Summary

  • RFC 9929 adds U and UP flags to IS-IS and OSPF so an ABR or ASBR can announce that a component prefix hidden beneath a summary is unreachable, including whether the reason is planned.
  • The announcement is a temporary, configurable input. The receiver decides what consumes it and what happens next; its disappearance cannot be treated as proof that reachability returned.

Summarization buys scale by withholding detail. A larger route represents several contiguous prefixes, reducing the state carried beyond an area or domain. The trade becomes operationally sharp when the summary remains reachable but one component does not. A remote node still sees the outer promise while the lost component state has been suppressed.

RFC 9929 supplies a narrow exception. An ABR or ASBR may originate an Unreachable Prefix Announcement when a previously reachable summarized prefix becomes unreachable. It may also do so for planned maintenance signals or after a configured metric threshold is crossed. Generation and propagation are optional, configuration-controlled and recommended to be disabled by default. Prefix filters and origination limits can further narrow the surface.

The new flags are not free-standing assertions. U means unreachable; UP adds that the reason is planned. UP without U is invalid and must be ignored. In IS-IS, the flags count only with a metric above 0xFE000000, building on the route-calculation exclusions in RFC 5305 and RFC 5308. In OSPFv2, the metric must be LSInfinity from RFC 2328. OSPFv3 also requires the NU-bit defined through RFC 5340. The planned context can arise from maintenance mechanisms such as those discussed in RFC 6987 and RFC 8770, but UP is not change approval or proof that traffic drained successfully.

UPA is explicitly an event-driven transition, not durable state. The originator must withdraw it when the reason ceases. It should withdraw it after a suitable lifetime even when the reason persists, so stale announcements do not remain indefinitely in every link-state database. That creates a critical evidentiary asymmetry: presence can report an observed transition under the configured rules; absence cannot report restoration. RFC 9929 requires a consuming application to verify remote reachability through its own mechanism.

Propagation preserves the same boundary. Older nodes can flood the advertisement without using it. A supporting boundary router may carry it across areas, levels or domains while preserving the unreachable metric and omitting cost to the originator. Delivery still does not equal interpretation. Processing is optional at the receiver, and the receiver chooses which application, if any, is notified. BGP PIC Edge appears only as an example. The RFC does not authorize a forwarding change or certify its result.

An operator therefore needs five linked receipts rather than one green label: the component and covering summary; the originator's metric, flags, policy, time and lifetime; each propagation boundary; the receiver's validation, configuration and selected consumer; then the local action and independent packet or endpoint evidence. RFC 8174 helps keep the specification's uppercase obligations precise. Heng Lu's minimum-specification rule keeps the shared signal equally precise: publish the smallest interoperable fact, leave the later decision local, and prove operational reality where code actually acts.

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