Administrative Distance¶

A router-local preference assigned to a route source so the routing information base can choose among candidate routes to the same prefix learned by otherwise incomparable mechanisms.

Version: v2 · 2026-09-07 · History
Domain-specific #: 1244
Origin domain: computer networking
Subdomain: route selection and routing information bases
Aliases: Route preference, Route-source preference, Administrative route distance

Core Idea¶

Administrative distance is a router-local preference attached to the source of a route. It lets a routing information base choose among candidate routes to the same destination prefix when those routes were learned through mechanisms whose native metrics are not directly comparable—for example, a static route, OSPF, RIP, or BGP.

The recurring operation is:

same-prefix candidate routes + source-specific local preference + lower-is-better comparison + platform tie rules → active route or route set in the local routing table.

Cisco calls the value administrative distance; Junos documentation commonly calls it route preference and explicitly identifies that as administrative distance^[1]. The number is not a measured geographic distance, hop count, latency, or universal probability of truth. It is a policy rank defined by the router implementation and configuration. Lower values are normally preferred within the same platform's comparison procedure.

Administrative distance bridges routing protocols because their metrics have incompatible meanings. An OSPF cost, RIP hop count, EIGRP composite metric, and BGP decision process cannot be safely compared as though they shared a unit. Each source first develops candidate routing information under its own rules. The local routing-information-base process then uses administrative distance or route preference to decide which source's candidate for a prefix is eligible to become active.

The value has local significance. It is not itself advertised as an end-to-end routing attribute. Changing it changes the receiving router's own source ordering; it does not rewrite the metric carried by the protocol or require another router to adopt the same ordering.

Structural Signature¶

The abstraction has ten roles:

local routing process — the router or routing instance making the installation decision;
destination key — the same address family, routing table, and destination prefix under comparison;
candidate route sources — connected, static, or dynamic mechanisms that produced eligible routes;
source-local best route — where applicable, the candidate already selected under a protocol's native decision procedure;
administrative-distance assignment — a default or configured preference associated with protocol, source, route class, peer, or prefix according to platform capabilities;
ordering convention — normally lower numeric distance means greater local preference;
RIB selection point — the stage at which candidates from different sources compete for active installation;
tie and multipath rules — vendor- and protocol-specific handling when the primary preference does not decide uniquely;
active-route result — the route or supported equal-cost route set installed for that prefix;
reevaluation trigger — withdrawal, next-hop failure, policy change, or new candidate that causes the choice to be recomputed.

Its invariant is:

administrative distance ranks the local source of otherwise comparable same-prefix candidates; protocol metrics rank paths within their own applicable decision process, and packet forwarding later uses the installed table.

Ranges and defaults are deliberately not part of the invariant. Cisco commonly uses an 8-bit range and treats 255 as unusable, while Junos documents a 32-bit preference range^[2]. Their default values differ sharply—for example, Junos documents OSPF internal preference 10 and BGP preference 170, while Cisco documents OSPF 110, external BGP 20, and internal BGP 200^[1]. The ordinal effect is portable; the literal table is not.

What It Is Not¶

Administrative distance is not a routing metric. A protocol metric compares routes according to that protocol's model—hop count, link cost, bandwidth/delay composition, path attributes, or another rule. Administrative distance compares route sources at a local RIB boundary.

It is not longest-prefix match. After routes are installed, forwarding selects the most specific matching prefix for a packet. A /24 can beat a /16 in forwarding even when their source preferences differ, because they are not competing candidates for the same prefix.

It is not BGP LOCAL_PREF. BGP local preference is a BGP path attribute used inside an autonomous system's BGP decision process and distributed to internal BGP peers^[3]. Administrative distance is a local cross-source RIB preference and is not advertised as that attribute.

It is not an objective or portable measure of trustworthiness. Vendor documentation often uses “trust,” but the number is a configurable policy order. Low distance does not certify that route information is factually correct, secure, fresh, loop-free, or optimal.

