- [Instructor] In this video, we're going to take a look
at the basic configuration of EIGRP for IPv4.
And we've got a three router topology
and we're going to visit each of these routers
and have all interfaces on all routers,
participate in the Edge ERP.
We're going to confirm that we have full reachability,
and we're going to give some show commands to take a look
at the different data structures at EIGRP maintains,
such as its interface table,
its neighbor table, its topology table.
Let's get started on router R1
by going into global configuration mode.
And we're going to start the EIGRP writing process.
I'm going to say router EIGRP
and I'm going to give not a process ID,
I'm going to give an autonomous system number.
I emphasize that because with OSPF,
we say router OSPF and we give a number,
that number with OSPF.
That's a locally significant process ID.
In other words, it does not have to match on my neighbor,
but with EIGRP, this is not a process ID,
this is an autonomous system number,
therefore it does have to match with our neighbor.
I'm going to use an autonomous system of one,
and I want to make interface gigabit 0/1 participate.
And to do that, I'm going to use the network command.
I'll say network 10.1.1.0.
And we give not a subnet mask, we give a wildcard mask.
Let's take a quick review
of how a wildcard mask is calculated from a subnet mask.
A wildcard mask is basically the inverse of a subnet mask.
And the way we calculate a wildcard mask
from a subnet mask is we take all 255s and all four octets
and we simply subtract the subnet mask from that.
And the result is going to be our wildcard mask.
Let's take a look at a couple of examples.
Let's start with all 255s,
and from that we're going to subtract a 24 bit subnet mask,
which is 255.255.255.0.
If you subtract on an octet by octet basis,
we've got 255 minus 255 is zero
and so on for the next two octets.
And then we have in the fourth octet, 255 minus zero is 255.
What if we have this sort of subnet mask though?
Let's say that we've got a subnet mask
of 255.255.255.252.
In other words we've got a 30 bit subnet mask.
In a case like that, if you do the subtraction,
you'll see that the wildcard mask is 0.0.0.3.
And that's a little bit of a review
about how wildcard masks are calculated.
Now let's go back to our demo
and take a look at how we apply this wildcard mask
to our network statement.
If you take a look at our topology gigabit 0/1
that interfaces on a network with the 24 bits of subnet mask
and I'm going to give this network statement
network 10.1.1.0 and the wildcard mask corresponding
to a /24 subnet mask is, as we just saw
on the calculation, 0.0.0.255.
But I want to make a big point here,
what I say network and I give the statement,
I am not telling EIGRP to advertise this network.
Same thing with OSPF,
When I give the network command,
I'm not saying advertise this network.
Instead I'm specifying a network address space.
And what I'm really saying with this command is,
if I have any interfaces which are up
and have an IP address in this network address space
then I want those interfaces to participate
in the Edge ERP, and by participate
I mean, I want them to advertise their network.
So I would not have to give a wildcard mask
that was perfectly the inverse
of the network's subnet mask,
that's what I typically do
because we're not really saying advertise this network.
We're just creating an IP address base
that contains the IP address of the interface,
and then we'll advertise that interface's network.
Let's take a look at gigabit 0/2 on R1.
I'll say network, and 172.16.1.0 is our network.
And we've got a /30 subnet mask
and that's going to have a corresponding wildcard mask
of 0.0.0.3, and we're done with our configuration on R1.
Let's move over to R2.
On R2 we'll do the very same thing.
We'll say router EIGRP,
I give a matching autonomous system of one,
and we'll say network 172.16.1.0.0.0.3.
In fact after I press Enter,
we might see an adjacency come up here
because, in fact there it is.
We have an adjacency form now between R1 and R2.
Let's get gigabit 0/2 participating in EIGRP.
We'll say network 192.168.1.0
with a wildcard mask of 0.0.0.3, we're done on R2,
and let's go over to R3.
on R3 we'll start the EIGRP writing process
the same way, router EIGRP 1.
But let's do the network statement a bit different.
I've been emphasizing that the network statement
does not say advertise this network.
