For protection of the area border router, the upstream node of the area border router acts as a point-of-local-repair (PLR), and the next-hop node to the protected domain border router is the merge-point (MP). Both manual and dynamic bypass are available to protect area border node.
Manual bypass protection works only when a correct completely strict path is provisioned that avoids the area border node.
Dynamic bypass protection provides for the automatic computation, signaling, and association with the primary path of an inter-area P2P LSP to provide ABR node protection. Figure: ABR node protection using dynamic bypass LSP illustrates the role of each node in the ABR node protection using a dynamic bypass LSP.
In order for a PLR node within the local area of the ingress LER to provide ABR node protection, it must dynamically signal a bypass LSP and associate it with the primary path of the inter-area LSP using the following new procedures:
The PLR node must inspect the node-id RRO of the LSP primary path to determine the address of the node immediately downstream of the ABR in the other area.
The PLR signals an inter-area bypass LSP with a destination address set to the address downstream of the ABR node and with the XRO set to exclude the node-id of the protected ABR node.
The request to CSPF is for a path to the merge-point (that is the next-next-hop in the RRO received in the RESV for the primary path) along with the constraint to exclude the protected ABR node and the include/exclude admin-groups of the primary path. If CSPF returns a path that can only go to an intermediate hop, then the PLR node signals the dynamic bypass and automatically includes the XRO with the address of the protected ABR node and propagate the admin-group constraints of the primary path into the Session Attribute object of the bypass LSP. Otherwise, the PLR signals the dynamic bypass directly to the merge-point node with no XRO object in the Path message.
If a node-protect dynamic bypass cannot be found or signaled, the PLR node attempts a link-protect dynamic bypass LSP. As in existing implementation of dynamic bypass within the same area, the PLR attempts in the background to signal a node-protect bypass at the receipt of every third Resv refresh message for the primary path.
Refresh reduction over dynamic bypass only works if the node-id RRO also contains the interface address. Otherwise the neighbor is not created when the bypass is activated by the PLR node. The Path state then times out after three refreshes following the activation of the bypass backup LSP.
Note that a one-to-one detour backup LSP cannot be used at the PLR for the protection of the ABR node. As a result, a PLR node does not signal a one-to-one detour LSP for ABR protection. In addition, an ABR node rejects a Path message, received from a third party implementation, with a detour object and with the ERO having the next-hop loose. This is performed regardless if the cspf-on-loose-hop option is enabled or not on the node. In other words, the router as a transit ABR for the detour path rejects the signaling of an inter-area detour backup LSP.