Pinging an MPLS LSP

You can ping a specific LSP. Echo requests are sent over the LSP as MPLS packets. The payload is a User Datagram Protocol (UDP) packet forwarded to the address 127.0.0.1. The label and interface information for building and sending this information as an MPLS packet is the same as for standard LSP traffic.

When the echo request arrives at the egress node, the receiver checks the contents of the packet and sends a reply containing the correct return value, by using UDP. The router sending the echo request waits to receive an echo reply after a timeout of 2 seconds (you cannot configure this value).

You must configure MPLS at the [edit protocols mpls] hierarchy level on the remote router to be able to ping an LSP terminating there. You must configure MPLS even if you intend to ping only LDP forwarding equivalence classes (FECs).

On the egress router (the router receiving the MPLS echo packets), you must configure the address 127.0.0.1/32 on its lo0 interface. If this is not configured, the egress router does not have this forwarding entry and therefore simply drops the incoming MPLS pings and replies with "ICMP host unreachable" messages.

The command you use to ping an MPLS LSP is ping mpls <count count> <ldp <fec>> <rsvp <exp forwarding-class> <lsp-name>>. To ping a secondary MPLS LSP, use the command ping mpls <count count> <rsvp <lsp-name>> standby path-name. For a detailed description of this command, see the JUNOS Protocols, Class of Service, and System Basics Command Reference.