A cooperative routing relationship between two or more ASes under common administrative control or explicit corporate affiliation in which the ASes exchange routes and traffic as if part of a single administrative domain, often allowing mutual route propagation and internal routing policies without commercial settlement between them.

Principle

Sibling ASes are operationally treated as a unified administrative entity for routing: they commonly export each other's prefixes broadly and accept routes with internal preference, so external observers see them behave similarly to a single AS for many routing decisions.

Demonstration

Illustrative scenario → A company operates two ASes for engineering or regulatory reasons. The ASes configure bilateral full‑route exchange and consistent import/export filters so that either AS can forward traffic for the other's customers and the outside world reaches their prefixes through either path as if a single network.

Misapplication

Mistaken interpretation: presuming sibling status implies legal merger or identical business responsibility. Semantic error: conflating technical routing cooperation with corporate, legal or regulatory identity; sibling routing permissions do not by themselves change contractual liabilities.

Consequence

Simplifies traffic engineering and redundancy inside the affiliated domain, enables aggregation or multihoming strategies, and reduces need for transit between the affiliated ASes; externally, siblings may present a combined routing footprint, affecting visibility and routing policy decisions by peers/providers.

Reversal

Qualification: affiliation can change (corporate divestiture, regulatory constraints), or siblings may intentionally apply filters or limits for commercial, legal or operational reasons—sibling behavior is contingent on administrative control and can be altered by organizational change.

Boundary

Clearly within: ASes under the same administrative control that exchange full or near‑full routing without settlement. Boundary case: affiliated ASes that maintain separate commercial relations and only partially exchange routes. Clearly outside: independent ASes that peer or buy transit but are not administratively linked.

Semantic Tension

Operational unity and routing simplicity ↔ legal, regulatory and commercial separation: treating ASes as siblings eases network operations but may conflict with legal or contractual separations that require distinct handling.

Synthesis

A Sibling Relationship collapses the technical distinction between ASes for routing purposes while leaving legal and commercial identities distinct; effective analysis requires separating operational routing behavior from corporate or regulatory status.