Application of LFA policy to backup node SID tunnel

The backup node SID feature allows OSPF to use the path to an alternate ABR as an RLFA backup for forwarding packets of prefixes outside the local area or domain when the path to the primary ABR fails.

This feature reduces the label stack size by omitting the PQ node label if a regular RLFA algorithm is run.

The backup node SID algorithm consists of the following steps:

1. Perform an SPF on the modified topology with the primary ABR removed.

   This action resolves the backup node SID using the path to the alternate ABR.

2. Install the ILM to use the backup node SID for transit traffic with the maximum ECMP next hops found in step 1.
3. Use the backup node SID as an RLFA backup for prefixes outside the local area or domain. This step is modified as follows to select the backup node SID by applying the LFA policy corresponding the primary next hop of these prefixes, as follows.

   1. For each neighbor (Ni) found in step 1, use the LFA policy to select the best next-hop interface.

   2. Among the remaining interfaces, use the LFA policy to select best (Ni) and select its interface.

   Note: A backup node SID is always preferred to a regular RLFA backup. This does not change after applying the LFA policy because the main objective of the backup node SID feature is to reduce the label stack size of the backup tunnel.