In Cisco IOS show ip route output for OSPF, which list correctly represents the six common OSPF route types?

Introduction / Context:
OSPF routes appear in the Cisco IOS show ip route output with specific codes that indicate the type of OSPF route. Knowing these codes is important for troubleshooting and for exams, because they reveal how a particular route entered the OSPF domain and how its metric is calculated.

Given Data / Assumptions:

• We are focusing on Cisco IOS route table codes for OSPF, not on LSA type numbers.
• The question asks for six OSPF route types as seen in show ip route.
• Common OSPF route codes include O, O IA, O E1, O E2, O N1, and O N2, but Cisco often abbreviates these to O, O IA, E1, E2, N1, and N2 in explanations.

Concept / Approach:
Cisco categorizes OSPF learned routes in six main types: intra-area (O), inter-area (O IA), external type 1 (E1), external type 2 (E2), NSSA external type 1 (N1), and NSSA external type 2 (N2). Intra-area routes are learned within the same OSPF area; inter-area routes cross area boundaries via ABRs. External routes are redistributed from other routing domains into OSPF and are represented as E1 or E2 depending on how their cost is calculated. NSSA external routes (N1 and N2) are similar, but they originate from a Not-So-Stubby Area.

Step-by-Step Solution:
Step 1: Identify intra-area routes, marked with code O. These are routes that originate and stay within a single OSPF area.Step 2: Identify inter-area routes, marked with O IA, which are learned from another area via an ABR.Step 3: Recognize external routes of type 1, marked E1, where the external metric is added to the internal OSPF cost as the route crosses the OSPF domain.Step 4: Recognize external routes of type 2, marked E2, where only the external metric is considered and the internal OSPF cost is not added beyond the ASBR.Step 5: Identify NSSA external routes N1 and N2, which correspond to external type 1 and type 2 routes originated inside an NSSA, then translated and flooded into the rest of the OSPF domain.

Verification / Alternative check:
On a Cisco router, issuing show ip route ospf in a lab that includes multiple areas and redistribution from another protocol will display entries starting with O, O IA, O E1, O E2, O N1, and O N2. These six codes confirm the list described in option A.

Why Other Options Are Wrong:
Option B lists general route codes such as C and S that are not specific to OSPF.Option C repeats the O prefix unnecessarily and mentions an extra type for default routes, which is not a separate OSPF route type.Option D refers to IS-IS style L1 and L2 designations, which belong to a different routing protocol, not to OSPF.

Common Pitfalls:
Candidates sometimes confuse LSA types with route codes, or they assume that all external routes in OSPF look the same. Remember that the route table codes O, O IA, E1, E2, N1, and N2 provide a quick summary of where the route came from and how its metric is treated, which is critical during troubleshooting.

Final Answer:
The correct list is O, O IA, E1, E2, N1, N2 (intra-area, inter-area, external type 1, external type 2, NSSA external type 1, NSSA external type 2).