Let's start off this module by covering what options are available with EIGRP to
summarize the different IP networks within an organization.
The use of summary routing is a common technique that is
used to limit the size of the routing tables of the
configured devices in a network.
It does this by aggregating multiple individual routes into a single or
a few route entries using a smaller prefix length.
This type of configuration is usually performed at the point
within a network where different networks converge.
On most enterprise networks this would be at the distribution and core layers.
In older versions of EIGRP the use of automatic summarization was
enabled by default and would try to advertise out at the classful
network boundary when possible.
This has since been disabled by default in favor of directed manual
summarization that is configured at the interface.
This type of configuration provides a more customizable solution,
especially in a time where classful network boundaries are rarely used.
With that said, let's take a look at this with an example topology.
From this module forward we are going to alter the topology that we are going
to be using for our examples and labs to make them a bit more realistic to
better show what these different features can do.
So here is the topology that we are going to use.
In this case, we have a simple hierarchical network with two core routers,
two distribution routers, and four access routers.
As you can see on the topology,
we have included some of the IP addressing information that we will be using.
For this topology we will be using /30 subnets for all WAN
lengths and /24 subnets for all LANs.
For this lab the LANs off the access devices will be
represented with loopback interfaces.
All of the networks that will be used,
both to and under the distribution one device,
will be in the 192.168 .0 .0 /17 network,
and all of the networks that will be used both to and under the distribution
two device will be in the 192.168 .128 .0 /17 network.
So if we look at this topology,
are there any points that you believe a summary may be useful?
Let's now move into the lab and start with the access devices and move up.
In this case,
we have preconfigured all of the devices with IP addressing information,
as well as EIGRP routing information.
As we can see from this routing table,
Access1 has a complete table with reachability information for all of
the different networks that we showed on the topology.
To verify a list let's move back to the topology for
a second and just take a glance, and as we can see from this,
we are currently looking at the Access1 router,
which has networks both in the 192.168 .15 network on its WAN side,
and the 192.168 .0 .0 /24 on its LAN side.
So if we wanted to make sure we had information everywhere we could
take a look and see if we could find information for those networks
that are connecting to Access129.
So, in this case,
that would be networks for the 192.168 .143 .0 network
and the 192.168 .129 .0 network.
Let's move back to the lab,
and if we look at this routing table we will see if we first
look at the 192.168 .143 network; 143 network,
there are currently 2 networks here.
one of them is the WAN link that goes between distribution 2 and Access128,
which would be this first one, /30, and the second one is this one,
which is the .4, which is another .30, so there are two .30 networks,
one of which connects between distribution 2 and Access128,
and the second one which connects between distribution 2 and access 129.
In this case,
it says go to 15.1 and if you remember the topology from Access1 back
into the network there's only one current link,
and that goes to distribution 1,
so 15.1 is distribution 1's IP address for it's WAN
link that connects to Access1.
So to reach either one of these networks it just goes to distribution 1,
which all of these routes will be because there's only one path
currently connecting Access1 to the rest of the network.
The other network that was on Access129 was the 192.168 .129 .0 /24 network,
and that you will see here.
In this case, it is a /24, and it still goes through distribution 1,
which is a 15.1 address, and if we look up one this is the 128 network /24,
which is the LAN or Access128.
But the question here is, since we're talking about summary,
does an access router at the edge of a network really have
to have this individualized knowledge, especially when,
in this case, Access1,
and actually for this topology that we're working with for this lab
specifically there's only 1 access link that goes between that
access device and the rest of the network,
and that should be very familiar to most of the people listening to
this because most of the people watching this,
at least at their home, are likely connected only through one link,
and when that happens you don't have individual routes on all of your
network devices that exist on your local network,
so you have a default gateway,
and that default gateway is usually a little home router,
and that's the one connection that connects you to the rest of the world.
So there's really no reason not to configure this access router the same way.
Now the two ways to configure this,
one would be to completely disable EIGRP on the access routers,
and just configure everybody with a static route.
This works fine as long as you want to manually
configure each one of the access routers,
and if you want to make any changes in the future you will have to
go out to every single one of these routers.
