- Advertised Distance = Distance from a routers neighbor to a network destination.
- Feasible Distance = Advertised Distance + The cost between your router and a neighboring router to get to a destiantion network.
So RouterB wil Advertise a Distance of 20 to get to network 192.168.1.0. RouterA will add the cost between itself and Router B and have a Feasible Distance of 30.
If RouterA has another path to get to network 192.168.1.0, let's say we have RouterD and RouterE, which are also connected to the 192.168.1.0 network and router A, the Router will first check if the pathcost is the same.
If so, both routes/paths will be used to send data to the 192.168.1.0 network and put into the routing table, if not, it will see if the alternative route can be used as an possible alternative route if the first path through RouterB and RouterC fails.
If the Advertised Distance of the alternate path is lower than the Feasible Distancee of the Succesors it is considered and alternative path.
The Feasible Distance includes the cost on the link between the router in question and the neighbor that is Advertising the route.
So router A is receiving an Advertised route from Router B.
Router B says I can get to that network in X (Bandwidth + Delay).
So router A, at this point now has an Adverstised Distance of X that it learned from B. It takes X and adds to X what ever the metric (bandwitdth + Delay) is from A to B. The sum is the Feasible Distance. Both the FD/AD are placed into the Topology Table.
The Feasible Successor is any route (up to 16) that had a "AD" that is less than the current FD of the "successor" (the route currently in the Routing Table).
The Feasible Successor is any route (up to 16)
I think you are confusing with the number of paths with same metric that can be installed in the routing table but the FD is not installed in the RIB until the successsor fails.Is there a limit to the number of feasible successor? I must admit that i've never thought about it.
Now concerning the calculation, it's not really additive because if it has a lower bandwidth it will choose this one for the FD and add only its delay. Brian posted a very good doc about this here on CLN.