400-007 · Question #323
Refer to the exhibit. The network 10.10.0.0/16 has been redistributed to OSPF processes and the best path to the destination from R1 has been chosen as R1-R2-R3. A failure occurred on the link…
The correct answer is D. The path reverts to R1-R2-R3 because this was the previous best path. When the R2-R3 link is restored, OSPF reconverges and selects R1-R2-R3 again because it has a lower cumulative metric to the redistributing router than the alternate path.
Question
Refer to the exhibit. The network 10.10.0.0/16 has been redistributed to OSPF processes and the best path to the destination from R1 has been chosen as R1-R2-R3. A failure occurred on the link between R2 and R3, and the path was changed to R1-R4-R5-R3 What happens when the link between R2 and R3 is restored?
Exhibit
Options
- AThe path R1-R4-R5-R3 continues to be the best path because the metric is better.
- BThe path reverts back to R1-R2-R3 because the route type is E1.
- CThe path R1-R4-R5-R3 continues to be the best path because OSPF does not compare the
- DThe path reverts to R1-R2-R3 because this was the previous best path.
How the community answered
(42 responses)- A21% (9)
- B5% (2)
- C12% (5)
- D62% (26)
Why each option
When the R2-R3 link is restored, OSPF reconverges and selects R1-R2-R3 again because it has a lower cumulative metric to the redistributing router than the alternate path.
R1-R4-R5-R3 does not retain a permanently better metric - OSPF recalculates costs after the link recovers and selects the path with the lowest current metric, which is R1-R2-R3.
E1 route type causes OSPF to add the internal path cost to the external metric, but the correct answer is specifically about OSPF reconvergence restoring the lower-cost path, regardless of whether E1 or E2 is used.
OSPF does compare metrics between competing paths to the same external destination, and the reconvergence process explicitly selects the path with the lower total cost.
OSPF runs SPF upon detecting the topology change when R2-R3 recovers, recalculating all path costs from scratch. The R1-R2-R3 path traverses fewer hops with a lower total cost to the ASBR redistributing 10.10.0.0/16, so OSPF reconverges back to that path as the mathematically best route.
Concept tested: OSPF external route reconvergence after link recovery
Source: https://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/18634-ospf-external-routes.html
Topics
Community Discussion
No community discussion yet for this question.
