- [Instructor] In this video, we're going to examine
BGP Route Summarization, otherwise known
as BGP aggregation.
First, let's look at our topology.
We have router 1, an autonomous system 65100
and we have routers 2 and 3 in autonomous system 65200.
You can see that on router 3, we have four networks in place
and all these prefixes are being advertised out.
These are represented with loop-back addresses.
We have networks 10.10.0.0/24
through 10.10.3.0/24.
Routers 2 and 3 are connected over the
3.1.1.0/24 network, all of these within the AS 65200.
R1 and R2 are connected over the 20.1.1.0/24 network.
R2 in this case is our autonomous system edge router
sitting at the edge of AS 65200.
This particular router, R2, is going to aggregate
all of our 10.10 networks into a summary
over to AS 65100.
Let's first connect to R3 and check out
our current configuration in place.
Let's say show IP interface brief
and here, we can see all of our designated interfaces.
You can see our four loop-back addresses.
Each one of those assigned to a 10.10 network.
We have loop-back zero also assigned as 3.3.3.3,
but these are the four networks, these 10.10 networks
that we want to advertise over BGP.
So let's go under global configuration mode
and let's say router bgp
and we are using 65200 and let's say
network 10.10.0.0,
255.255.255.0 mask
we'll do the same thing for our other three
loop-back networks, 10.10.1.0
10.10.2.0,
and 10.10.3.0.
Let's end this and let's say
show ip bgp
and we see our prefixes that we're going to be
advertising here, so that looks good.
Let's go to R2, now.
And from here, let's also say,
now remember, this is our edge router
on autonomous system 65200.
So let's look at our BGP table, show ip bgp.
And we do see all four of those prefixes listed here,
we see the next hop address
of course, being that of router 3,
so that looks good.
So now, let's aggregate our routes
or let's summarize our routes and let's advertise these
to our neighboring autonomous system
which is 65100 where router 1 is.
Now, in order to do that, we need to know
how to calculate the correct route summary address.
There are certain calculators online
that will do this for you, but it's still important
that you understand exactly how this works.
First, we want to convert each of our network addresses
to a binary value.
If we take all of our 10.10 networks
and convert those to binary, we get the following values.
What we want to look at are the common bits.
In other words, how many bits do these network addresses
have in common that are exactly the same?
In this case, we have 22 bits in common.
The first 16 bits plus an additional 6 bits,
which totals 22 bits.
That means our network addresses will share
the first 16 bits which would be 10.10.0.0
with a slash 22 mask.
So in other words, 10.10.0.0 with a mask
of 255.255.252.0.
So this is a prefix summarization
of all four networks on R3.
So that's what we want to use as we aggregate our routes.
So let's go back on R2 where we're at,
and let's go under global configuration mode.
Router BGP
65200, and the key word we want to use
is aggregate-address
followed by the summary address that we've calculated
which is 10.10.0.0
with a mask of 255.255.252.0.
So let's hit Enter there, let's end
and let's say show ip bgp.
And now we also see our aggregate address
being advertised just as we would expect, here.
Let's go now, into router 1 and verify
the local BGP table in autonomous system 65100.
Let's say show ip bgp.
And notice we do see our aggregated address,
our bgp summarization address, but we also see
our four original /24 networks here, as well.
So in order to suppress these, let's go back
to our edge router which his router 2.
We'll clear off a little bit of space, here.
And let's go back under router bgp 65200,
and let's use a variation of our aggregate command.
This time, let's say aggregate-address 10.10.0.0,
same subnet mask 255.255.252.0
but at the end, let's also append
the key word summary-only.
So this is going to only advertise our aggregate address.
We won't see those /24 networks any longer on router 1.
So let's break out of here, and let's say show ip bgp.
Notice on router 2, we do still see our /24 networks,
all four of those, along with our aggregate network.
If we go back to router 1
and let's again say, show ip bgp
and now this time, we only see our bgp summarization,
our aggregate address 10.10.0.0/22.
We see the next hop is, of course, router 2.
Let's also say show ip bgp 10.10.0.0/22
and this is going to give us just a little bit
more detailed information.
We can see here, that the aggregator
of this route is router 2.
That's 2.2.2.2, we see that, here.
It's been aggregated in autonomous system
65200 by router 2.
And we also see again, that router ID, 2.2.2.2
and of course, we want to point out, also,
the automic aggregate that we see here, in this output.
This attribute is listed.
This is one of our well known discretionary attributes.
So this is informing R1 or any other BGPers
that we're using a less specific route to a destination.
So in other words, it lets this know
that we using an aggregated route.
So this essentially alerts BGP routers
that this actual route technically does not exist,
that it's a generated route representing
a contiguous group of addresses.
So that's a look at BGP summarization
through an aggregate address.