In prior implementation, an inter-area LSP path would have been re-routed if a failure or a topology change occurred in the local or a remote area while the ABR loose-hop in the path definition was still up. If the exit ABR node went down, went into IS-IS overload, or was put into node TE graceful shutdown, the LSP path remains down at the ingress LER.
One new behavior introduced by the automatic selection of ABR is the ability of the ingress LER to reroute an inter-area LSP primary path via a different ABR in the following situations:
When the local exit ABR node fails, There are two cases to consider:
The primary path is not protected at the ABR and, so, is torn down by the previous hop in the path. In this case the ingress LER retries the LSP primary path via the ABR which currently has the best path for the destination prefix of the LSP.
The primary path is protected at the ABR with a manual or dynamic bypass LSP. In this case the ingress LER receives a Path Error message with a notification of a protection becoming active downstream and a RESV with a Local-Protection-In-Use flag set. At the receipt of first of these two messages, the ingress LER then performs a Global Revertive Make-Before-Break (MBB) to re-optimize the LSP primary path via the ABR which currently has the best path for the destination prefix of the LSP.
When the local exit ABR node goes into IS-IS overload or is put into node TE Graceful Shutdown. In this case, the ingress LER performs a MBB to re-optimize the LSP primary path via the ABR which currently has the best path for the destination prefix of the LSP. The MBB is performed at the receipt of the PathErr message for the node TE shutdown or at the next timer or manual re-optimization of the LSP path in the case of the receipt of the IS-IS overload bit.