Exam 350-501 topic 1 question 119 discussion

correct answer: A. An LSP is not required for the reply to reach the ingress MPLS router. An MPLS echo reply is sent in response to an MPLS echo request. The reply is sent as an IP packet and it is forwarded using IP, MPLS, or a combination of both types of switching. Head-end router will receive an echo-reply from the last router (before the broken binding), and that reply will be: 'U' - unreachable

A is 100% correct: https://www.cisco.com/c/dam/en/us/td/docs/routers/ncs4200/configuration/guide/mpls/mp-em-mibs-ncs4200-book.html#:~:text=and%20AToM%20VCCV-,MPLS%20LSP%20Ping%20Operation,-MPLS%20LSP%20Ping

Actually, the purpose of MPLS LSP Ping is to verify if LSP is working well, that is why you do need LDP. So for this reason it should be “B”. https://www.cisco.com/c/dam/en/us/td/docs/routers/ncs4200/configuration/guide/mpls/mp-em-mibs-ncs4200-book.html “MPLS LSP Ping uses Multiprotocol Label Switching (MPLS) echo request and reply packets to validate a label switched path (LSP). Both an MPLS echo request and an MPLS echo reply are User Datagram Protocol (UDP) packets with source and destination ports set to 3503.”

A - agree with blizzlondon An MPLS echo reply is sent in response to an MPLS echo request. It is sent as an IP packet and forwarded using IP, MPLS, or a combination of both types of switching.

i think answer B is correct also https://www.cisco.com/c/en/us/td/docs/routers/asr920/configuration/guide/mpls/mp-basic-xe-3s-asr920-book/mp-basic-xe-3s-asr903-book_chapter_011.html

A is correct, for those like me that thought about a pseudowire of sorts, the LSP is the tunnel that a customer's traffic would take. The question is to test MPLS OAM, so it will use the label forwarding database regardless of if there is an LSP even built to test reachability.

'A' is the correct answer. An Echo reply message is encapsulated in plain unicast.

Agree correct is A