In prior implementations, an inter-area LSP path would have been rerouted if a failure or a topology change occurred in the local area or in a remote area while the ABR loose hop in the path definition was still up. If the transit/inter-area (exit) ABR failed or was put into node TE graceful shutdown, or if IS-IS went into overload mode, the LSP path would remain down at the ingress LER.
With automatic ABR selection, the ingress LER can reroute an inter-area LSP primary path via a different ABR in the following situations:
When the local exit ABR fails, there are two cases to consider:
If the primary path is not protected at the ABR, and is therefore torn down by the previous hop in the path, then the ingress LER retries the LSP primary path via the ABR that currently has the best path for the destination prefix of the LSP.
If the primary path is protected at the ABR with a manual or dynamic bypass LSP, the ingress LER will receive a PathErr message with a notification of protection becoming active downstream and a RESV message with a Local-Protection-In-Use flag set. At the receipt of the first of these two messages, the ingress LER performs a Global Revertive MBB procedure to optimize the LSP primary path via the ABR that 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, the ingress LER performs an MBB procedure to optimize the LSP primary path via the ABR that 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 optimization of the LSP path if the IS-IS overload bit is received.