This feature modifies the LFA next-hop selection algorithm. The SRLG and admin-group criteria are applied before running the LFA next-hop selection algorithm. In other words, links which do not include one or more of the admin-groups in the include-group statements and links which belong to admin-groups which have been explicitly excluded using the exclude-group statement, and the links which belong to the SRLGs used by the primary next hop of a prefix are first pruned.
This pruning applies only to IP next hops. Tunnel next hops can have the admin-group or SRLG constraint applied to them under MPLS. For example, if a tunnel next hop is using an outgoing interface which belongs to a specific SRLG ID, the user can enable the srlg-frr option under the config>router>mpls context to be sure the RSVP LSP FRR backup LSP does not use an outgoing interface with the same SRLG ID. A prefix which is resolved to a tunnel next hop is protected by the RSVP FRR mechanism and not by the IP FRR mechanism. Similarly, the user can include or exclude admin-groups for the RSVP LSP and its FRR bypass backup LSP in MPLS context. The admin-group constraints can, however, be applied to the selection of the outgoing interface of both the LSP primary path and its FRR bypass backup path.
The following is the modified LFA selection algorithm which is applied to prefixes resolving to a primary next hop which uses a specific route next hop policy template.
Split the LFA next hops into two sets:
IP or direct next hops.
Tunnel next hops after excluding the LSPs which use the same outgoing interface as the primary next hop.
Prune the IP LFA next hops which use the following links:
Links which do not include one or more of the admin-groups in the include-group statements in the route next hop policy template.
Links which belong to admin-groups which have been explicitly excluded using the exclude-group statement in the route next hop policy template.
Links which belong to the SRLGs used by the primary next hop of a prefix.
Continue with the set indicated in the nh-type value in the route next hop policy template if not empty; otherwise continue with the other set.
Within IP next hop set:
Prefer LFA next hops which do not go over the Pseudo-Node (PN) used by the primary next hop.
Within selected subset prefer the node-protect type or the link-protect type according to the value of the protection-type option in the route next hop policy template.
Within the selected subset, select the best admin-groups according to the preference specified in the value of the include-group option in the route next hop policy template.
Within selected subset, select lowest total cost of a prefix.
If same total cost, select lowest router-id.
If same router-id, select lowest interface-index.
Within tunnel next hop set:
Select tunnel next hops which endpoint corresponds to the node owning or advertising the prefix.
Within selected subset, select the one with the lowest cost (lowest LSP metric).
If same lowest cost, select tunnel with lowest tunnel-index.
If none is available, continue with rest of the tunnel LFA next hop set.
Prefer LFA next hops which do not go over the Pseudo-Node (PN) used by the primary next hop.
Within selected subset prefer the node-protect type or the link-protect type according to the value of the protection-type in the route next hop policy template.
Within selected subset, select lowest total cost of a prefix. For a tunnel next hop, it means the LSP metric plus the cost of the LSP endpoint to the destination of the prefix.
If same total cost, select lowest endpoint to destination cost.
If same endpoint to destination cost, select lowest router-id.
If same router-id, select lowest tunnel-index.