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Is there a way that I can find out (using a terminal cmd/util), if across an internet path that a datagram may travel, if there are routers that utilize MPLS?

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You cannot really tell the path that a packet may travel or has traveled. The packet has no record of that, and the path could change from one packet to the next because routing is dynamic. Routers route packets one at a time, regardless of any packets that came before, and an Internet path can change with no notice.

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  • What if the IP imposed max MTU for packets? Would that be a telltale sign?
    – Michael
    Aug 24 '17 at 16:05
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    There could be many reasons for a restriction on the path MTU. It could be a sign of MPLS, encryption, tunnel, etc.
    – Ron Maupin
    Aug 25 '17 at 1:17
  • Given the modern Internet, the answer to "does my traffic pass through MPLS routers?" is almost assuredly, Yes. It would be unusual if the answer was No.
    – Ron Trunk
    Aug 26 '17 at 16:06
  • I agree, but proving that from a command line is not really something that you can do.
    – Ron Maupin
    Aug 26 '17 at 16:07
  • I guess I could devise an experiment that sends packets to relatively similar geographic regions through different ISPs and tests for differences but still there are some many factors that are at play which will make it impossible to know for sure. I mean it would be surprisingly if they didn't use like @RonTrunk mentioned but still there may a case that an ISP doesn't.
    – Michael
    Aug 27 '17 at 2:14
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Yes, use traceroute to check what routers are along the route, use w3m (or your preferred command line web browser) to find what ISPs own those routers and their contact details and then use mail (or elm or mutt or whatever) to get in touch with them and ask if those routes use MPLS. It's not guaranteed to work, of course; it depends on them replying to tell you.

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