It is not inherently a fallback path. Every source comparison uses administrative distance, while fallback behavior arises only when a less-preferred valid candidate is retained or can be reoffered after the active route disappears. A floating static route is a deliberate fallback use of the mechanism.

It is not a guarantee that only one route exists. Routers can retain inactive candidates and may install multiple paths under supported equal-cost or multipath rules. The exact RIB and FIB behavior is platform-specific.

Scope of Application¶

Administrative distance applies to IP routing implementations that maintain multiple candidate sources for a routing table. It appears in enterprise routing, service-provider routers, virtual routing and forwarding instances, network migrations, route redistribution, static/dynamic coexistence, and controlled failover designs.

The abstraction includes default source preferences, protocol-wide overrides, and finer-grained source or prefix adjustments where the platform supports them. It includes the use of a high-distance static route as a standby for a dynamically learned route. It also includes operational inspection of RIB entries that display both source distance and protocol metric.

The node does not prescribe a universal default table. Cisco IOS, IOS XR, NX-OS, Junos, EOS, FortiOS, and other systems use different names, numeric ranges, route categories, tie breakers, and configuration scopes^[4]. A statement is valid only for the platform, release, address family, and routing instance documented.

Clarity¶

Use this diagnostic sequence:

Confirm that the candidates are for the same routing table and destination prefix.
Identify how each route was learned and whether each source has already run its own best-path logic.
Read the effective local administrative distance or route preference after all policy overrides.
Prefer the lower value under the platform's documented comparison order.
If values tie, follow the platform's explicit tie or multipath rules rather than inventing a universal rule.
Distinguish the installed RIB result from the later longest-prefix forwarding lookup.
Recalculate after withdrawal, next-hop invalidation, or configuration change.

This avoids a common classroom shortcut: “the lowest AD route wins” is incomplete unless the routes compete for the same prefix in the same local table. It also avoids comparing the bracketed values in Cisco output incorrectly: in [90/3072], 90 is administrative distance and 3072 is the protocol metric; their numerical sizes are not competing on one scale^[2].

Manages Complexity¶

A router running several routing protocols receives claims produced by different algorithms and measurement units. Administrative distance reduces this heterogeneous arbitration problem to a small local ordering over sources. The router need not translate OSPF cost into RIP hops or BGP attributes into EIGRP composite metric.

The mechanism also makes staged change possible. During migration from OSPF to IS-IS, for example, an operator can run both protocols and adjust preferences so one source is active while the other is observed. A floating static route can remain less preferred during normal operation and become eligible after dynamic withdrawal.

The simplification introduces concentrated risk. A distance change can redirect many prefixes at once. If neighboring routers use inconsistent preferences, the resulting control-plane choices can produce asymmetry, suboptimal paths, loops, or black holes. Vendor documentation therefore cautions that distance changes require topology-wide reasoning even though the value is locally applied.

Abstract Reasoning¶

Administrative distance licenses ordinal predictions. If two valid same-prefix candidates reach a RIB from different sources and all earlier eligibility checks pass, lowering one candidate's effective distance below the other normally makes it the preferred source. Raising a static route's distance above a dynamic source can make the static route float; withdrawal of the preferred route then triggers reevaluation and possible standby installation.

The abstraction also supports counterfactual diagnosis. If a route with an apparently attractive protocol metric remains inactive, inspect source distance before tuning that metric. If packets use a more-specific route despite another route's lower distance, the issue is forwarding specificity, not a failure of administrative-distance comparison. If two routers make different choices from similar protocol inputs, compare their local policies and platform defaults; the value is not exchanged in routing updates.

It does not license cardinal arithmetic. A route at distance 20 is not “twice as trusted” as one at 40, and subtracting two distances has no standardized operational meaning. Only the ordering and special platform-defined endpoints matter.

