One thing I've noticed about the metric in EIGRP
is that if you have to cross a slow link,
that metric is going to be very, very large.
In this video, I'd like to share with you what the two
main factors are that go into the calculation
for that metric in EIGRP.
[MUSIC PLAYING]
And as we embark on this calculation
of this composite metric for EIGRP,
it's not too important to memorize
all the details for the algorithm, for the formula WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
all the details for the algorithm, for the formula
that it uses to calculate it, but it
is important to know the impact of the two major factors that
go into that calculation.
So we can be aware when we change
either one of those factors that the metric is
going to drastically change.
So let's use this topology as we discuss the calculations
for the metric.
Now, if you've been with me in these videos on EIGRP up
to this point, you're aware that we're communicating
certain things as we propagate routes through the network,
including we're propagating the slowest link,
and it can only go down.
For example, if we have this network over here, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
For example, if we have this network over here,
this is 10 dot-- and let's see, it's
10.1.7.0 with a slash 24 mask, what's
advertised as that route goes up, is, hey, it's gigabit
and when R5 advertises it says, hey, it's gigabit.
But when R3, because it has to go over fast ethernet,
it's going to report that the lowest link is now
100 megabits.
And so all the way across the top route and this route
is going be advertised that the lowest
common denominator for link speed is fast ethernet.
And then, as it goes down here, R6,
when it advertises that same network, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
when it advertises that same network,
because it had to go across a serial link, which
is really slow, it's going to report that the slowest link is
now 1.54 megabits per second.
So with EIGRP, we only know what our neighbors are telling us.
But our neighbors are taking the worst news in the path.
And that's the information they're propagating.
Each router is also propagating the delay.
And what they're actually doing is
the sum of the delay on each router, each device.
So if router 7 advertises and router 5 advertises,
maybe router 5 adds a 10 microsecond delay. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
maybe router 5 adds a 10 microsecond delay.
And then R3 adds a larger value, because it
has to go over fast ethernet.
Each of these routers is adding a delay value.
And so when R6 advertised the R8, it's going to say, hey,
OK, regarding this route, this lowest link is--
and it's going to be 1.54 megabits per second.
And sum of the delays up to my point
is-- and whatever that value is, at which point
R8 over this fast ethernet link is going to add
its own delay to that equation.
So its calculation, R8's calculation,
is going to be the slowest link.
It's going to involve the slowest WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
It's going to involve the slowest
link and the sum of all the delays between R8
and this network.
That's how it works.
Now, here's the formula--
and it's not too critical that you
memorize all the details for the formula, but here it is.
It is 256 times--
and then I'm going to put this part of the equation
in brackets, because it's going to be processed first.
And then it's the reference bandwidth for EIGRP.
And it currently uses the bandwidth equivalent
to a 10 gig link.
So that's 10 million k. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So that's 10 million k.
So that's represented here as 10 million
divided by the slowest bandwidth in the path.
So we're going to take that value and add it to the delays.
So that's the total delays, the reported delay
from R8's perspective, the reported delay from R6,
and R8's own delay to get to R6.
And that's can be divided by 10.
And that is going to equal our metric for EIGRP.
And I think for this lab I think what we ought to do
is let's focus on this network, 10.1.7, way over here. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
is let's focus on this network, 10.1.7, way over here.
And let's take a look at the details on R8.
And then we'll do this calculation
by doing some show commands here on R8.
We'll verify what those values are and see
if we, as humans, can come up close to the metric
that R8 is generating on its own regarding the route to 10.1.7.
So here in the lab, let's go over to R8.
And let's make sure we have the route to begin with.
That's always a good idea.
Show ip route.
And the route we're looking for is 10.1.7, that guy
right there.
So I'm going to actually highlight that. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So I'm going to actually highlight that.
And if we do a show ip eigrp--
and so the metric right there is 2,176,256.
So that's the feasible distance for the successor
route regarding 10.1.7.
So then if we do a show ip eigrp topology, and we do a paste
and we put in that same value, this
is what gives us the juice, the details,
about what's going into the equation.
So this indicates that the slowest link in the path WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So this indicates that the slowest link in the path
is 1.54 megabits per second, or 1,544 kilobits per second.
And it's also showing us that the total delay
from R8's perspective is 20,250 microseconds.
So the calculation would be 256 times, and then in parentheses
because we're going to do this part of the equation first,
it'd be 10 million divided by 1,544, plus the delays.
That's the sum of the delays is 20,250, divided by 10. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
That's the sum of the delays is 20,250, divided by 10.
And so we do inside the parentheses,
we do that equation first.
And then we multiply by 256.
And then that gives us our metric, or hopefully,
very close to the metric.
And what we're shooting for is this value right here,
2,176,256.
If we get really, really close to that, that's a good sign.
Also, in EIGRP there's other factors
that could be calculated or could be used.
But they're not by default, including reliability and load.
Those are factors that could go into EIGRP. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
Those are factors that could go into EIGRP.
But we'd have to retrain and customize our routers
with configuration to tell each of the EIGRP routers
that we want to use the reliability and the load
as part of the equation.
By default, it's just the lowest bandwidth
in the path and the sum of the delays
that go into our equation.
So I currently have a spreadsheet
that we can go ahead and use as part of our calculations here.
And let me show you the formula that I
have for this spreadsheet.
It says that we're taking B2 divided by B3.
So B2 is this. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So B2 is this.
We'll divide it by the slowest bandwidth in the path
right here.
We'll plug that value in.
And then we'll add that to B4, which is going
to be the sum of our delays.
We'll plug that value in, divide it by 10.
And then we'll multiply that result by 256.
And so this should show us the metric
that we're calculating right here in this field
once we're done.
So let me scooch this over just a little bit,
so we can see the values.
Fantastic.
And let's put them in.
So the reference bandwidth for EIGRP, that's baked in.
That's part of the formula.
Our slowest bandwidth in the path is 1,544. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
Our slowest bandwidth in the path is 1,544.
So we'll put that in.
And then the sum of the delays is 20,250 microseconds.
20,250, press Enter.
And that's showing us the result of 2,176,000
and change, which is very, very close--
2,176,000.
And EIGRP calculated it with a 256 for the last three digits.
We got 431.
You know, between friends, that's like a rounding error. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
You know, between friends, that's like a rounding error.
And if we wanted to do this again for a different route,
we could.
Let's go ahead-- let's pick another route.
Let's imagine-- how about 10.2.6 right there?
So we'll do a show ip eigrp topology for 10.2.6 slash 24.
Press Enter.
So here it's specifying that for that route, the 10.2.6.8,
that the minimum bandwidth is 100,000.
So we could just replace that there.
I added too many. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
I added too many.
There we go, 100,000.
And the total delay is only 110 microseconds.
So I'll go ahead and replace that with 110.
Press Enter.
And now, let me scooch this over just a little bit.
So in our calculations, it's showing a metric
based on that data of 28,416.
And right here, it's showing that that's
the feasible distance.
It calculated it as well, using the factors
of bandwidth and delay in the formula for the EIGRP
composite metric.
So our takeaway from this video is twofold.
Number one, there's a calculation WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
Number one, there's a calculation
that's done with EIGRP.
The actual math for that calculation
isn't the critical part.
What is the critical part is the fact that bandwidth,
the lowest bandwidth in the path, and the sum of the delays
are the two major factors that go
in to that calculation for the EIGRP composite metric.
I hope this has been informative for you,
and I'd like to thank you for viewing.