It is a great thing, when possible, to have a backup,
and the same is true with our routing protocols.
And EIGRP is no exception.
[MUSIC PLAYING]
And in this video, you and I will have opportunity
to both predict and then verify if a backup route exists
in EIGRP, and more importantly, why.
And lets you and I use this topology
as we discuss a backup route.
Now in this topology, I have the yellow links here
that represent fast ethernet, the green links WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
that represent fast ethernet, the green links
between R3, R1, and R2, and R4 represent gigabit ethernet.
And then this serial link is a 1.54 megabits per second.
I've also drawn in this color right here,
as I write over that, this lime green colored--
the successor route.
So R3 would be going this direction.
R1 would be going that direction.
R2 would be going this direction.
And R4 would be going this direction.
And I also, on this diagram in this color here,
put the feasible distance to this network of 10.2.68.
So R1 is a feasible distance of 110. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So R1 is a feasible distance of 110.
And they use more simple numbers because saying 2 million
and change every time is a lot of work.
So just for the purpose of education and backup routes,
we're going to imagine that R1 has
a feasible distance for its successor route, and it's 110.
R2 has 105, R4 has 100, and R3 has 115.
Now here are the rules about how the router believes,
or believes there is not, a backup route possible
and that is called the feasibility condition. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
and that is called the feasibility condition.
And you might ask yourself, what exactly
is the feasibility condition?
Imagine somebody that is purchasing something
at a store, and maybe it's $50, or 50 euros,
or 50 whatevers for the item.
But there's a special deal.
If you buy a second one, you can get the second one at half off
at a lesser price.
Some people go for that.
I can see that.
If you wanted two of them, you could get one for full price
and then one for less. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
and then one for less.
How about this though?
Let's say we walk into a store one day and they say hey,
we've got a deal for you.
You can buy one for $50, and you can
buy another one, a second one, for $60.
We'd say, you're crazy.
There's no way I'm going to pay more for the second one.
That doesn't make any sense.
In fact, I wouldn't even buy a second one at the same price.
That second one has to be a lower cost.
Otherwise, I'm not even going to consider it.
And that's also what goes in the brain of a Cisco router
running EIGRP regarding what's called a feasible successor. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
running EIGRP regarding what's called a feasible successor.
The successor is the winning route.
And a feasible successor is the backup route
who could be used if their primary route goes away.
So successor is the winner.
And the feasible successor is the backup guy
who could be used.
But the rule is that feasible successor
has to have a lower advertised distance
from a neighbor than our current feasible distance.
Let's take R1 as an example.
R1 has a feasible distance of 110. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
R1 has a feasible distance of 110.
If it is to believe that it has a backup route that it can use,
the advertised distance, the reported distance
from its neighbor has got to be less
than its own feasible distance.
It's not as a, yep, my current feasible distance is 110.
And I'll use a backup that's being
advertised at 110 or higher.
It just won't go for it.
The advertised distance has to be lower than the router's
current feasible distance.
Otherwise, there's no backup route.
So let's use this topology as an example of
whether or not a router would believe WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
whether or not a router would believe
it has a feasible successor, which
is another fancy name for a backup route.
And the rule, again, is that the advertised distance
for the route has to be less than the current feasible
distance on that router or, otherwise,
it just says, no way.
It can't be equal to or greater than.
It has to be less for it to be considered.
So let's take a look at R2.
R2 has a feasible distance of 105 for this route,
10.2.68.0, which is way down here.
So its successor route, which is going southbound, that's WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So its successor route, which is going southbound, that's
this area right here, it has a feasible distance, the metric,
for that winning route is 105.
So besides the reported distance that R4 is already
giving to R2, which is 100, and that's
causing R2 to use that path, let's
take a look at the only other neighbor that R2 has,
and that's R1.
R1 is advertising a reported distance,
or advertised distance, of 110.
And so when R2 sees that it says,
my current feasible distance is 105,
you're advertising, Mr. R1, that your reported distance
for that route is 110, not lower. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
for that route is 110, not lower.
[WRONG ANSWER NOISE] No.
The answer's no.
--doesn't meet what's called the feasibility condition, where
the advertised distance from a neighbor
is less than my current feasible distance for the successor
route.
So R2 does not have a backup route.
It's not receiving any advertised distances
for that same network that are below its current feasible
distance.
And therefore, it says no, I can't consider it.
If it did consider it, if it did try
to forward pack us in direction, there could be a loop incurred.
And that's one of the reasons that the feasibility condition WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And that's one of the reasons that the feasibility condition
exists is to avoid loops.
So R2 does not get a backup route.
Let's take a look at R1.
R1 has a successor route of just one.
It goes this way, or at R2.
R1 has one other neighbor.
That neighbor is advertising its advertised
or reported distance of 115.
Router 1 says, well, my current feasible distance is 110.
And as much as I would like to consider your path, he says,
to R3, your advertised distance is 115.
It's greater than my current feasible distance. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
It's greater than my current feasible distance.
The answer is no, not going to happen.
So R1 also does not have a backup route.
Well is anybody in this topology going to have a backup route?
The answer's yes.
I built this so that we would have one.
And let's take a look at R3 and take a look at it.
It has a successor route at the moment that's
going up to R1, that direction.
And it's feasible distance for that successor route is 115,
great.
Now the question is, is R3 receiving any advertisements
from other neighbors, other than R1? WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
from other neighbors, other than R1?
We're already using that for our primary path.
Is R3 receiving any advertised distances
that are currently less than its current feasible distance?
Well over here, R4, it's feasible distance is 100.
So R4 is advertising, or reporting, a distance of 100.
And as a result, R3 says, woah, my current feasible distances
is 115.
That's for the successor route.
And this reported distance is 100 going through R4,
gets a big happy face on, says, you
know what, if that primary route ever goes away, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
know what, if that primary route ever goes away,
I now have a backup option going through you
Mr. R4, because that meets the feasibility condition.
And the feasibility condition relates to backup routes.
And it determines whether or not the router will consider
having a backup router.
And if the advertised distance from a neighbor
is lower than our current feasible distance,
we'll use that second path as a backup.
We'll keep it in check right there.
And then we can start using it if the primary route ever
goes away.
In this video, we've taken a look
at the concept of a backup route and how WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
at the concept of a backup route and how
to predict whether or not a router will
have a backup route.
And that's if the feasibility condition
is met, which simply means that a router takes
a look at its current routes that it has,
that current feasible distance.
And if another neighbor is advertising that same network
with a lower advertised distance, boom.
--that means the feasibility condition,
and we now have a backup route that we
can turn to if the primary route goes away.
So in the next video and lab, we'll
take a look at the live topology and verify the backup route
and whether or not that backup route will kick in WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
and whether or not that backup route will kick in
if the primary route goes away.
So I'll see you there in just a moment.
Meanwhile, I hope this has been informative for you,
and I'd like to thank you for viewing.