BFD

Hi, please your help

Which of the following statements about BFD for MPLS LSP are true?

a. In BFD for MPLS LSP, LSP Ping is used to negotiate the identifier and parameters of a BFD session.

b. BFD for MPLS LSP detects faults only on the forwarding plane. LSP Ping, however, can not only detect faults on the forwarding plane, but also find out inconsistencies between the forwarding plane and the control plane. Therefore, they are often combined for a better fault detection solution.

c. An MPLS LSP may be unidirectional. If BFD packets are transmitted along an MPLS LSP in the forward direction, these packets may be forwarded over IP in the reverse direction. Then the forward and reverse paths of a packet may be different. As a result, when a fault occurs on the reverse path, BFD detects and reports the fault even if the MPLS LSP is normal.

d. If MPLS LSPs work in load balancing mode, a BFD session should be triggered on each LSP in principle. Only one BFD session, however, can be initiated for load-balancing MPLS LSPs to an intermediate node. In this case, the forwarding path of BFD packets may be different from what expected. As a result, faults may be falsely reported or missed.