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4. Supporting UPA in OSPF

This section preserves the RFC text for Unreachable Prefix Announcement (UPA), including IS-IS and OSPF signaling, U-Flag and UP-Flag semantics, propagation rules, IANA assignments, and security considerations.

4.  Supporting UPA in OSPF

   Appendix B of [RFC2328] defines the following architectural constant
   for OSPFv2:

   |  LSInfinity
   |     The metric value indicating that the destination described by
   |     an LSA is unreachable.  Used in summary-LSAs and AS-external-
   |     LSAs as an alternative to premature aging (see Section 14.1).
   |     It is defined to be the 24-bit binary value of all ones:
   |     0xffffff.

   Appendix B of [RFC5340] states:

   |  Architectural constants for the OSPF protocol are defined in
   |  Appendix B of [OSPFV2].

   indicating that these same constants are applicable to OSPFv3.

   [RFC2328], Section 14.1 also describes the usage of LSInfinity as a
   way to indicate loss of prefix reachability:

   |  Premature aging can also be used when, for example, one of the
   |  router's previously advertised external routes is no longer
   |  reachable.  In this circumstance, the router can flush its AS-
   |  external-LSA from the routing domain via premature aging.  This
   |  procedure is preferable to the alternative, which is to originate
   |  a new LSA for the destination specifying a metric of LSInfinity.

   In addition, the NU-bit is defined for OSPFv3 [RFC5340].  Prefixes
   having the NU-bit set in their PrefixOptions field are not included
   in the routing calculation.

   UPA in OSPFv2 is supported for prefix reachability advertised via
   OSPFv2 Summary-LSA [RFC2328], AS-external-LSAs [RFC2328], Not-So-
   Stubby Area (NSSA) AS-external-LSA [RFC3101], and OSPFv2 IP Algorithm
   Prefix Reachability Sub-TLV [RFC9502].

   UPA in OSPFv3 is supported for prefix reachability advertised via
   OSPFv3 E-Inter-Area-Prefix-LSA [RFC8362], E-AS-External-LSA
   [RFC8362], E-Type-7-LSA [RFC8362], and SRv6 Locator LSA [RFC9513].

   For prefix reachability advertised via Inter-Area-Prefix-LSA
   [RFC5340], AS-External-LSA [RFC5340], NSSA-LSA [RFC5340], UPA is
   signaled using their corresponding extended LSAs.  This requires
   support of the OSPFv3 Extended LSAs in a sparse mode as specified in
   Section 6.2 of [RFC8362].

4.1.  Advertisement of UPA in OSPF

   If an ABR or ASBR advertises UPA in an advertisement of an inter-area
   or external prefix inside OSPFv2 or OSPFv3, then it MUST set the age
   to a value lower than MaxAge and set the metric to LSInfinity.

   UPA flooding inside the area follows the existing standard procedures
   defined by OSPFv2 [RFC2328] and OSPFv3 [RFC5340].

4.2.  Signaling UPA in OSPF

   A prefix can be advertised (in OSPFv2) with the LSInfinity metric or
   (in OSPFv3) with the NU-bit set in PrefixOptions, for various
   reasons.  While OSPFv2 and OSPFv3 specify the treatment of the
   LSInfinity metric and the NU-bit, explicit signaling is required to
   distinguish UPA from other scenarios using the LSInfinity metric or
   NU-bit.

   OSPFv2 and OSPFv3 Prefix Extended Flags Sub-TLVs been defined in
   [RFC9792] for advertising additional prefix attribute flags in OSPFv2
   and OSPFv3.

   Two new bits in the Prefix Attribute Flags Sub-TLV are defined:

   U-Flag:  Unreachable Prefix Flag (bit 0).  When set, it indicates
      that the prefix is unreachable.

   UP-Flag:  Unreachable Planned Prefix Flag (bit 1).  When set, this
      flag indicates that the prefix is unreachable due to a planned
      event (e.g., planned maintenance).

   The originating node MUST NOT set the UP-flag without setting the
   U-flag.

   The receiving node MUST ignore the UP-flag in the advertisement if
   the U-flag is not set.

4.2.1.  Signaling UPA in OSPFv2

   The OSPFv2 Prefix Extended Flags Sub-TLV [RFC9792] is a sub-TLV of
   the OSPFv2 Extended Prefix TLV [RFC7684].

   The prefix that is advertised with U-Flag or UP-Flag MUST have the
   metric set to a value LSInfinity.  If the prefix metric is not equal
   to LSInfinity, both of these flags MUST be ignored.  Therefore, for
   the prefixes in default algorithm 0 that are advertised with U-Flag,
   or Up-Flag, it is REQUIRED to advertise the unreachable prefix in the
   base OSPFv2 LSA, e.g., e.g., OSPFv2 Summary-LSA [RFC2328], or AS-
   external-LSAs [RFC2328], or NSSA AS-external LSA [RFC3101].

4.2.2.  Signaling UPA in OSPFv3

   OSPFv3 Prefix Extended Flags Sub-TLV is defined as a sub-TLV of the
   following OSPFv3 TLVs that are defined in [RFC8362]:

   *  Intra-Area-Prefix TLV

   *  Inter-Area-Prefix TLV

   *  External-Prefix TLV

   The prefix that is advertised with U-Flag or UP-flag MUST have the
   metric set to a value LSInfinity.  For default algorithm 0 prefixes,
   the LSInfinity MUST be set in the parent TLV.  For IP Algorithm
   Prefixes [RFC9502], the LSInfinity MUST be set in OSPFv3 IP Algorithm
   Prefix Reachability sub-TLV.  If the prefix metric is not equal to
   LSInfinity, both of these flags MUST be ignored.

   The prefix that is advertised with U-Flag or UP-Flag MUST have the
   NU-bit set in the PrefixOptions of the parent TLV.  If the NU-bit in
   PrefixOptions of the parent TLV is not set, both of these flags MUST
   be ignored.

4.3.  Propagation of UPA in OSPF

   OSPF ABRs, which would be responsible for propagating UPA
   advertisements into other areas, need to recognize such
   advertisements.  Failure to do so would prevent UPA from reaching the
   routers in the remote areas.

   Advertising prefix reachability between OSPF areas assumes prefix
   reachability in a source area.  Such a requirement of reachability
   MUST NOT be applied for UPAs, as they are propagating unreachability.

   OSPF ABRs or ASBRs MAY advertise received UPAs between connected
   areas or domains.  When doing so, the original LSInfinity metric
   value in UPA MUST be preserved.  The cost to reach the originator of
   the received UPA MUST NOT be considered when readvertising the UPA to
   connected areas.
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