Also known as: AD, distance
Quick Definition
A value (0–255) that tells a router how trustworthy a routing source is when multiple sources advertise the same prefix.
Administrative Distance (AD) is a Cisco concept used to rank routing protocol sources. When a router receives the same route from multiple sources (e.g., OSPF and a static route), it installs the route from the source with the lowest AD. Key AD values: Connected = 0, Static = 1, EIGRP summary = 5, EIGRP internal = 90, OSPF = 110, RIP = 120, EIGRP external = 170, Unknown/unreachable = 255.
AD is used to choose between different routing sources advertising the same prefix. It is not used to choose between routes learned from the same protocol — that uses the metric. Remember: lower AD wins.
A link-state routing protocol that calculates the best path using cost based on bandwidth.
The value a routing protocol uses to measure the desirability of a route to the same destination.
A manually configured route that does not update automatically when the network changes.
A backup static route with a higher AD than the primary dynamic route, installed only if the primary fails.
Administrative Distance (AD) is a Cisco concept used to rank routing protocol sources. When a router receives the same route from multiple sources (e.g., OSPF and a static route), it installs the route from the source with the lowest AD. Key AD values: Connected = 0, Static = 1, EIGRP summary = 5, EIGRP internal = 90, OSPF = 110, RIP = 120, EIGRP external = 170, Unknown/unreachable = 255.
AD is used to choose between different routing sources advertising the same prefix. It is not used to choose between routes learned from the same protocol — that uses the metric. Remember: lower AD wins.
Administrative Distance falls under the Routing domain of the 200-301 exam. Understanding it in context with related terms like metric and ospf is essential for answering scenario-based questions correctly.