The BDI flow is injected on a return path (such as a return LSP) to inform the upstream LSR (which is the source of the forward LSP) that there is a defect at the downstream LSP's LSR sink point.BDI and loopback transactions use a return path. A return path could be: 1: A dedicated return LSP; 2: A shared return LSP, which is shared between many forward LSPs.