Calculating the Best Routes with SPF
As you can see from these LSAs, they contain useful information, but they do not contain the specific information that a router needs to add to its IPv4 routing table. In other words, a router cannot just copy information from the LSDB into a route in the IPv4 routing table. Instead, the router must do the SPF math, choose the best route, and add a route: a route with a subnet number and mask, an outgoing interface, and a next-hop router IP address.
Although engineers do not need to know the details of how SPF does the math, they do need to know how to predict which routes SPF will choose as the best route. The SPF algorithm calculates all the routes for a subnet—that is, all possible routes from the router ...
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