Exam 400-007 topic 1 question 244 discussion

OSPF 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

B. 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.

C. 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.

I 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

https://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.

E1 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

obviously 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.

D is right. E1 is always chooses before E2 even with worst metric