To assist with problem determination in an MPLS (Multiprotocol Label Switching) network, LSP ping support is included as a CLI ping option in the ExtremeXOS software. The LSP ping support is based on draft-ietf-mpls-lsp-ping-13.txt. This draft includes support for both connectivity verification and fault isolation for transport LSPs. Connectivity verification is supported using a modified ping packet that is sent over the specified transport LSP.
LSP ping is designed to catch failures where a transport LSP appears to be operational but is actually not functioning correctly. LSP data plane corruption is far less likely to occur than an LSP control plane failure, but the LSP ping is also useful for detecting possible latency issues.
MPLS pings are sent to the well-known UDP port number 3503 with an IP in the 127.0.0.0/8 IP subnet. The source IP address is set to the sender.
The time stamp field is supported for calculating round trip times and is accurate to 1/100 of a second. When replying to a ping, the LSP ping response (MPLS echo reply) sequence number and time-stamp fields are set to the LSP ping request (MPLS echo request) values. One MPLS echo response is sent for each MPLS echo request received. An MPLS echo reply is sent out-of-band as a natively IP routed IPv4 UDP packet. The normal IP routed path might or might not use an LSP.
To reduce the possibility of fragmentation problems on the return path, MPLS echo reply packets do not include any padding that was sent in the MPLS echo request. Because each LSP is unidirectional, the return path is not directly relevant for verification of the LSP's functionality. What is important is that the LSP ping results are returned to the source of the MPLS echo request.
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