- In this video,
we're going to be considering an EIGRP route summarization.
And the idea behind route summarization is,
instead of sending individual route advertisements
for individual subnets, what we can do is say,
you know, these subnets, a lot of bits in common.
Instead of advertising them all individually,
why not just send a single advertisement
that encompasses the IP address space all of those subnets.
And that's going to give us some obvious benefits
and maybe some non-obvious benefits.
Obviously it's going to reduce the size
of our routing table.
Maybe I can summarize, what say 25 networks
with a single summary advertisement.
That's going to dramatically reduce the size
of my routing table.
But if you had the option of doing so,
if you're laying out sub-net addressing
for a new installation, it's best if you consider
if you're going to be using summarization now
or in the future.
What you don't want to have happen is,
maybe have the 10-Dot address space,
that private IP address space scattered
throughout your network.
You've got some 10-Dot subnets going in one direction
out of a router and out of another interface on the router,
you've got other subnets of the 10-Dot network.
So if possible, if we've got the option
of setting up the subnet addressing from the beginning,
we want to have contiguous IP address spaces close together,
where they can all be summarized together.
Something that's unique about EIGRP as opposed to OSPF,
is that we can do this summarization
on any EIGRP speaking router.
That's unlike OSPF,
where we have to do summarization at the ASBR
as we're coming into the OSPF autonomous system
or at the ABR
as we're going from one OSPF area into another OSPF area.
Another benefit that might not be so obvious is that,
this can reduce the scope of a query.
In other words how far a query can travel
throughout our network.
Remember that an EIGRP router has a successor route,
that's its primary route to get to a destination network.
And if we have a backup route
and that backup route has passed the feasibility condition,
then it's a feasible successor.
But if our successor rock goes down
and we do not have any feasible successor,
then the rattler is going to send out a query
to see if there's another way to get to this network.
Maybe a route that has failed the feasibility condition.
And to do that, it's going to send out a query
and this query can be propagated
through a very large network being replicated over and over.
If we don't want that to happen,
EIGRP route summarization can help us out,
because an EIGRP speaking router
is not going to afford a query for network
if the network that is being queried for,
if that network is not explicitly
in that routers IP routing table.
It has to be an exact match.
So a summarized route is not going to be an exact match
and that's going to stop the query right there.
But remember, we can't have suboptimal routing if we have
for example, contiguous IP address spaces
scattered throughout our network.
So what you want to do when you're setting this up,
is ask yourself,
is this summary route that I'm about to advertise,
does it encompass any IP address space
that lives somewhere else in the network?
If it does, you need to rethink your summary advertisement
or maybe not do summarization.
And with EIGRP, we can set up summarization manually,
and we're going to do that in this video,
or we can set it up automatically
using EIGRP auto summary feature.
Now specifically in this demonstration,
we're going to work with this topology.
And you see that we've got
some networks off of the switches SW1, 2, 3 and 4.
In fact, here are those networks
that we're going to be summarizing.
And you can see that they've got a lot of bits in common.
They all begin with 172.16,
so we know that the first 16 bits are in common,
but now let's look at that third octet
up close and personal.
If we look at that third octet
and we break those decimal values down into binary,
we see that third octet has six bits in common.
So we've got 16 plus 6,
we've got 22 bits in common among all of these networks.
So what we're going to be able to do
is to summarize these networks
as a single route advertisement.
We're going to take all those 22 bits that are in common,
and we're going to set the remaining 10 bits
in this IP address to zeros.
And we're going to use a 22 bit subnet mask.
That's going to give us 172.16.0.0/22.
That's what we're going to set up in this video.
And then, we're going to see how auto summarization
can be used to summarize all those 10-Dot networks
between R1, 2,3 , and 4, feeding into R5.
Now let's hop out to the live interface
and configure both manual and automatic summarization.
Let's begin by taking a look at the IP routing table
on router R6.
Let's do a show IP route command.
And we see that we've got some networks
that we might want to summarize.
First we've got all these 10-Dot networks,
10.1.1.0,
.1.4, .1.8, .1.12.
We're using a 30 bit subnet mask.
So we're counting by four in that fourth octet
to come up with the network addresses.
We'll summarize those networks
in a few moments using auto summarization,
but let's begin by doing a manual summarization
of the 172.16 networks.
Notice that we've got 172.16.0.0, .1.0, .2.0, .3.0,
and we saw just a moment ago
that these four networks, these subnets,
they have 22 bits in common.
So let's do this.
Let's go to router R5 and tell router R5,
when you're advertising over to router R6,
instead of sending these individual route advertisements,
to reduce the size of the routing table
instead, let's send a summary route.
Let's take those 22 bits that are in common.
Let's set the remaining 10 bits to zeros.
Let's use a 22 bit subnet mask,
which means we're going to be advertising a network of
172.16.0.0
with a 22 bit subnet mask.
In other words it's going to be 255.255.252.0.
Let's go over to router R5 and see exactly how we do that.
On router R5, we need to go into the interface
that's going to be sending the summary advertisement
over to router R6.
And here's the command we give,
we'll go into interface, gigabit 0/5,
that's the one leaving R5 going into R6,
and I'm going to say IP summary-address.
And this is going to be for EIGRP autonomous system one.
And the summary network that I'm going to be sending,
is 172.16.0.0.
And our subnet mask is 255.255.252.0.
That's a 22 bit subnet mask.
And when I do that, it's going to re-sync with the neighbor.
And let's go over to router R6
and see if things look different now.
Over on router R6, I'll do a show IP route.
Look at this.
Instead of seeing
those four individual network advertisements
for those 172.16 networks,
now we're advertising a single route.
We're being told that it's subneted,
but we're advertising the single route
of 172.16.0.0/22.
And we see that to get to any of those subnets
within that summary route,
we're going to be going to 192.168.1.1, that's R5.
But as we look at the output of this IP routing table
we see there's another opportunity to do some summarization.
Look at these 10-Dot networks.
We could summarize all of those.
And we're going to do that in this example,
using the auto summary feature.
Now the auto summary feature isn't going to do
the due diligence that we did,
which was determine exactly how many bits
all of these subnets have in common.
No, it's going to say
"Oh, we've got some subnets
that all belong to the same classful network.
I'm just going to advertise that classful network."
In other words it's going to advertise 10.0.0.0/8,
because an eight bit subnet mask is the default subnet mask.
It's the classful mask for a class eight network.
And we know it's a class eight network
because we've got a 10 in the first octet.
So the caution is, make sure you don't have other subnets
of the 10-Dot network somewhere else in your network
if you're going to be doing this.
Let's go over to R5 and see exactly how to set that up.
Oh, by the way, we can see how we're currently set up
for summarization by doing a show IP protocols command.
And it tells us right now
that we're doing address summarization out of gigabit 0/5,
we advertising 1702.16.0.0/ 22
and we're summarizing four components.
In other words, four subnets.
But you'll notice that automatic summarization is disabled.
Let's turn on automatic summarization,
and we do that within router configuration mode.
Let's go into global configuration mode
and then we'll say router EIGRP autonomous system one.
And I'll say auto-summary and that's it.
That turns on auto summarization
and notice that we did a re-sync with all of our neighbors.
Let's go over to R6 now and see how things look there.
Let's once again do a show IP route command.
And now of those four individual 10-Dot networks,
we have 10.0.0.0/8.
We're advertising the classful network
that encompasses all of those subnets.
And that's a look at how we can configure
EIGRP route summarization,
both manually and automatically.