. "Further in accordance with this aspect of the present invention, the following five types of nodes may be included in a correlation graph: LINK UP or LKU; LINK DOWN or LKD; LSP UP or LPU; LSP DOWN or LPD; or LSP REROUTED or LPR. LKU and LKD events are regarded as root cause events and therefore LKU and LKD nodes do not include outgoing edges." . . .