Knowledge Transfer¶

Within networking, the abstraction transfers across vendors as a role pattern: source label, local ordinal preference, same-prefix arbitration, active installation, and reevaluation. Literal commands and numbers must be remapped to the target operating system.

The portable skeleton is Prioritization: competing claims receive an ordering that determines which proceeds under a constraint. Selection describes the resulting survivor set. Fallback Path explains one engineered use when a secondary route is intentionally given a worse preference. These primes do not carry routing-specific separation among protocol metrics, RIB source arbitration, and forwarding lookup.

Outside networking, “administrative distance” should not be used as a general synonym for policy priority. That borrowing would discard the routing-source and RIB semantics that make the node precise.

Examples¶

OSPF versus RIP on Cisco. Cisco documents defaults of 110 for OSPF and 120 for RIP^[2]. If both offer a valid route to the same prefix, OSPF is normally installed because 110 is lower. This says nothing about comparing an OSPF link cost directly with a RIP hop count.

OSPF versus BGP on Junos. Junos documents internal OSPF route preference 10 and BGP preference 170. The literal numbers differ from Cisco, but the lower-is-preferred source ordering has the same structural role.

Floating static route. A static route normally has a highly preferred default. Configuring it with a distance worse than the active dynamic route prevents it from displacing normal routing. When the dynamic candidate is withdrawn and the static next hop remains valid, RIB reevaluation can install the static route as backup.

Longest-prefix nonexample. A packet matches an installed /16 and /24. The /24 is used because it is more specific. Administrative distance was relevant when candidates for each individual prefix competed for installation, not when the packet chose between different prefix lengths.

Metric nonexample. OSPF knows two paths to the same prefix. Its own cost calculation selects or multipaths them before or within the protocol-to-RIB interaction. Administrative distance does not choose between OSPF cost 10 and OSPF cost 20 merely because both are numbers.

Misconfiguration. An operator lowers a redistributed route source on only part of a network. Each router follows its local ordering correctly, yet their inconsistent choices create a loop. Local correctness therefore does not imply global routing safety.

Structural Tensions¶

Local policy vs. global outcome. The number is local, but interacting local choices shape paths. Diagnostic: model the entire redistribution and failover topology before changing it.
Portability vs. literal defaults. The mechanism recurs, while numbers and ranges differ. Diagnostic: use current vendor documentation, not a memorized cross-vendor table.
Source preference vs. path quality. AD makes incomparable protocols arbitrable but can prefer a poorer path. Diagnostic: identify whether the policy intends source authority or performance.
Primary vs. standby. Keeping a backup less preferred enables failover, but a bad next hop can still be installed later. Diagnostic: verify candidate validity and failure detection as well as distance.
Single route vs. multipath. The simple model returns one active source; platforms may retain or install multiple paths. Diagnostic: inspect tie and ECMP support.
Simple scalar vs. configuration scope. A displayed number looks global, yet policy can vary by protocol, peer, prefix, route class, or instance. Diagnostic: trace the effective value after all overrides.
Control plane vs. forwarding plane. RIB source selection precedes FIB lookup. Diagnostic: separate same-prefix installation from longest-prefix packet forwarding.

Structural–Framed Character¶

Administrative Distance is strongly structural within routing. Its source-ranking, local comparison, installation, and reevaluation roles recur across major platforms and support direct configuration and troubleshooting predictions.

It remains domain-specific because the identity depends on route sources, protocol-native metrics, a RIB, an active-route state, and the separation between control-plane installation and forwarding. Removing these leaves generic prioritization, already represented by a prime. It is therefore not a prime abstraction.

Structural Core vs. Domain Accent¶

The structural core is an ordinal policy that reconciles claims produced by incompatible evaluators: assign each source a local priority and let the best-ranked eligible claim proceed.

The domain accent is essential: destination prefixes, routing protocols, static and connected sources, native route metrics, RIB installation, inactive candidates, and FIB longest-prefix lookup. The special lower-is-better convention and vendor default tables also belong to the domain frame.