The other option is you can only advertise a default route from the
distribution down to all the access devices that are connected to it,
and in this case we only have two access routers
that connect off each distribution,
but the reason that I went from Access1 and Access2 to Access128
and so on was to sort of represent that there could be many
routers that connect off distribution 1.
It could be connected to 128 other access routers,
and there could be 128 other access routers potentially
connecting off a distribution 2 route.
So I don't think in most circumstances if you add a large
enough network that you would want to individually go out
to each one of these routers.
So one other option, as I noted,
was to actually advertise only an EIGRP default route.
In this case, we would do that on the distribution router,
and if we look, we just cheat a little bit,
and look at CDP information we'll see that distribution 1's
gigabit 0/3 interface is connected to Access1.
So if we go onto the gigabit 0/3 interface,
and we configure a manual summarization,
and in this case all we're doing is advertising a default route to Access1,
and what this command does is it takes all the individualized
routes that would be covered under that summary,
in this case it would be all the individualized routes,
and it will only advertise this one.
So if we hit Enter on this one, and it'll take a second because it'll resync,
and go back to the Access1 router and see the routing table.
We'll see now instead of having all these individualized routes that
are shown here with the D character up front here,
which means it's an EIGRP route.
So all of these that have a D on them all can go away in
favor of this one advertised default route.
Now if we just do a quick test and said,
okay, can we still get to the other side of the network?
So, in this case, 129.1 is the Access129 router,
and we can still get there if you looped back and looked at the topology.
This jumps through distribution 1, through the core,
back to distribution 2, and out to Access129.
So this is one position that you can configure summary.
You can configure summary on the distribution routers in order to
summarize down to a default or at very least a smaller portion of
your network to the access routers, and you can control that at that point.
So if we wanted to be consistent here we could go up the distribution router.
I can go to the 0/4 interface, which is the one that connects to Access2,
configure a summary on him, go to the distribution 2 router,
and just like distribution 1,
he's connected via those same interfaces to Access 128 and 129.
So what we did here is we jumped to 129,
just to show an example and show his routing table.
We will see that he has a default route and everybody else has default route.
The access routers will only have a default route
being advertised from the EIGRP, and I can still reach the other access routers,
but it greatly simplified the amount of routes that are
required to be known on the access router,
and that is especially helpful because, as some people may know who watch this,
an access router typically wants to be as small of hardware footprint as
physically possible because you're trying to save cost.
So basically you try to pick out an access router that has enough
power in order to perform its duties at that specific site and
with the minimum amount of original capital expenditure for each
one of those access devices,
since especially if you're in a large network where you
have lots of different access sites, you know,
buying a really expensive router on each one of those sites can be
prohibitively expensive if you can get away with a smaller router because it
only needs to know one route that is usually the best option.
Now if we move back to the distribution router and then we're
going to jump back to the topology for a second.
Now that's one point of summary.
Are there any other parts of this network that could benefit from
summarizing the knowledge from one point to another because those
devices really don't need to know.
And another place to do this would be instead of going from the
distribution down we could go from the distribution up.
Now, in this case, the distribution 1 device,
if we look on the left side of this topology,
all of the networks that are below it are within the 192.168 .0 .0 /20 network.
So there's no real reason that anybody north of him,
anybody above him needs to know about
individualized routes that come from there.
So the Core1 does not need to know the individualized knowledge
and status of the access routers below it,
because if you're really thinking about it the core routers
really want to have a limited view of the individual routes
that go on in the network.
It's better to keep their routing tables as skinny as possible,
but as many summaries as possible because just like at the access router
you're trying to have a limited number of route entries that exist there
because the smaller the tables that they have to search whenever they're
doing the routing logic the faster they are,
and since one of the primary purposes of the core is to
try to move traffic as fast as possible,
and you can move traffic fast as possible whenever the
tables are as small as possible.
So if we looked at this topology we can show that on the distribution 1
router he has two links up to the cores and you can summarize that
192.168 .0 .0 /20 network up to Core1 and Core2,
and on the distribution 2 device you can do a similar summary for the 192.168 .128 .0 /20 network.