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A is incorrect because it says to pick the route with the lowest bandwidth, it should be with the highest bandwidth. The higher the bandwidth, the less the cost
OSPF-running routers use these criteria to select the best route to be installed in the routing table:
When there are multiple routes available to the same network with different route types, routers use this order of preference (from highest to lowest):
Intra-area routes
Inter-area routes
External Type-1 routes
External Type-2 routes
If there are multiple routes to a network with the same route type, the OSPF metric calculated as cost based on the bandwidth is used for selecting the best route. The route with the lowest value for cost is chosen as the best route.
If there are multiple routes to a network with the same route type and cost, it chooses all the routes to be installed in the routing table, and the router does equal cost load balancing across multiple paths.
C says how to calculate ospf metric for a specific intf
A says how to choose the correct path to dest with the lowest cumulative metric.
A is correct answer.
A takes every exiting interface from source to destination into account
C Only takes 1 exiting interface into account
Cumulative cost is the sum of the all costs of the outgoing OSPF interfaces in the path.
While calculating cumulative cost, OSPF consider only outgoing interfaces in path. It does not add the cost of incoming interfaces in cumulative cost.
If multiple routes exist, SPF compares the cumulative costs. Route which has the lowest cumulative cost will be chosen for routing table.
https://www.computernetworkingnotes.com/ccna-study-guide/ospf-metric-cost-calculation-formula-explained.html
metric should be cumulative cost and it should say lowest cost or highest bandwidth instead of lowest bandwidth (totally misread (past tense) it).
So C should be more correct... but it needs cumulative cost of every exiting/outgoing interface on the path from source to destination to actually be able to calculate which path is best=lowest cost. Trip up question.
I would go for A
The destination could be after several hops ,and each of those has a cost based on BW , they needed do be all added to calculate the cost to that destination
C :
Cisco routers determine what to base the cost versus bandwidth on a 'reference bandwidth' which defaults to 100mbps. The reference bandwidth is then divided by the link bandwidth to generate the link cost
In fact C is misleading. This is because cost compared is not the cost of one interface (obtained by division of ref BW/interface BW). It is rather the accumulation of all the divisions on all interfaces, e,g. if the first interface 100Mbps ==> cost=1 and the second interface is 10 Mbps==> Cost =10, the total cost is 10+1=11, which is the one compared with the total costs of other paths calculated similarly.
Hence, I would choose A as it is more descriptive of the process except for that last 2 words "lowest bandwidth" is incorrect and should be "lowest cost".
C is correct. https://networktechstudy.com/home/learning-ospf-path-selection
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