MPLS Control Plane Recovery with SSO and NSF
Which control plane process allows the MPLS forwarding state to recover when a secondary RP takes over from a failed primary RP?
Community Votes
100% of anonymous learners picked answer A. Votes are pick records left by other test-takers — they are not the verified answer.
Community Insight
The question asks specifically about the control plane process allowing MPLS forwarding recovery, which relies on LDP's integration with SSO rather than just protocol-level restarts.
This question tests the mechanism for MPLS forwarding state recovery during a Route Processor (RP) failover. The correct answer identifies that LDP utilizes Stateful Switchover (SSO) to maintain forwarding continuity when the secondary RP takes over.
Candidates often select D (LSP/NSF) because NSF is the general term for non-stop forwarding, but they miss that LDP SSO is the specific control plane service described in Cisco documentation for this scenario.
Community Discussion (3 comments)
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Expert Analysis
Why the Answer Is Correct
The correct answer is A because it accurately describes how the Label Distribution Protocol (LDP) integrates with Stateful Switchover (SSO). According to Cisco documentation, LDP uses SSO along with Non-Stop Forwarding (NSF) and graceful restart mechanisms. This combination allows the router to maintain its MPLS forwarding table even if the primary Route Processor (RP) fails and a secondary RP takes over. The key is that the forwarding state is preserved via SSO, ensuring no disruption to data plane traffic.Why the Other Options Are Wrong
Option B is incorrect because MP-BGP is primarily used for distributing labeled routes in MPLS VPNs (Layer 3), not for the basic label prefix bindings in the local MPLS forwarding table handled by LDP in this context. Option C is incorrect because FEC (Forwarding Equivalence Class) is a classification concept, not a control plane service or process that distributes information between processors. Option D is partially true in that LSPs use NSF, but the question specifically asks about the control plane process allowing recovery when an RP fails; LDP SSO is the precise mechanism cited for maintaining the control plane's forwarding state association during RP switchover.Community Comment Notes
Community consensus strongly supports option A. One user noted that "A makes sense as the question implies router route-processor failure," highlighting that the protocols themselves don't fix hardware failures but rely on HA features like SSO. Another commenter provided a direct link to Cisco documentation confirming that "LDP uses SSO, NSF, and graceful restart to allow a Route Processor (RP) to recover... without losing its MPLS forwarding state." This confirms that LDP is the specific protocol component leveraging SSO for this purpose.Official Reference
Exam Strategy
When reviewing MPLS HA features, distinguish between the general concept of Non-Stop Forwarding (NSF) and the specific protocol integrations (like LDP SSO). Focus on how different control plane protocols (OSPF, BGP, LDP) interact with SSO to preserve their respective state tables during an RP failover.
Frequently Asked Questions
What is the difference between LDP SSO and LDP Graceful Restart?
SSO maintains the forwarding state during an RP switchover internally within the router. Graceful Restart helps neighbors understand that the adjacency is being maintained despite a control plane restart, preventing neighbor flapping.
Why isn't MP-BGP the correct answer for MPLS forwarding state?
MP-BGP distributes labeled routes for VPNs. LDP is responsible for the basic label bindings in the MPLS forwarding table. The question refers to the fundamental MPLS forwarding state recovery via RP failover, which is LDP's domain.
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