Which control-plane process allows MPLS forwarding state recovery when a secondary RP takes over from a failed primary RP?

The correct answer is LDP uses SSO to recover from disruption in control plane service. Label Distribution Protocol (LDP) is a control-plane protocol used to distribute labels in an MPLS network. Stateful Switchover (SSO) allows the standby Route Processor (RP) to maintain control-plane information, including LDP sessions and label bindings, from the active RP. When the primary RP fails, the secondary RP can take over without re-establishing LDP sessions or re-distributing labels, ensuring fast recovery of the MPLS forwarding state. MP-BGP (Multiprotocol BGP) is used for VPNv4 or VPNv6 label distribution, not for general MPLS forwarding state recovery during RP failover. FEC (Forwarding Equivalence Class) is a concept, not a protocol distributing information between processors. LSP (Label Switched Path) is the path, and NSF (Nonstop Forwarding) is a mechanism for data plane forwarding during control plane disruption, but SSO is specifically for control plane state recovery like LDP sessions.