It's time to recap a few basics of MPLS, and in particular of CSPF. The Constrained Shortest Path First (CSPF) algorithm allow an ingress LSR to compute a Label Switched Path (LSP) out of a Traffic Engineering (TE) database, the latter includes various constraints or requirements on how a LSP must be signaled. As you may wonder, CSPF is widely use for traffic engineering purpose, but it's also a prerequisite for two protection mechanisms, namely Fast Reroute (FRR) and link/node protection. In fact, these two, uses the TE database to compute and later signal backup tunnels (or bypass LSPs). CSPF is therefore an important piece on the MPLS chessboard.