In networking we use many times different terms to define the same thing. MPLS tunnel label or transport label are just two of those. Not only transport and tunnel labels but also other terms are used to define the same thing which these labels provide. Let me explain first why and where MPLS tunnel label is used.
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In the above figure, traditional MPLS nodes are shown. CE, PE and the P nodes are the core nodes which are used in MPLS. Tunnel label is only used to provide reachability between PE devices. You can think MPLS tunnel label as GRE tunnel. With point to point GRE tunnels, you create a connection between two nodes. With MPLS tunnel label, exactly the same thing is done. But the tunnel endpoints are PE (Provider Edge devices) . In different resources you can find the different terms to describe this reachability. Instead of MPLS tunnel label, you can see; MPLS transport label , topmost label , outer label , outmost label. Many people don’t know that they are all the same thing. All these terms are used to define the same thing and they are used for the reachability between two MPLS PE devices. There are many other terms which you should know if you are in networking field and you can reach to most commonly used and important ones from here.