Administrative distance (AD) expresses how preferred or trustworthy a route source is. Routers compare AD when different routing protocols offer routes to the same destination; the lower value wins. A metric instead compares routes learned through the same protocol, so metric values from different protocols are not directly comparable. section 17.3 section 17.3.2
Key Cisco IOS defaults and selection rules include:
- Connected routes have AD 0, static routes 1, EIGRP 90, OSPF 110, RIP 120, and unusable routes 255. section 17.3.2
- A route with AD 255 is unusable and is removed from the routing table. section 17.3.2
- AD selects which routes enter the routing table; the longest-prefix rule selects a route already in the table for packet forwarding. section 17.3.3
EIGRP versus OSPF
R1 learns 10.0.0.0/24 through EIGRP with metric 3584 and OSPF with metric 4. EIGRP wins because its AD is 90, lower than OSPF’s 110; the metric values are irrelevant across protocols. The routing table displays the selected route as D 10.0.0.0 [90/3584]. A floating static route can be configured with a higher AD, such as 111, so it remains unused while the OSPF route is available and enters the table after that route disappears.
