The scenario
Host-A (10.0.1.10) and Host-B (10.0.3.10) are joined by three routers in a line: PE1, P and PE2. PE1 has a static route to 10.0.3.0/24 via P and an LSP bound to that route, so it pushes label 16001 onto packets for Host-B. P looks up 16001 in its label table, swaps it for 16002 and sends the packet to PE2. PE2 pops 16002 and delivers the plain IP packet to Host-B. Replies use a second LSP the other way: PE2 pushes 16010, P swaps it for 16011, PE1 pops it. P has only its two connected networks in its routing table and no route to 10.0.1.0/24 or 10.0.3.0/24; no router here runs OSPF, and every label is configured by hand.
What you will work out
Understand the three MPLS operations (push, swap, pop), why a label switched path runs in one direction only, and how a core router can forward traffic it has no IP route for.
Why this matters
MPLS carries traffic across many service-provider and large enterprise networks, and push, swap and pop are the operations everything else is built on: L3VPNs, traffic engineering and fast reroute all come down to which labels a router pushes and what each router does with them. Remembering that an LSP runs one way explains a common puzzle, a path that works in one direction and silently fails in the other.
Running it
MPLS: Label Switching Basics opens on a canvas, already wired and addressed. Trace a packet through it hop by hop, or run the same topology live on real Linux network namespaces. How the two modes differ.
Open MPLS: Label Switching Basics
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