This is achieved by sending LSP ping requests
along an LSP but to an IP destination address in network 127/8 (i.e., in the range
127.0.0.1 to 127.255.255.254). Network 127/8 is guaranteed never to be forwarded by
428 Chapter 14
an IP router, so if the LSP terminates prematurely, the packet will be dropped. Packets
addressed to network 127/8 are also typically discarded by hosts (reducing the risk of
LSP ping packets being misdelivered to hosts). Finally, by sending packets to varying
destination addresses in 127/8, the sender can exercise all ECMP paths for an LDPsignaled
LSP (since routers typically do a hash of the IP destination address, or source
and destination address, when performing ECMP).
LSP ping packets are sent to UDP port 3503 (requests) or from UDP port 3503
(replies). The TTL is set to 255 for LSP ping and starts at 1 when being used in traceroute
mode (being incremented to probe to subsequent LSRs). Note that a response will
only be sent if the destination implements LSP ping??”in the traceroute case, this may
result in ???missing hops??? from routers that don??™t implement the standard. Each packet
indicates how its recipient should send its reply; because MPLS LSPs are unidirectional,
the reply can??™t be sent back over the LSP.
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