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Difficulty: HardRouting Concepts and Dynamic Routing Protocols

An enterprise edge router participating in an OSPF dynamic routing domain learns three distinct paths to reach the remote subnet 10.10.0.0/1610.10.0.0/16:

- Path 1: Learned as an Inter-area route (O IA) via Router West with a calculated total metric of 4545.
- Path 2: Learned as an External Type 1 route (O E1) via Router East with a calculated total metric of 1515.
- Path 3: Learned as an External Type 2 route (O E2) via Router North with a fixed external metric of 55.

Assuming all links are operational and default administrative distances apply, which route will the router install into its active routing table for traffic destined to 10.10.15.110.10.15.1?

  1. The Inter-area route (O IA) via Router West, because OSPF prioritizes Inter-area routes over External routes regardless of metric cost.Answer
  2. B
    The External Type 2 route (O E2) via Router North, because it presents the absolute lowest metric value of 5.
  3. C
    The External Type 1 route (O E1) via Router East, because Type 1 routes include internal path cost to achieve a lower cumulative cost than Inter-area routes.
  4. D
    Equal-cost multi-path (ECMP) traffic splitting across all three routes because they originate from the same dynamic routing protocol instance.

Answer

The Inter-area route (O IA) via Router West is selected and installed into the routing table because OSPF path-type precedence (Intra-area > Inter-area > External Type 1 > External Type 2) takes precedence over metric calculations.
In OSPF path selection, routes are evaluated according to a strict order of preference: Intra-area (O) > Inter-area (O IA) > External Type 1 (O E1) > External Type 2 (O E2). Because an Inter-area (O IA) route exists, OSPF selects it immediately over any External routes (E1 or E2), regardless of whether the external routes have lower metric costs.

Step-by-Step Solution

1
Analyze the prefix lengths of all candidate routes.
All three routes cover the identical prefix length of 10.10.0.0/1610.10.0.0/16, so Longest Prefix Match (LPM) does not eliminate any path.
Prefix length comparison always occurs first in IP routing selection.
2
Evaluate administrative distance across protocol sources.
All routes originate from the same OSPF process (Administrative Distance 110110).
Administrative Distance is only compared across different routing sources.
3
Apply OSPF route preference rules based on path type hierarchy.
OSPF path type order is: 1. Intra-area (O), 2. Inter-area (O IA), 3. External Type 1 (O E1), 4. External Type 2 (O E2).
OSPF path type precedence is evaluated before comparing metric costs.
4
Determine the winning path for installation into the Routing Information Base (RIB).
The Inter-area route (O IA) via Router West beats External Type 1 (O E1) and External Type 2 (O E2) paths, even though its metric cost (4545) is higher.
Inter-area routes outrank any external route types in OSPF.

Key Concept

OSPF Route Selection Hierarchy (Path Type Precedence)
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Topics

Routing Concepts and Dynamic Routing Protocols

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