When the switches fail to establish an LDP session or LSP, the possible causes are:
The switches use S series boards that do not support the MPLS feature.
The MPLS feature is unavailable because the license for this feature is not loaded.
The peer device is unreachable, so the LDP session cannot be set up.
The local and remote devices are indirectly connected, not LDP is not enabled between every two devices on the path. A complete LSP cannot be established this case even when the remote session has been established.