question regarding feasible successor in CBT nuggets video about EIGRP

question regarding feasible successor in CBT nuggets video about EIGRP.

If anyone has it I am a little confused - at around 1300-14:00 minutes he draws a diagram explaining the backup route rule however he doesn't add up the 500 cost plus the 10, he just uses the 10 as the example. So in theory wouldn't his backup route be 510<100 and in this case NOT true?

I believe i understand the theory which is important but he didn't add up all of his routes like what he did from r1 and r4, and he was just using r3

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Comments

Jon_Cisco

I believe what he is referring to is determining which routes can be used as the Feasible successor.

The Successor is the lowest cost route on that router. Which includes the final link to the router (the 500).
The Feasible successor looks for the next best route using the advertised distance only (the 10).

The math behind this is the look for another link that will not cause loops.

So in the example there are two paths to the network R3 and R4 both list 10 for the advertised distance.
R3 has a 500 for the next step for 510 total. R4 has a 90 for the next step for 100 total.
R4 wins and becomes the successor with 100 as the final number.

Now to see if R3 is a feasible successor you compare it's advertised distance of 10 to the successor distance of 100 to see if it is lower.
If it is the router knows it can safely use that as a feasible successor without causing a loop.

So the total 510 was used in the first part to pick the successor and the 10 was used in the second part to pick the backup feasible successor.

Hope this makes sense.
Jon

gbdavidx

Why isn't it added up though? It technically takes two hops to get there such as the successor does, that part doens't make sense to me, does it not mater just because it is the backup route?

OfWolfAndMan

Feasible distance is the metric of the neighboring router PLUS the metric between that next hop to the local router you are currently on. Advertised/Reported distance is the metric from the neighboring router to the destination network. You do not add the feasible and reported distance together as this is an irrelevant calculation. However, if you subtract the feasible distance from the advertised distance, you get the metric from the local router to the neighboring router.

Jon_Cisco

gbdavidx wrote: »

Why isn't it added up though? It technically takes two hops to get there such as the successor does, that part doens't make sense to me, does it not mater just because it is the backup route?

Without looking into it much I know that what your missing is how it uses this information to ensure a loop free route.
I would search for something explaining the loop prevention when selecting the feasible successor and it will probably clear up your question.

Good Luck!