Let's demonstrate that.
What if I said network 0.0.0.0
with a wildcard mask of all 255s.
Will that represents an IP address base
that contains all possible IP addresses?
In other words, both of my interfaces have IP addresses
that fall within this address space of all IPv4 addresses.
That means this one network statement
is going to make both of my interfaces participate in EIGRP.
Let's prove it, let's press Enter
and we should see an adjacency come up, there it is.
Now let's give some verification.
Let's see if we have reachability throughout the network.
If I do a show IP route,
we see that I have learned network 10.1.1.0/24.
That's at the far end of R1
that gets me all the way across the network.
So I've learned about that.
I've also learned about 172.16.1.0/30,
that was learned via EIGRP, the D indicates EIGRP.
And my guess as to why they use the D to represent EIGRP
is EIGRP uses the dual algorithm,
the diffusing update algorithm.
That's just a guess but I've never heard a good explanation
as to why D is used for the code of an EIGRP learned route.
Now let's take a look at EIGRP structures.
Let's say show IP EIGRP interfaces,
and we can see both interfaces participating in EIGRP.
We can take a look at our neighbors.
In fact, we'll just have one neighbor on R3
I'll do a show, IP EIGRP neighbors command.
And we took a look at this and our timers video.
We've got the whole time counting down,
and when it hits 10 it goes back
and starts counting back down from 15
although we'll only see 14, as we saw in our timers video.
And we see that I've got this neighborship
with 192.168.1.1, and that's off of gigabit 0/1.
Let's take a look at our typology table now.
We'll do a show IPE IGRP topology,
and these are networks that EIGRP has learned.
That does not mean that these networks
are necessarily going to appear
in our router's IP routing table
because the router may know a better way
to get to some of these networks.
For example, if the router's directly connected
to a network, that's going to be a better way.
And that's the case with 10.2.2.0/ 24 as an example,
it's directly connected to us.
But things that we have learned via EIGRP,
notice we have a couple of numbers here.
This one is 3072 / 2816.
What do those represent?
Well, this network 172.16.1.0, that's the network
between R1 and R2 and we're on R3 right now.
What we're seeing is the feasible distance
of a 3072 and the reported distance of 2816.
In other words router R2 is reporting to R3
that its distance from this network is 2816
based on the bandwidth and the delay
associated with gigabit 0/1.
But it's going to cost R3 something
to go out of its gigabit 0/1, to get over the R2.
And when we add the cost to get the R2
to the reported cost that R2 gave us,
that's going to give us a total of 3072.
So that's the grand total distance
to get to that network from our perspective.
And we talked about how the feasibility condition worked
and that condition tried to prevent us
from accidentally selecting a backup path
that we're going to switch over to almost immediately
that was dependent on our primary path.
It wasn't a perfect test, but it helps as we discussed.
And we said that any backup router
that passed the feasibility condition,
we could cut over to very quickly,
and those feasible successors, they would show up
in the output of the show IPE IGRP typology.
Does that mean that this is the only way
that EIGRP knows how to get to these networks?
Not necessarily, on our topology there is only one way
to get from any one place to any other one place.
But if we did have a more complex network
with lots of interconnections and backup paths,
we might have an alternate way to get to, for example,
10.1.1.0/ 24, but not show up here because maybe that route
had failed the feasibility condition.
That does not mean it's not a valid route to get there.
As we talked about with the query process,
if the router does not have a feasible successor
to go to a network, it will send out a query
throughout the network
to see if anybody else knows how to get there.
And if we had a valid backup path
which had failed the feasibility condition
we would discover that with the query process.
But here's a question,
what if I want to know about those backup routes
that failed the feasibility condition?
I'd like to see them as well in this output.
What we can do is say show IPE IGRP topology,
all hyphen links.
In this case, the output is not going to be any different
but the all hyphen links keyword
that's going to add in any additional path to a network,
if that path had failed the feasibility condition.
And that's a look at a basic EIGRP for IPv4 configuration.