The router LSP diagnostics are implementations of LSP ping and LSP trace based on RFC 8029, Detecting Multi-Protocol Label Switched (MPLS) Data Plane Failures. LSP ping provides a mechanism to detect data plane failures in MPLS LSPs. LSP ping and LSP trace are modeled after the ICMP echo request or reply used by ping and trace to detect and localize faults in IP networks.
For a specific LDP FEC, RSVP P2P LSP, or BGP IPv4 or IPv6 label route, LSP ping verifies whether the packet reaches the egress label edge router (LER), while in LSP trace mode, the packet is sent to the control plane of each transit label switched router (LSR) which performs various checks to see if it is actually a transit LSR for the path.
The downstream mapping TLV is used in lsp-ping and lsp-trace to provide a mechanism for the sender and responder nodes to exchange and validate interface and label stack information for each downstream hop in the path of an LDP FEC or an RSVP LSP.
Two downstream mapping TLVs are supported. The original Downstream Mapping (DSMAP) TLV defined in RFC 4379, Detecting Multi-Protocol Label Switched (MPLS) Data Plane Failures, (obsoleted by RFC 8029) and the new Downstream Detailed Mapping (DDMAP) TLV defined in RFC 6424, Mechanism for Performing Label Switched Path Ping (LSP Ping) over MPLS Tunnels, and RFC 8029.
When the responder node has multiple equal cost next-hops for an LDP FEC prefix, the downstream mapping TLV can further be used to exercise a specific path of the ECMP set using the path-destination option. The behavior in this case is described in the ECMP sub-section below.