From a router's perspective, if it can know 10 roots
and do a great job or know 1,000 routes
and do the same great job, we should probably
opt for the 10 routes.
And in this video, we're going to do exactly that.
We're going to take a look at how we can summarize networks
with EIGRP at each interface, and in that process create
and describe null routes to help make sure we don't
have loops in our network.
So for this demonstration, let's imagine
that we have this topology. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
that we have this topology.
And over on this left-hand side of the network
we've got some 10.1 networks.
Over here on the right we have some 10.2 networks.
Up here we have some 10.0 networks.
And one of the benefits of doing summarizing
is that we can have a router.
Let's pick on R4.
And R4 on this interface, which is serial 3/1, we could say,
we've got the slow serial link done to all you guys.
Why bother teaching all the details about
the 10.1's and the 10.0's?
I can just send a summary.
So if we wanted to summarize a bunch of networks that began
with 10.0 and 10.1, what we could do is create a summary WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
with 10.0 and 10.1, what we could do is create a summary
that looks like this, 10.0.0.0/8,
and that would be a summary of the 10.1's and the 10.0's.
And we can just have R4 push that summary
down to its neighbor R6, who would then push it down to R8.
And when we create a summary and an interface with the EIGRP,
we're actually suppressing the more detailed routes.
So what that means is that the 10.1's, and the 10.0's, and all
those networks that start with 10,
they wouldn't bother being pushed
or advertised down further than R4.
And that means R6 and R8 wouldn't WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And that means R6 and R8 wouldn't
have to be aware of it.
Now, they would have a summary.
So if this PC here said, I need to send a packet to 10.1.7,
which is way over here on this side of the network,
it would use this default gateway.
And R8, its best match might be the 10 network,
the summary address that it got from R4.
And so it could route the packet that way and that way.
And then R4, when he gets it, he could route it appropriately
till it gets down to this PC.
And then the PC gets it and replies.
Now, here's the challenge for us.
What if PC-8 sent a packet to 10.40.1.2? WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
What if PC-8 sent a packet to 10.40.1.2?
And he spits it out-- sends it to this default gateway.
This router, Router 8, says, well, my best match
is the 10 network.
So it routes it this way.
R6, same thing, routes it this way.
And R4, if it doesn't have that route,
if it has a default route, it could forward it
based on its default route, which may not
be the optimal thing to do.
What also happens when we create a summary
route on the interface of R4, it also
creates a route for 10.0.0.0, that 8-bit route, that summary, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
creates a route for 10.0.0.0, that 8-bit route, that summary,
and it sends it over to the bit bucket, which we affectionately
call null 0.
Null 0 is basically like a trash can.
And if you have a route that says forward this packet
to the trash can, it just does.
Boom.
Dead.
Next packet, please.
Now, what about the customer's real traffic?
If this customer is trying to go to 10.1.7.10, 10 this PC,
and it forwards it up here, when it
gets to R4, if R4 has this route to null 0,
it's going to do the bit bucket.
But remember, in a routing table, WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
But remember, in a routing table,
it's always the longest match.
So if R4 has a route for the 10.1.7 network over here,
that's going to be the longest match.
It's going to match that route.
That packet's going to match that route.
And then R4 will use that route and forward it.
So the null 0 is just a nice safety measure
that Cisco does with EIGRP in the background automatically
so that if there's a packet that's
going for a network that's not known that's
within the summary, it'll just go ahead and be disposed
of without potentially creating a loop
or forwarding a packet to another router who may not
know what to do with it. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
know what to do with it.
So let me share with you how to create the summary.
We'll do it on serial 3/1.
And when we create that summary on this interface for 3/1
we'll also see how it automatically with EIGRP
is going to create that route for that summary address
that goes to null 0.
So before I demonstrate the summary here on R4,
let's first take a look at R8.
And in our topology, that's way down here.
I just want to make sure that he has roots-- he or she-- has
roots for the 10.1 networks, the 10.2 networks, the 10.0
networks before we do the summary.
That way we can do a before and after picture. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
That way we can do a before and after picture.
So let's go up to R8.
And we'll do a show IP route for EIGRP-learned routes.
And sure enough, we've got 10.0's, which are coming
from the top of our topology.
We've got 10.1's, which are coming from the left-hand side
of our topology.
And we've got 10.2's, which are local.
That's like R6 and R8.
They're playing with those 10.2 addresses.
Super.
So let's go to R4.
And the way we create a summary in EIGRP
is we go to the interface where we
want to advertise the summary.
So in our topology, let's take a look together. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So in our topology, let's take a look together.
In our topology, that would be this interface right here,
serial 3/1, where we want to cause that summary for the 10
network with an 8-bit mask to be advertised down this direction.
So here at R4, we'll go into configuration mode
and interface serial 3/1.
Also, let's just verify we're on the right router.
Do show IP interface brief.
Do show CDP neighbor.
Just to verify that we are.
OK.
Great.
So this is R4 connected down to R6 off of serial 3/1. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So this is R4 connected down to R6 off of serial 3/1.
So interface serial 3/1.
If I wasn't already there, we are now.
And then to create a summary in EIGRP,
we're going to use the command IP summary address.
Then we're going to specify EIGRP.
And then our autonomous system number,
which is 1 in this case.
And then we specify the summary that we want to go ahead
and put in.
So if we want to create a summary for 10 anything,
it would go something like this 10 with a slash 8.
And we are done. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And we are done.
So it's going to resync with its neighbor,
push that information out, and if we rush down
to R8, who just moments ago had tons of routes--
it had a whole bunch of 10.1's.
It had a whole bunch of 10.0's.
Now if we do an up arrow just to show
IP route for EIGRP-learned routes again and press Enter,
now it's pretty thin pickings.
It has this summary address that R4 caused to be advertised.
And it has some 10.2 networks, but it
doesn't have any of the other detailed 10.0's WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
doesn't have any of the other detailed 10.0's
or 10.1's, because those are all caught up in the summary
that R4 is pushing out on its serial 3/1 interface.
And if we go up to router 4 again
and on R4 if we do a show IP route,
here it created this route of 10.0.0.0
to slash 8 as a summary.
And it's sending it to null 0.
And that's to catch all the strays.
Any packets that come into R4, if it does not
have a more detailed route, something longer than 8 bits
for the 10 networks, it's going to go ahead and use this route
and just send it straight to the bit
bucket, sort of a loop prevention mechanism WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
bucket, sort of a loop prevention mechanism
that it has built in.
But in a production network, we can also
create static routes that point to null 0 anytime we
want take packets for a given destination address based
on that static route pointing to null zero and just dump it
right in the bit bucket.
So this video and demonstration, we
identified how we can do summary addresses at interfaces
in EIGRP with the command IP summary-address, EIGRP,
our autonomous system number, and then the summary
that we want to go ahead and advertise out that interface. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
that we want to go ahead and advertise out that interface.
And any more detailed routes that we
have in EIGRP that we're learning
we're going to suppress and not push down out
of that interface, rather just giving them
the summary that we created.
I hope this has been informative for you.
And I'd like to thank you for viewing.