- [Instructor] We said that one of the big benefits
of EIGRP is that if it loses a primary path to a destination
it can in many cases very quickly reconverge
to a backup path.
Let's see how EIGRP determines the best path
and how it determines the backup path.
And in this video we want to define a couple of terms.
The first term is RD for Reported Distance
and FD is for Feasible Distance.
In this example let's say that router R3
is wanting to get to the 10.1.1.0/24 network.
Well, obviously there're two ways to get there;
we can go via R1 or we can go via R2.
But R1 has to go out of one of its interfaces,
it's directly connected
but still it has to go out of one of its interfaces
to get to that network.
So let's say that it has a metric of 1000
to get to that network.
We going to enter that
and R1 is going to report that information to R3
and R3 says, this is the reported distance to this network
by my neighbor R1, that's the reported distance.
But R3 says, hold on there's a metric to get to R1.
We have to consider that link going to R1.
And here we're saying that that link has a metric of 10000.
Well, what's the grand total cost to get to this network
of 10.1.1.0/24?
Well, it's going to be R1's reported distance of 1000
plus the distance for us to get to that neighbor,
which is 10000.
1000 plus 10000, that's 11000.
That's the total distance
which we call the feasible distance.
What about R2?
Let's say that R2 also has a metric of 1000
to get to this network and it's going to report that to R3.
We going to enter that in the reported distance
for our neighbor R2
but what is the distance or the metric to get to R2?
In this case we're saying it's 5000.
Let's add those up.
We're going to add the reported distance
from R2 of 1000 plus the distance to get to R2
which is 5000, 1000 plus 5000 that is 6000.
Now we've got the feasible distances from our neighbors.
We're going to go as you might guess
with lowest feasible distance.
And we're going to call that route via R2 in this case
because it has the lowest feasible distance.
We're going to call that route the successor route
but if the successor route is no longer available,
maybe R2 goes down for some reason.
We want to go to that network via R1,
we'll say that is a feasible successor route.
And that's the way that EIGRP
makes its path selection decisions.
However, in order for this to work as explained,
there's a criteria under that feasible successor
has to meet.
And we're going to talk about that in our next video.
It's called EIGRP's feasibility condition.