Exam 4A0-112 topic 1 question 4 discussion

To determine the number of entries for the prefix 192.168.3.0/24 in router R3's routing table, you need to find all the equal-cost paths from R3 to R5. Since all links have the same cost, we can count the number of hops for each path from R3 to R5. Since all links have the same cost, we can count the number of hops for each path from R3 to R5. Path 1: R3 -> R1 -> R5 This path has two links: R3-R1 and R1-R5. Total cost = 2 hops (or if these all had a metric of 10 for ease of addition we could say a cost of 20 depending on the IGP being used whether it is protocols such as OSPF or IS-IS, but with them all being the same cost it really comes down to hop count). Path 2: R3 -> R2 -> R5 This path has two links: R3-R2 and R2-R5. Total cost = 2 hops (metric of 20). Path 3: R3 -> R4 -> R2 -> R5 This path has three links: R3-R4, R4-R2, and R2-R5. Total cost = 3 hops (metric of 30). Path 4: R3 -> R1 -> R2 -> R5 This path has three links: R3-R1, R1-R2, and R2-R5. Total cost = 3 hops (metric of 30). Path 5: R3 -> R4 -> R1 -> R5 This path has three links: R3-R4, R4-R1, R1-R5. Total cost = 3 hops (metric of 30). Path 6: R3 -> R2 -> R1 -> R5 This path has three links: R3-R2, R2-R1, R1-R5. Total cost = 3 hops (metric of 30). The paths with the minimum cost are Path 1 and Path 2, both of which have a total cost of 2 hops or a metric of 20. Since the ECMP value is 4, R3 can install up to four equal-cost paths. In this case, there are only two equal-cost paths with the minimum cost. In the routing information base (RIB), all 6 paths would be present. However, the routing table which has the active paths chosen by the route table manager (RTM), and is what is stated in the question would only have 2 paths present and active. In order for D to be the correct answer, at least four paths identified would need to have the exact same lowest cost, which they do not. In a real network, a router's SPF algorithm would correctly identify that the paths with a cost of 3 (metric 30) are not equal to the paths with a cost of 2 (metric 20). It would only install the two minimum-cost paths into its routing table. Therefore, the correct answer is B. 2.