The automatic selection of the ABR allows the ingress LER to reroute an inter-area LSP primary path through a different ABR in the following situations:
When the local exit ABR node fails, there are two possibilities to consider:
The primary path is not protected at the ABR and is therefore torn down by the previous hop in the path. In this case, the ingress LER retries the LSP primary path through the ABR that 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 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 make-before-break (MBB) to re-optimize the LSP primary path through the ABR that 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 to re-optimize the LSP primary path through the ABR that 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 manual re-optimization of the LSP path in the case of the receipt of the IS-IS overload bit.