- [Man] Let's consider EIGRPs metric.
Remember when he has EIGRP has more than one path
to get to a destination network,
it's going to choose the path that has the lowest metric.
And the metric with EIGRP is a bit more complicated
than OSPF or RIP.
With RIP our metric is simply hop count.
How many routers do we have to hop through
to get to this destination network?
with OSPF it's cost which is based entirely on bandwidth.
But with EIGRP we can base it on several things.
And I've got an acrostic for you as a memory aid.
And that acrostic is big dogs really like me.
Big dogs really like me.
And the B in big reminds us of the B in bandwidth
and the D dog's reminds me of the D in delay.
And the R in really reminds me of the R in reliability.
And the L in like reminds me of the L in load,
and the M in me reminds me of the M in MTU.
Big dogs really liked me, bandwidth, delay,
reliability, load and MTU.
In fact when I talk about this,
it reminds me of my interview down at Walt Disney world
when they were interviewing me for the job,
because they ran EIGRP pretty much exclusively
on the Disney world property,
they asked me during the interview,
"By default what does EIGRP use to calculate its metric?"
And I thought, Oh, I've got this because I knew
this acrostic, big dogs really liked me.
So I listed them off. I said,
"Bandwidth, delay, reliability, load, MTU."
and they said, "No, that's not correct."
They ask by default, what does EIGRP consider.
And by default EIGRP is only considering
bandwidth and delay.
Let me show you why that is.
If we take a look at the metric formula
for EIGRP, it's a lot more convoluted than OSPF or RIP
as we were saying.
Notice that we've got these constants,
these K values, and here are the default K values.
Most of them are zeros.
And if you plug in all the zero values
a lot of this just multiplies out to zero
and we're left with just a bandwidth an a delay component.
Oh, by the way if you notice that K five is zero,
and we've got that value of K5 divided
by the sum of K4 plus reliability,
if K5 is zero, then that term is zero
and we're multiplying everything by zero.
So everything's going to zero.
So this formula has a caveat.
The caveat is if K5 equal zero then
K5 divided by the sum of K4 in reliability
that is considered to be a one.
So if K5 is a zero then that term equals a one.
But other than that, this formula holds true.
You might also notice that there's no mention
of the maximum transmission unit in this formula.
Where's the MTU?
Well the MTU is not part of the formula,
rather it's designed to be a tie breaker,
if we have two paths and their metric calculation
comes out the same.
We'll go with the one that has the largest MTU
if the MTUs happen to be different.
Now let's talk a bit more about those other parameters.
Bandwidth is actually the weakest link.
If we're trying to get from one router
to a destination network,
we may be going through different link speeds
on the way to that destination network.
We're going to take the bandwidth of the slowest link.
In other words the weakest link.
that's going to be the value that we use.
And we're going to measure that in kilobits per second.
So if we had a gig link,
that would be 1 million kilobits per second
or 10 to the sixth.
And the way we put this factor into the formula
is if you notice on screen,
we're going to take the minimum bandwidth any of our links,
and we're going to say 10 to the second seventh,
which is 10 million divided by your weakest link.
And if it's a gig link, that's 10 to the seventh
divided by 10 to the sixth, which is a 10.
That would be our bandwidth value
that we plugged into the formula.
Now delay is not the weakest link, delay is cumulative.
Every time we exit a router interface,
there's a delay associated
with the exiting of that interface.
And that delay is measured in tens of microseconds.
It's a bit of an odd unit of measure we're using
because when we look at the interface
and we'll do that in a moment, add on some live gear,
when we look at the interface,
it says, "Here's the delay in microseconds."
But when you plug it into the formula,
you have to divide by 10 to get tens of microseconds.
So let's go back to our gig link.
On a gig link, the delay is 10 microseconds.
But the number that we plug into this formula for delay
is 10 microseconds divided 10 which is one .
The delays going to be a value of one 10 microseconds
when we plug it into this formula
because of that somewhat odd unit measure we're using.
And remember it's cumulative.
So if I go out of two gig interfaces,
that's okay going to be 10 microseconds
plus 10 microseconds for 20 microseconds,
we have to divide that by 10
and that will be a total delay of two.
Reliability is a number over 255.
And if the network is considered to be 100% reliable,
the value of reliability is going to be 255.
That's going to mean we have maximum reliability.
Load is also a value over 255.
The minimum value is one, that would mean that this link
is minimally loaded.
If we were using the link that was
typically saturated or near saturation,
we might want to give it a larger number than one.
That might not be accurate.
So load is a number over 255 and that value
is representative of the load on that link.
And of course, MTU is the number of bytes.
That's the maximum transmission unit allowed over a link.
That's something we can set on an interface by default.
It's going to be 1500 bytes, but we can change that.
And again that's not part of the formula,
it's used as a tie breaker.
But let's go through an example calculation
and we'll go out to this topology and a few moments.
And we'll make sure the calculation that
matches what we see in real life.
Here we're sitting on R3
and we're trying to calculate the metric
to get to the 192.0.2.0/30 network.
If I'm on R3, what is the weakest link?
What is the weakest link that I have to go out off
to get there?
Well it's a gig link up to R2 and then it's a gig link
out of R2 to get to that network.
So the weakest link is a gig link.
They're all the same, but remember, it's not cumulative.
A gig is 1 million K,
and we have to say 10 to the seventh
or 10 million divided by our weakest link in K.
So it's 10 million divided by 1 million.
That's going to be our bandwidth value,
then the delay, the sum of the interface delays
is going to be 10 microseconds as we exit that
top interface on R3.
It's going to be another 10 microseconds
as we exit that top interface on R2,
for a grand total of 20 microseconds.
However the unit of measure for our formula
is in tens of microseconds.
So we say 10 plus 10 is 20 but,
we've got to divide that by 10 because our unit of measure
is in tens of microseconds.
We add those two together. It's going to to be 10 plus two.
We take that sum and we multiply that
by 256 to give us a metric of 3072.
And the reason this formula looks a lot simpler
than what we saw just a moment ago,
is this is with our default set of K values.
Everything else just kind of zeroes out
and we're left with just this formula by default,
unless we go in and change things.
Now let's go out to this typology.
I've got it set up in my lab right now,
and let's go to R3 and see if the feasible distance
or the metric to get to 192.0.2.0/30 truly is 3072.
We're sitting here on rider R3, and I'm going to do a
show IP route command,
and let's take a look at the feasible distance or the metric
to get to the 192.0.2.0/30 network.
Here. We see it. Here's the network,
and we've learned it via EIGRP is indicated with the D
and the 90 that represents the EIGRP administrative distance
compared to OSPF it's more believable,
OSPF has an AD of 110, ISS has an ID of 115,
RIP has an AD of 120 EIGRP is considered
to be a very believable protocol.
But the big number we're looking for is the metric
or the feasible distance.
And that number is 3072 which is exactly
what we calculated on paper earlier.
And that's all look at how we calculate
EIGRP metrics.