers will not be able to exchange labels, causing the LSP (Label Switched Path) to break. E. MPLS not enabled: If MPLS is not enabled on either of the PE routers, the LSP cannot be established, as MPLS relies on the ability to swap labels across routers to forward packets along a path.

tubirubs 👍 1 Selected: AE

Prefix mismatch would more likely cause routing issues, but it is not directly related to the LSP breaking, as LSPs are built using label information rather than IP prefix information directly.

[Removed] 👍 3 Selected: AC

A & C are correct The label bindings must correspond to the prefixes and subnet masks in the routing tables. If there's a mismatch, the labels will not be used properly, causing the LSP to fail. i think E is incorrect, if MPLS is not enabled on the routers, there won’t be any LSPs to break. https://www.cisco.com/c/en/us/support/docs/multiprotocol-label-switching-mpls/mpls/23565-troubleshoot-mpls-vpn.html#cause:~:text=If%20label%20bindings,devices%20will%20fail.

jabal93 👍 2

AC A: Like many other protocols, LDP first establishes a (neighbor adjacency) before it exchanges label information. and each router has unique LSR. C: When we use LDP on Cisco IOS, we locally generate a label for each prefix that we can find in the RIB, except for BGP prefixes. This information is then added to the LIB (Label Information Base). The information in the LIB is used to build the LFIB (Label Forwarding Information Base). When the router has to forward a packet with a MPLS label on it, it will use the LFIB for forwarding decisions. I don't think E is accurate as if MPLS not enabled the Adjacency between the PEs will not form the start, my hint the question asking about already formed session and got broken.

Pietjeplukgeluk 👍 1 Selected: AE

I thought A and E. Anyway, it do not think LSP break with prefix mismatch, so personally C is wrong. A & E seem best here

dapardo 👍 1

Im not sure on A because of the word "adjacency". I mean LDP issues yes, would cause the break between PE routers but adjacency?

Deejay18 👍 1

https://www.cisco.com/c/en/us/support/docs/multiprotocol-label-switching-mpls/mpls/23565-troubleshoot-mpls-vpn.html#cause I think its CE

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Expert Analysis

Why the Answer Is Correct

An LSP between two PE routers is built from label bindings exchanged by the Label Distribution Protocol, so anything that stops that exchange tears the path down end to end. Option A is correct: if the LDP adjacency (the UDP discovery and TCP session between the two label switch routers) goes down, the peer's label bindings are withdrawn from the LIB/LFIB and the end-to-end LSP no longer exists. Option C is also correct: Cisco's own MPLS VPN troubleshooting guide states that label bindings must correspond to the prefixes and subnet masks in the routing table, so a prefix mismatch means no usable outgoing label is installed for the destination. Both conditions are listed as classic causes of an LSP break between PE routers, which is exactly the exam doctrine the question targets.

Why the Other Options Are Wrong

Option B is wrong because matching labels are the desired state — if local and remote label bindings agree, the LSP works; matching is never a failure cause. Option D is wrong because an IGP hello adjacency is a routing-protocol construct, not an MPLS one; losing it can disturb the route to the BGP next hop, but the question asks what breaks the LSP between the PEs, and LDP does not rely on IGP hellos to keep its session alive. Option E is the most tempting distractor but fails logically: if MPLS is not enabled on the routers, no LSP is ever established, so there is nothing to break — a cause must act on an LSP that previously existed. That prerequisite-versus-failure distinction is the whole point of the item.

Community Comment Notes

Kojy made the decisive point that "if mpls is not enabled there's no LSP in first place", and joker-style reasoning aside, that matches the exam logic exactly. Another learner quoted the Cisco troubleshooting doc, noting that "The label bindings must correspond to the prefixes and subnet masks" in the routing table, which is the documented basis for option C. dapardo doubted option A over the word "adjacency", but LDP does form a neighbor session before exchanging label information, so the term is used correctly here. Pietjeplukgeluk and tubirubs argued for AE but, as the vote split (AC 67 vs AE 33) shows, the prerequisite argument for E does not survive scrutiny.

Official Reference

https://www.cisco.com/c/en/us/support/docs/multiprotocol-label-switching-mpls/mpls/23565-troubleshoot-mpls-vpn.html

Exam Strategy

When an MPLS item asks what 'breaks' an LSP, filter out any option that is a prerequisite for the LSP existing at all — those describe a path that never formed. Then look for the two conditions that destroy an established label-switched path: loss of the LDP session/adjacency and a mismatch between label bindings and the routing table.

Frequently Asked Questions

Why is "MPLS not enabled" (E) wrong if the LSP is broken?

If MPLS or LDP is not enabled on the PE routers, no LSP is ever built, so there is nothing that can break. A cause of an LSP break must act on a path that previously existed.

How does a prefix mismatch actually break the LSP between the PEs?

LDP only generates a local label for prefixes it finds in the routing table, so if the prefix or mask does not match, no label binding is installed and the LFIB has no outgoing label for that destination.

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