For protection of the ABR, the upstream node of the ABR acts as a 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 the ABR.
Manual bypass protection only works when a proper completely strict path is provisioned that avoids the ABR.
Dynamic bypass protection provides for the automatic computation, signaling, and association with the primary path of an inter-area point-to-point LSP to provide ABR protection. Figure: ABR Protection Using Dynamic Bypass LSP illustrates the role of each node in ABR protection using a dynamic bypass LSP.
In order for a PLR within the local area of the ingress LER to provide ABR protection, it must dynamically signal a bypass LSP and associate it with the primary path of the inter-area LSP using the following procedures.
The PLR must inspect the RRO node-id 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 and with the exclude route object (XRO) set to exclude the node-id of the protected ABR.
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 message for the primary path) along with the constraint to exclude the protected ABR 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 signals the dynamic bypass and automatically includes the XRO with the address of the protected ABR and propagates 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 in the PATH message.
If a node-protect dynamic bypass cannot be found or signaled, the PLR attempts a link-protect dynamic bypass LSP. As with the 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 RRO node-id also contains the interface address. Otherwise, the neighbor is not created once the bypass is activated by the PLR. The Path state then times out after three refreshes following the activation of the bypass backup LSP.
A one-to-one detour backup LSP cannot be used at the PLR for the protection of the ABR. As a result, a 7705 SAR, acting as a PLR, will not signal a one-to-one detour LSP for ABR protection. In addition, an ABR will reject 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 whether the cspf-on-loose option is enabled or not on the 7705 SAR. In other words, the 7705 SAR, working as a transit ABR for the detour path, rejects the signaling of an inter-area detour backup LSP.