1. Introduction

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

1.  Introduction

   IGP protocols historically computed the best paths over the network
   solely based on the IGP metric assigned to the links.  An IGP Flex-
   Algorithm as specified in [RFC9350] allows IGPs to compute
   constraint-based paths.  Several mechanisms to include or exclude the
   link during the Flex-Algorithm path calculation have been defined
   already:

   *  link inclusion or exclusion based on the presence of a specific
      Admin Group(s) [RFC9350]

   *  link exclusion based on the presence of a specific Shared Risk
      Link Group (SRLG) [RFC9350]

   *  link exclusion based on minimum bandwidth [RFC9843]

   *  link exclusion based on maximum delay [RFC9843]

   This document extends IGP Flex-Algorithm with additional constraints
   for inclusion or exclusion of links in the path based on Admin Groups
   associated with the reverse direction of the Shortest Path First
   (SPF) computation.

   This document updates [RFC9350] and [RFC9843] by creating the new
   IANA registry that specifies the ordered set of rules that are used
   to prune links from the topology during the Flex-Algorithm path
   computation (refer to Section 12.3.1).

   The term OSPF in this document is used to cover both OSPFv2 and
   OSPFv3 protocols.