In an MPLS network, if an LSP fails to forward data, the MPLS control plane on which the LSP is set up cannot detect the forwarding failure, which complicates the network maintenance.


MPLS ping is a mechanism to detect an LSP failure and locate the faulty node. Similar to ordinary IP ping, MPLS ping detects availability of an LSP through MPLS Echo Request messages and MPLS Echo Reply messages. These two types of messages are encapsulated through UDP and sent through port 3503.


An MPLS Echo Request message carries information about a FEC to be detected. Similarly to a packet belongs to this FEC, the MPLS Echo Request message travels along an LSP to implement detection.


After the MPLS Echo Request message reaches the egress of the LSP and the control plane on the egress confirms that the outgoing interface of the FEC resides on the local LSR, the MPLS ping finishes.


To prevent the egress from forwarding the Echo Request message to another node, in the IP header of the Echo Request message, the destination address is set to an IP address (as the local loopback address) within the segment 127/8 and the TTL is set to 1.

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