Routing Loop¶
A destination-specific cycle in installed router next hops that permits a packet to revisit a router instead of reaching its destination.
Core Idea¶
A routing loop is a cycle in the forwarding choices installed for one destination or prefix. Following the chosen next hop from router to router eventually returns to a router already visited, so a packet for that destination can revisit it instead of reaching the destination. The loop can be identified from the forwarding entries even without an observed packet trace. An individual packet may stop when its hop limit expires, while the entries causing the loop remain.[ref-7dc9671703d9][ref-1ff0bdb53e73][^ref-0f2655573cff]
Scope of Application¶
Use the test where installed router next hops can be traced for one destination or prefix. These are the two source-backed settings here; another setting needs its own forwarding-state evidence:
- RIP routing: RFC 2453 gives a possible A→B→C→A route cycle for one destination.[^ref-7dc9671703d9]
- Link-state convergence: RFC 8333 gives a conditional S→B→S microloop after an S–D link failure, if S updates its next hop before B does.[^ref-1ff0bdb53e73]
Clarity¶
Hold the destination and time fixed, then inspect the installed next hop at each router. Physical links forming a ring do not establish a routing loop; the installed choices might follow an acyclic route. Advertised or computed routes alone also do not prove which forwarding entries were in effect. A cycle assembled from choices for different destinations is not one packet's routing loop. The cited RFC patterns are worked cases, not reports of observed outages or packet traces; another delivery failure needs its own forwarding-state evidence.[ref-7dc9671703d9][ref-1ff0bdb53e73][^ref-0f2655573cff]
Manages Complexity¶
The useful ledger is one destination → installed next-hop relation → cyclic closure → possible packet revisit. It separates recognition of the loop from the protocol details explaining how it formed. Split horizon and count-to-infinity matter in the RIP case, while update ordering matters in the link-state example. Those mechanisms are not requirements for every routing loop.[ref-7dc9671703d9][ref-1ff0bdb53e73]
Abstract Reasoning¶
Starting at a router, follow its installed next hop for a fixed destination, then repeat at the next router. Reaching the destination or leaving the examined path without repetition does not demonstrate a loop. Revisiting a router before arrival identifies the closed segment. In RFC 8333's worked state, S→B paired with B→S closes that relation; if B has already changed its entry away from S, that particular loop is absent.[^ref-1ff0bdb53e73]
Knowledge Transfer¶
The same four-role test applies to the RIP and link-state cases despite their different route-update mechanisms. The prerequisite Relation is the destination-specific association between routers established by installed next-hop entries; composing it reveals a return path. An acyclic relation can exist without a routing loop. Generic Cycle shares closure, but the live Prime's additional commitments are not established by these routing examples. A cyclic project dependency may be a cycle without being a routing loop.[ref-7dc9671703d9][ref-1ff0bdb53e73]
Relationships to Other Abstractions¶
Current abstraction Routing Loop Domain-specific
Parents (1) — more general patterns this builds on
-
Routing Loop presupposes Relation Prime
A routing loop presupposes a destination-specific installed next-hop relation whose composition closes back to a router.
Hierarchy path (1) — routes to 1 parentless root
- Routing Loop → Relation
Neighborhood in Abstraction Space¶
Routing Loop sits in a sparse region of the domain-specific corpus (97th percentile for distinctiveness): few abstractions share its structure, so a faithful description tends to retrieve it precisely.
Family — Unclustered & Miscellaneous (2721 abstractions)
Nearest neighbors
- Administrative Distance — 0.76
- In-Place Adjacent-Block Rotation — 0.76
- A-star algorithm — 0.76
- Floyd's Cycle-Finding Algorithm — 0.76
- Hop-by-hop transport — 0.75
Computed from structural-signature embeddings · 2026-10-17
References¶
[^ref-7dc9671703d9]: Gary Malkin, RFC 2453, “RIP Version 2” (1998), especially §3.4.4. Official Standards Track RFC; its three-router loop is a possible route pattern, not an observed incident. [^ref-1ff0bdb53e73]: Stéphane Litkowski, Bruno Decraene, Clarence Filsfils, and Pierre Francois, “Micro-loop Prevention by Introducing a Local Convergence Delay,” RFC 8333 (2018), especially §1 Figure 1 and §6. https://www.rfc-editor.org/rfc/rfc8333.html. Official RFC; the S/B loop is conditional on forwarding update order. [^ref-0f2655573cff]: Mike Shand and Stewart Bryant, “A Framework for Loop-Free Convergence,” RFC 5715 (2010), especially §2. https://www.rfc-editor.org/rfc/rfc5715.html. Official Informational RFC; its discussion of transient microloops does not make every routing loop transient.