Administrative Distance Calculator
Compare administrative distance values across routing sources to determine which route a router will trust.
Inputs
Preferred (Trusted) Route
OSPF (AD 110) — lower AD wins
Source A Administrative Distance
110
Source B Administrative Distance
120
AD Difference
10
Step by step
Values used
Route Source A = OSPF (110); Route Source B = RIP (120)
Route source preference
lower administrative distance wins, when prefix lengths are equal
Preferred (Trusted) Route
= OSPF (AD 110) — lower AD wins
Source A Administrative Distance
= 110
Source B Administrative Distance
= 120
AD Difference
= 10
How it works
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.
Formula
Route source preference
lower administrative distance wins, when prefix lengths are equal
- AD(r)
- Administrative distance of the source that learned route r
Frequently Asked Questions
What are the standard Cisco administrative distance values?
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.
Why does eBGP have a much lower AD than iBGP?
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.
Does administrative distance ever get overridden by a metric or prefix length?
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.