Automatic ABR selection provides the following functionality:
Path bandwidth reservation and admin-groups operate within the scope of all areas because they rely on propagating the parameter information in the Path message across the area boundary.
The TE graceful shutdown and soft preemption functionality support MBB of the LSP path to avoid the link or node that originated the PathErr message, provided the link or node is in the local area of the ingress LER. If the PathErr originated in a remote area, the ingress LER cannot avoid the link or node when it performs the MBB because it computes the path to the local ABR exit router only. There is, however, an exception to this for the TE graceful shutdown case only: the upstream ABR nodes in the current path of the LSP record the link or node to avoid and use it in subsequent ERO expansions. This means that if the ingress LER computes a new MBB path that goes through the same exit ABR router as the current path, and all ABR upstream nodes of the node or link that originated the PathErr message are also selected in the new MBB path when the ERO is expanded, the new path avoids this link or node.
MBB avoids the ABR node when the node is put into TE graceful shutdown.
The use-te-metric option in CSPF cannot be propagated across the area boundary and operates within the scope of the local area of the ingress LER node.
The srlg option on bypass LSP operates locally at each PLR within each area. The PLR node protecting the ABR checks the SRLG constraint for the path of the bypass within the local area.
The srlg option on the secondary path is allowed to operate within the scope of the local area of the ingress LER node.
The PLR node must indicate to CSPF that a request to a one-to-one detour LSP path must remain within the local area. If the destination for the detour, which is the same as that of the LSP, is outside the area, CSPF must not return a path.
The propagate-admin-group option under the LSP must be enabled on the inter-area LSP if the user wants to have admin-groups propagated across the areas.
Using automatic ABR selection, timer-based resignal of the inter-area LSP path is supported and resignals the path if the cost of the path segment to the local exit ABR changes. The cost shown for the inter-area LSP at ingress LER is the cost of the path segments to the ABR node.