What Is Administrative Distance? (AD Table)

WHAT IS ADMINISTRATIVE DISTANCE?_

When two routing sources both claim to know the way, the router needs a tiebreaker. That's administrative distance.

[ ▶ SHORT ANSWER ] Administrative distance (AD) is a number from 0 to 255 that ranks how trustworthy a route's source is. When a router learns the same destination network from more than one source — say both OSPF and a static route — it installs the one with the lowest AD into the routing table. Key values to memorize: directly connected = 0, static route = 1, EIGRP (internal) = 90, OSPF = 110, RIP = 120, and an unreachable/unusable route = 255. AD only breaks ties between different sources; within a single protocol, metric decides the best path. Lower AD always wins.

A router often hears about the same network from multiple places: a connected interface, a static route you typed, and a routing protocol like OSPF. It can only install one of them as the best route. Administrative distance is the tiebreaker — a trust ranking where lower is better.

The AD table (memorize this)

Connected interface ....... 0
Static route .............. 1
EIGRP (internal) .......... 90
OSPF ...................... 110
IS-IS ..................... 115
RIP ....................... 120
External EIGRP ............ 170
Unusable (never installed)  255

How it's used

Say a router learns 10.5.0.0/16 from both OSPF (AD 110) and a static route (AD 1). The static route wins and goes in the table; the OSPF route is held in reserve. If the static route's next hop goes away, OSPF's version can take over.

AD vs metric — don't confuse them

AD compares different sources. Metric compares routes within the same protocol (OSPF cost, EIGRP composite, RIP hop count). The router first picks the source by lowest AD, then — among that source's routes — picks the best by metric.

Floating static routes

You can deliberately raise a static route's AD above a protocol's (e.g. set it to 200) so it only installs when the protocol-learned route disappears. That's a floating static — a clean backup path, and a favourite exam scenario.

★ RELATED QUESTIONS

[ Does lower or higher administrative distance win? ] Lower wins. A lower AD means a more trusted source, so the router installs that route. Connected (0) beats static (1), which beats EIGRP (90), which beats OSPF (110).

[ What is the difference between administrative distance and metric? ] AD chooses between different route sources (e.g. OSPF vs static). Metric chooses the best path within a single protocol (e.g. lowest OSPF cost). AD is checked first, then metric.

[ What does an administrative distance of 255 mean? ] 255 means the route is considered unreachable and will never be installed in the routing table. It's used to effectively disable a route.

WHAT IS ADMINISTRATIVE DISTANCE?_

The AD table (memorize this)

How it's used

AD vs metric — don't confuse them

Floating static routes

★ RELATED QUESTIONS

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