The abstraction ceases to be administrative distance when it ranks paths inside one routing protocol, compares different destination prefix lengths during forwarding, or becomes an advertised inter-router attribute.

The minimal prospective parent is Prioritization. Several eligible sources make competing claims on the active route slot for a prefix; a configured value orders those claims and determines which is installed first.

Selection describes the resulting active route or multipath set. Fallback Path is instantiated by the floating-static use case but is not the general parent. Preference is related at the abstract ordering level, while Local Autonomy & Tiered Escalation helps explain why platform-local policy can differ across routers.

Prospective DAG placement:

parent: prime:prioritization type: composition flavor: instantiates qualifier: strict

Relationships to Other Abstractions¶

Current abstraction Administrative Distance Domain-specific

Parents (1) — more general patterns this builds on

Administrative Distance is a kind of Prioritization Prime

The minimal prospective parent is Prioritization.

Hierarchy paths (3) — routes to 3 parentless roots

Administrative Distance → Prioritization → Optimization

Show alternative paths (2)

Neighborhood in Abstraction Space¶

Administrative Distance sits in a sparse region of the domain-specific corpus (89^th percentile for distinctiveness): few abstractions share its structure, so a faithful description tends to retrieve it precisely.

Family — Unclustered & Miscellaneous (1565 abstractions)

Nearest neighbors

Computed from structural-signature embeddings · 2026-09-08

Not to Be Confused With¶

Do not confuse administrative distance with routing-protocol metric, BGP weight, BGP LOCAL_PREF, MED, AS-path length, OSPF cost, RIP hop count, ECMP hashing, or longest-prefix match. Each occupies a different stage or comparison domain.

Do not assume Cisco defaults on Junos or another vendor. Do not infer that lower distance measures actual security or reliability. Do not assume the value is propagated to neighbors. Do not assume tied distances have one universal outcome.

Finally, do not equate administrative distance with a floating static route. The latter is a configuration pattern that deliberately uses administrative distance to realize backup behavior.

References¶

[1] Juniper Networks, Inc. “Understanding Route Preference Values (Administrative Distance)”. Junos OS Documentation (Routing Overview), Junos OS Release 26.2. Juniper's own documentation, whose title and text gloss the Junos term 'route preference' as administrative distance; the Cisco half of the naming contrast rests on Cisco's document, not this one. Juniper's default route-preference table, source of the Junos constants quoted here (OSPF internal 10, BGP 170); the Cisco figures in the same sentence come from Cisco's default administrative-distance table. registry ↩a ↩b

[2] Cisco Systems, Inc. “Understand Administrative Distance”. Cisco Support Documentation (Document ID 15986). Cisco's administrative-distance document, which publishes the default distance table and states that 255 marks a route the router will not install; it states no explicit range, and the Junos 32-bit figure comes from Juniper's documentation. Cisco's administrative-distance document, which states that bracketed routing-table values read as [administrative distance/metric] and works the reading through an EIGRP example. Cisco's default administrative-distance table, which lists OSPF at 110 and RIP at 120. registry ↩a ↩b ↩c

[3] Rekhter, Yakov, Li, Tony, and Hares, Susan. A Border Gateway Protocol 4 (BGP-4). Internet Engineering Task Force (IETF), 2006. The BGP-4 specification, which defines LOCAL_PREF as a well-known attribute carried in UPDATE messages to internal peers only and consumed as degree of preference in the Decision Process. registry ↩

[4] Juniper Networks, Inc. “Route Preferences Overview”. Junos OS Documentation (Routing Overview), Junos OS Release 26.2. Junos OS's route-preference overview, which documents a 32-bit range, lowest-preference-wins selection and Junos-specific tie breakers (preference2, color, color2); it evidences the cross-platform divergence by example only, not the full list of platforms named. registry ↩