What is Administrative Distance?

Understanding Administrative Distance

Administrative distance (AD) is a value a router uses to rank the trustworthiness of different routing information sources. When multiple routing protocols or methods offer a path to the same destination, the router installs the route from the source with the lowest administrative distance. It is a tie-breaker between protocols, not a measure of path length.

Each routing source has a default AD. On Cisco routers, directly connected interfaces are 0, static routes are 1, eBGP is 20, internal EIGRP is 90, OSPF is 110, RIP is 120, and external EIGRP is 170; a route with AD 255 is considered unusable and is never installed. The router compares AD first to choose which protocol's route enters the routing table, and only within a single protocol does it then compare that protocol's metric (such as OSPF cost or hop count). Administrators can override defaults to engineer failover or preference.

Administrative distance matters for both reliability and security. It controls which routing source wins, so misconfiguring it - or an attacker injecting routes via a more-trusted protocol - can redirect traffic, enabling interception or denial of service. Correctly tuned AD provides deterministic failover (for example, preferring a dynamic route but falling back to a backup static "floating" route) and helps prevent less-trusted or rogue information from overriding legitimate paths.

For example, a router learns the path to 10.0.0.0/8 from both OSPF (AD 110) and a static route (AD 1). Because the static route has the lower AD, the router installs it and ignores the OSPF route while the static path is valid. An administrator wanting the static route to serve only as backup would configure it as a floating static with an AD higher than 110, so OSPF is preferred and the static route activates only if OSPF withdraws the route.