MPLS Control Plane Recovery with SSO and NSF

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.