1. Introduction

この節では Unreachable Prefix Announcement (UPA) の RFC 原文を保持し, IS-IS and OSPF signaling, U-Flag and UP-Flag semantics, propagation rules, IANA assignments, security considerations を含めます.

1.  Introduction

   Link-state Interior Gateway Protocols (IGPs) like Intermediate System
   to Intermediate System (IS-IS) [ISO10589], Open Shortest Path First
   version 2 (OSPFv2)) [RFC2328], and Open Shortest Path First version 3
   (OSPFv3) [RFC5340] are primarily used to distribute routing
   information between routers belonging to a single Autonomous System
   (AS) and to calculate the reachability for IPv4 or IPv6 prefixes
   advertised by the individual nodes inside the AS.  Each node
   advertises the state of its local adjacencies, connected prefixes,
   capabilities, etc.  The collection of these states from all the
   routers inside the area form a Link State Database (LSDB) that
   describes the topology of the area and holds additional state
   information about the prefixes, router capabilities, etc.

   The growth of networks running a link-state routing protocol results
   in the addition of more state, which leads to scalability and
   convergence challenges.  The organization of networks into levels/
   areas and IGP domains helps limit the scope of link-state information
   within certain boundaries.  However, the state related to prefix
   reachability often requires propagation across a multi-area/level
   and/or multi-domain IGP network.  IGP summarization is a network
   engineering technique for combining multiple smaller, contiguous IP
   networks into a single, larger summary route.  Techniques such as
   summarization have been used to address the scaling challenges
   associated with advertising prefix state outside of the local area/
   domain.  However, this results in suppression of the individual
   prefix state that is useful for triggering fast-convergence
   mechanisms outside of the IGPs -- e.g., Border Gateway Protocol (BGP)
   Prefix-Independent Convergence (PIC) [BGP-PIC].

   Similarly, when a router needs to be taken out of service for
   maintenance, the traffic is drained from the node before taking it
   down.  This is typically achieved by setting the OVERLOAD bit
   together with using a high metric for all prefixes advertised by the
   node in IS-IS.  The mechanisms available for that purpose are (in
   OSPFv2) using the cost of MaxLinkMetric for all non-stub links in the
   router-LSA [RFC6987] or using the H-bit [RFC8770], or (in OSPFv3
   [RFC5340]) using the R-bit.

   When prefixes from such nodes are summarized by an Area Border Router
   (ABR) or Autonomous System Boundary Router (ASBR), nodes outside of
   the area or domain are unaware of these summarized prefixes becoming
   unreachable.  This document proposes protocol extensions to carry
   information about such prefixes in a backward-compatible manner.

   This document does not define how to advertise a prefix that is not
   reachable for routing.  That has been defined for IS-IS in [RFC5305]
   and [RFC5308], for OSPFv2 in [RFC2328], and for OSPFv3 in [RFC5340].

   This document defines a method to signal a specific reason for which
   the prefix was advertised with the metric that excludes it from the
   route calculation.  This is done to distinguish it from any other
   possible cases, where such metric advertisement may be used.

   IGPs typically only advertise the reachability of the prefix.  A
   prefix that was previously advertised as reachable is made
   unreachable just by withdrawing the previous advertisement of the
   prefix.  Some of the use cases mentioned earlier in this section
   require that unreachability be signaled for a prefix for which the
   reachability was not explicitly signaled previously, because it was
   covered by the reachability of the summary prefix.

   This document defines two new flags in IS-IS, OSPFv2, and OSPFv3.
   These flags provide the support for advertising prefix
   unreachability, together with the reason for which the unreachability
   is advertised.  The functionality being described is called
   Unreachable Prefix Announcement (UPA).

   This document also defines how the UPA is propagated across IS-IS
   levels and OSPF areas.

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

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.