When you start working with routers, especially
on larger networks, you'll find that your routing table starts
getting larger and larger and larger.
And one of the things you can do to optimize this routing table
is to summarize the routes through something
like route aggregation.
You may see this referred to as supernetting, prefix
aggregation, route summarization.
It's effectively all the same thing.
We're taking a list of routes and we're summarizing them down
to perhaps one single route that we can then
use to make our routing tables much more efficient.
This is going to simplify the methods that the routing table
uses to make a decision.
So our routing is going to be faster.
We're also going to use less memory
since every entry in the routing table
takes up space inside of the memory of our router.
And if we're doing dynamic routing
there will be fewer routing advertisements
that we're sending out.
To be able to take advantage of this route summarization
we're going to want to either statically route
or use a routing protocol that understands CIDR block
notation.
An example of these might be RIP version two, OSPF, EIGRP,
or BGP.
To summarize our routes we would look
at all of the routes in our routing table.
And we want to know what part of these routes
are similar across all of these.
The best way to see this is if we take each route
and we write it out in binary.
So I've done that with my routes of 192.168.98.0
is 192.168.98.0.
And we're going to write out all of our addresses, all
the way down, for all of the routes inside of our device.
Then we're going to look at all of these
that we've written down.
And we're going to try to find the things that are similar
and the things that are different between them.
And I've highlighted this in my list
by making them green and orange.
And you can see that you've got a similar octet 1.
Octet 2 is exactly the same.
Octet 3 is a little bit similar.
Notice that the first four bits of octet 3 are identical.
But the last four bits begin to deviate.
So everything from the last four bits of octet 3,
and of course everything in octet 4,
are going to be different.
So we've separated those by colors.
And now we can decide where we need to summarise.
So we're going to take this separation that we've done.
And at the dividing point everything else
is going to be a 0.
So if we take that binary and we convert it back to decimal
we're able to summarize all of this as 192.168.96.0.
And we need to determine how many bits
are going to be identical for all of these.
And if we count eight, 16, 17, 18,
19 and 20 bits in total are going to be identical between
all of these routes, which means that we could use a mask of /20
to summarise this.
So instead of putting all of these separate addresses
in a routing table we can simply put
one single route in a routing table
and point that to the next hop.
Sometimes you have to calculate this aggregation yourself
and configure your router manually.
But a number of routers and operating systems
are able to aggregate these routes automatically
inside the operating system.
So you may request a list of all the routes
that a device knows about.
But it is giving you a summarized view
so that it's able to minimize the number of entries
in its routing table.
One of the great things about having
all of these choices of dynamic routing protocols
is that we can find the one that fits perfectly
for our organization and use it inside of our company.
But other companies may choose to use other dynamic routing
protocols.
And if we ever need to communicate
to one of those other companies what we'll find
is that these different routing protocols will not
talk to each other.
For instance, we have RIP in this company.
This company's using EIGRP.
This company down here is using RIP Version 2,
Company B using OSPF.
And connecting them all together is BGP.
Our OSPF can't update the routes that are inside
of Company C's EIGRP.
Because these routers are effectively
communicating with completely different routing protocols.
So now we've got this problem.
We have these autonomous systems that
are managed by completely different people that
have decided on their own sets of dynamic routing protocols.
And of course, none of these routing protocols
can communicate with each other.
You might have RIP Version 2 on one side
and you might have OSPF on the other side.
So one of the ways that you can take routes from one system
and convert them so that they can
be used in a different system is to use something
like route redistribution.
This can redistribute these routes
that we're learning about via one method
and redistribute them through another method entirely.
You might take routes that are being determined through RIP
Version 2 and change them so that they
can be advertised through OSPF, or take our OSPF routes
and redistribute so those can be seen on a BGP network.
Route redistribution also allows us
to take routes that aren't normally advertised
on our internal network and advertise them to the outside.
This may be useful if we need to advertise a service so
that people can gain access to a web server or a file server
inside of our organization.