OSPF External Route Selection: E1 vs E2 Metric Comparison
Short answer
B The path reverts back to R1-R2-R3 because the route type is E1.
Question
Key Takeaways
The core concept tested is the OSPF external route preference hierarchy; the common trap is assuming the lower metric path wins, ignoring the type precedence rule.
This question examines OSPF external route selection logic, specifically the tie-breaking rules between External Type 1 (E1) and External Type 2 (E2) routes. It establishes that E1 routes are always preferred over E2 routes regardless of metric magnitude.
Common mistake: Most learners choose C or D, mistakenly believing that metrics from different OSPF processes cannot be compared or that historical best paths persist without recalculation.
Expert Analysis
Why the Answer Is Correct
The correct answer is B because OSPF has a strict hierarchy for selecting among multiple external routes to the same destination. When a router receives both an E1 and an E2 route for the same network, it will always prefer the E1 route, even if the E2 route has a significantly lower cost. This behavior occurs because E1 routes accumulate internal AS costs plus the external cost, making them generally more representative of the true distance, whereas E2 routes only use the external cost. Therefore, when the link restores and R1-R2-R3 becomes available again, if it is advertised as an E1 route while the backup path remains E2, R1 will select the E1 path.
Why the Other Options Are Wrong
Option A is incorrect because OSPF does not simply pick the lowest metric across all types; type takes precedence over metric in this specific comparison. Option C is incorrect because while OSPF processes maintain separate LSDBs, the router's routing decision process compares valid candidates from all configured processes (assuming equal Administrative Distance), and type is a primary discriminator. Option D is incorrect because OSPF is a dynamic protocol that recalculates paths based on current topology and metrics; it does not revert to previous paths out of loyalty or history.
Community Comment Notes
Community comments largely support the E1 preference logic. As blurain noted, "Intra-Area... External Type 1... would still be selected over the E2 route." Another user, Weltbuerger, pointed out that intra-area is preferred over inter-area which is preferred over external, but emphasized that within external types, the type itself is the key differentiator here. Kalulosu correctly identified that E1 includes internal cost, reinforcing why it might be chosen, though the primary reason is the type preference rule.
Official Reference
https://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/4170-ospfprocesses.html
This link opens the official vendor documentation in a new tab.
Exam Strategy
When analyzing OSPF external routes, always check the Type (E1/E2) before comparing Metrics. Remember the hierarchy: Intra > Inter > External E1 > External E2. If types differ, the lower type number wins regardless of cost.
Frequently Asked Questions
Why doesn't the lower metric path win?
Do OSPF processes compare metrics?
Community Votes
76% of anonymous learners picked answer B. Votes are pick records left by other test-takers — they are not the verified answer.
Discussion 8 contributions
Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.
From public community threads 8
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blurain 👍 10 Answered BOSPF path selection: Intra-Area (O) Inter-Area (O IA) External Type 1 (E1) External Type 2 (E2) NSSA Type 1 (N1) NSSA Type 2 (N2) even if metric is higher on E1, it would still be selected over the E2 route
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pavelma 👍 3 Answered Cobviously B or D. I vote B, because strictly speaking it doesn't matter which route was the best in the past. When the link comes back a new route calculation happens and R1-R2-R3 wins because of the metric, not for historical reasons.
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Weltbuerger 👍 1 Answered Dhttps://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/4170-ospfprocesses.html states "The OSPF route selection rule is that intra-area routes are preferred over inter-area routes, which are preferred over external routes. However, this rule can apply to routes learned via the same process. In other words, there is no preference between external routes from one process compared to internal routes of other process." It is implementation dependent which criteria is selected when two OSPF routes with same AD need to be compared, so it reverts back as the same rule (be it lowest router ID or whatever) is again selected as tie-braker. Metric type/metric does not play a role here.
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sandccie 👍 1 Answered BB. I change my mind after more research. when an identical external route received from two OSPF instances on a router, the tie-breaker is the External Type (E1 vs E2). If they both were the same then it would be the OSPF instance ID. The outcome would be the same but the reason R1-R2-R3 path is preferred would be different.
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4f12874 👍 1 Answered BE1 includes – the internal cost to ASBR added to the external cost, E2 does not include – internal cost. it is same as external cost. Cost ospf1 : 10 + 10 +100 = 120 Cost ospf2 : 100 If both were the same type, R4-R5-R3 would be chosen based on cost. However, E1 is chosen before E2, even if E2 has a better metric
Show 3 more comments
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kalulosu 👍 1 Answered BI think ans is B. In OSPF, E1-type routes take into account not only the external cost but also the internal cost in the route metric; when the link between R2 and R3 is restored, the R1-R2-R3 path is again chosen as the best path because it has a lower metric including the internal cost
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sandccie 👍 1 Answered CC. the router compares possible paths for a route only within each OSPF process (domain). So both OSPF processes have only one possible path for the route. However, only one path can be installed to the forwarding table at a time, even they have the same Administrative Distance.
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Rollizo 👍 1 Answered DD is right. E1 is always chooses before E2 even with worst metric
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