Compare administrative distances across routing protocols to determine the preferred route.
Administrative distance is a per-source trust ranking (0-255, lower is more trusted) that a router uses to choose between multiple routes to the exact same destination prefix learned from different routing sources — it only applies when two routes have identical prefix length; a more specific prefix always wins via longest prefix match regardless of AD. Connected interfaces (AD 0) and static routes (AD 1) are trusted most, since they are directly configured by an administrator, while dynamically learned protocols are ranked by how reliable their information is generally considered, with iBGP (200) and unknown sources (255) trusted least.
Route source preference
lower administrative distance wins, when prefix lengths are equal
Connected = 0, Static = 1, EIGRP summary route = 5, eBGP = 20, EIGRP internal = 90, IGRP = 100, OSPF = 110, IS-IS = 115, RIP = 120, EIGRP external = 170, iBGP = 200, and an unknown/unreachable source = 255. Lower values are more trusted and preferred when multiple sources offer a route to the identical destination prefix.
Routes learned from an external autonomous system (eBGP, AD 20) are generally treated as more authoritative for reaching that external destination than routes redistributed internally via iBGP (AD 200), which is often carrying information that originated elsewhere in the network and could be less direct or more prone to internal misconfiguration.
Yes, in two ways: administrative distance only decides between routes of the exact same prefix length from different sources — a route with a longer (more specific) prefix always wins regardless of AD. And metric (like OSPF cost or EIGRP composite metric) only matters for choosing between multiple routes within the same protocol/source, not across different sources, where AD applies instead.