Administrative distance (AD) of OSPF: What is the default administrative distance value used by OSPF routes in Cisco IOS?

Introduction / Context:
Administrative distance (AD) is a Cisco metric that ranks the trustworthiness of a route source. When multiple protocols know a path to the same destination, the route with the lowest AD is installed in the routing table.

Given Data / Assumptions:

• Default Cisco AD values are assumed.
• Protocol in question: OSPF (an open standard link-state IGP).

Concept / Approach:

Common defaults: Connected 0, Static 1, eBGP 20, EIGRP internal 90, IGRP 100, OSPF 110, IS-IS 115, RIP 120, EIGRP external 170, iBGP 200. Knowing these helps predict which protocol’s route will win in a tie for prefix length and metric.

Step-by-Step Solution:

Verification / Alternative check:

show ip route marks each route source with a code; the installed route aligns with the lowest AD among candidates of equal prefix specificity.

Why Other Options Are Wrong:

90 is EIGRP internal; 100 is IGRP; 120 is RIP. These are not the OSPF default.

Common Pitfalls:

Confusing metric values (cost, hop count) with AD; assuming AD varies per vendor—defaults can differ outside Cisco.

Final Answer:

110