Given the following OSPF network diagram, what router will be added to R1’s routing table as a next-hop to get to R8?

(A) R2
(B) R4
(C) R7
(D) R5

R4 is the correct answer. The SPF algorithm calculates all the routes for a subnet—that is, all possible routes from the router (R1) to the destination subnet (R8). If more than one route exists, the router compares the metrics, picking the best (lowest) metric route to add to the routing table. Once SPF has identified a route, OSPF calculates the metric based on the sum of the OSPF interface costs for all outgoing interfaces in the route.

The three possible routes to 175.25.30.0/24 are:

• Route 1: R1-R2-R3-R8
  Cost: 10 + 80 + 40 = 130
• Route 2: R1-R4-R5-R6-R8
  Cost: 20 + 30 + 10 + 20 = 80
• Route 3: R1-R7-R8
  Cost: 60 + 100 = 160

As a result of the SPF algorithm’s analysis of the LSDB, R1 adds a route to subnet 175.25.30.0/24 to its routing table, with the next-hop router of R4.