The scenario
Host-A (10.0.1.10) is behind PE1 and Host-B (10.0.3.10) behind PE2, with P in between. Everything is configured for two LSPs except one number. Toward Host-B, PE1 pushes 16001, but P's table has an entry for 16099 (swap to 16002 toward PE2) and none for 16001, so P drops the packet. The return LSP is correct: PE2 pushes 16010, P swaps it for 16011, PE1 pops it. P has no route to either host network and no router runs OSPF, so nothing can carry the packet past P except a label P recognises.
What you will work out
See how a label mismatch between two neighbouring routers drops traffic, and how to find it by comparing what one router sends with what the next expects.
Why this matters
Label mismatches happen when labels are configured by hand, during migrations, and when one router is changed without its neighbour. The symptom is traffic that disappears in the core with every interface up. The quickest diagnosis is the one in this lab: find the last router the packet reached, note the label it arrived with, and compare it with that router's label table.
Running it
MPLS: Wrong Label Mapping 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: Wrong Label Mapping
Runs in your browser. No install, no signup, and the simulation starts instantly.
Open the lab →