3. Use Case Example

This section preserves the RFC text for IGP Flex-Algorithm reverse affinity constraints, including FAERAG, FAD, IS-IS and OSPF Sub-TLVs, reverse Administrative Groups, IANA registrations, and path pruning rules.

Original RFC Text

3.  Use Case Example

   The Flexible Algorithm Definition (FAD) can specify Extended
   Administrative Groups that are used by the operator to include or
   exclude links during the Flex-Algorithm path computation [RFC9350].
   These link Extended Administrative Groups are checked in the path
   direction of the SPF computation, e.g., in the direction from the
   root vertex toward vertices of increasing distance.

   In certain scenarios, it is beneficial to evaluate the Extended
   Administrative Groups associated with the reverse direction of a
   link, rather than solely those in the direction of path computation.
   Consider a point-to-point link represented as a pair of directed
   edges between two nodes, A and B.  When computing a path from A to B,
   issues such as input errors on the link, detectable only at the
   receiving node B, may be operationally significant.  An operator
   might monitor metrics like Cyclic Redundancy Check (CRC) errors or
   other input-related faults at node B and apply thresholds over a
   defined observation period.  If such a threshold is exceeded, node B
   may locally assign specific Extended Administrative Groups to the
   link in the direction from B to A.

   To accommodate this operational intent, the Flex-Algorithm can be
   extended to inspect the Extended Administrative Groups of the
   reverse-direction edge (from B to A) when evaluating the forward-
   direction edge (from A to B) during path computation.  This enables
   the exclusion of links from the computed topology based on conditions
   detected at the far end of the link, improving network reliability
   and policy control.