In Cisco routing, what is the default administrative distance value assigned to OSPF routes?

Introduction / Context:
Administrative distance is a Cisco specific concept used to rank the trustworthiness of different route sources. When multiple routing protocols or static routes offer paths to the same destination, administrative distance determines which route is preferred. Each protocol has a default value. Knowing the administrative distance of OSPF is important for predicting how it will interact with other protocols such as EIGRP, RIP, and static routes.

Given Data / Assumptions:

• You are using Cisco IOS, which assigns default administrative distance values to route sources.
• OSPF is configured and running on the router.
• No custom administrative distance values have been configured for OSPF routes.
• You want the default numeric distance that Cisco uses for OSPF.

Concept / Approach:
Administrative distance is a simple integer where lower values indicate more trusted routes. For example, directly connected networks have distance 0, static routes typically have 1, and EIGRP internal routes have distance 90 by default. OSPF is assigned a default administrative distance of 110. When a router learns about the same destination from multiple protocols, the route with the lowest administrative distance is placed in the routing table, assuming metrics and reachability are otherwise valid.

Step-by-Step Solution:
Step 1: Recall the most common administrative distances used in Cisco IOS: 0 for connected, 1 for static, 90 for EIGRP internal, 110 for OSPF, and 120 for RIP.Step 2: Focus on OSPF specifically and remember that it is generally considered more trustworthy than RIP but less than EIGRP internal routes and static routes by default.Step 3: From this list, identify 110 as the default value assigned to OSPF.Step 4: Understand that this means if a destination is learned via OSPF and RIP, the router will prefer the OSPF route because 110 is lower than 120.Step 5: Recognize that administrative distance can be changed manually for policy reasons, but the question asks specifically for the default value.

Verification / Alternative check:
On a Cisco router, you can use show ip route to view OSPF routes, which appear with code O in the table. The summary at the top of the routing table output lists route sources and their administrative distances, including OSPF with a distance of 110. This confirms the default value.

Why Other Options Are Wrong:
Option A, 90, is the default administrative distance for EIGRP internal routes.Option B, 100, is not the standard default distance for OSPF in Cisco IOS.Option D, 120, is the default administrative distance for RIP, not OSPF.Option E, 1, is the typical administrative distance for static routes added without further parameters.

Common Pitfalls:
Candidates often mix up the numeric values for administrative distances. A useful mnemonic is to remember the ordering from most to least preferred among common dynamic protocols: EIGRP internal (90), OSPF (110), and RIP (120). Connected and static routes are even more preferred with distances 0 and 1.

Final Answer:
The default administrative distance of OSPF routes in Cisco IOS is 110.