- [Instructor] Earlier in this course,
we took a look at route summarization for EIGRP.
In this video, we going to consider route summarization
for OSPF.
The benefits are similar,
specifically we can take multiple route entries
in an IP routing table and consolidate them
into a single route advertisement
making it easier for us to interpret
what's in the routing table
and making it easier to troubleshoot.
For example, let's say that we've got
these four networks, 192.168.0.0.1.0.2.0.3.0.
Those all seem fairly similar, don't they?
It's like they're contiguous networks.
So instead of sending four separate route advertisements
and having four different route entries
in our IP routing table,
what if we created a summary- address,
that encompassed all four of those networks?
If we were to take those networks
and write them out in binary,
this is what we would get.
We know the first 16-bits are the same
because they all begin with 192.168,
but take a look at that third octet.
They have the first six bits in common, in that third octet.
So here's the dividing line.
If we take a look at this, we see that
they all have the first 22-bits in common.
So what we can do,
instead of sending those four advertisements separately,
we can send a single route advertisement.
Now, what is that route advertisement?
Well, we going to take those first 22-bits
and we'll set the remaining 10-bits,
the host bits to zeros.
And that's what we advertise.
If we're advertising a 22-bit network,
then our subnet mask in binary would be 255.255.252.0
Again, we took the 22-bits that we had in common,
and we say will put 22 ones in the subnet mask
because they all have those bits in common
and we'll set the remaining 10-bits,
the host bits to zeros.
And that's our subnet mask.
Now, if I take the actual 22-bits that we have in common
and I write those out, and I put zeroes in the last 10-bits,
this is what we have in binary, and then in decimal,
it's going to be 192.168.0.0,
but we're using a /22 subnet mask.
In other words, 255.255.252.0.
That is the single advertisement that we could send,
instead of those four individual advertisements.
So the four networks are advertised into R1,
but then it summarizes those as a single advertisement.
Now, with EIGRP, we could have done summarization
on pretty much any of our routers.
However, there are some restrictions with OSPF.
If we want to do route summarization with OSPF,
we can do it either, at an area border router
an ABR, as we're going from one OSPF area
into another OSPF area,
or we can do it at the ASBR
the autonomous system boundary router,
as we're going from one autonomous system
into another autonomous system.
In this topology that you see on screen
R2 is the ABR, it's bordering areas zero and one
and router R3 is the ASBR.
It's interconnecting the OSPF autonomous system,
and the EIGRP autonomous system.
And the syntax is different,
depending on whether we're configuring summarization
at the ABR or the ASBR,
we're going to use the area range command,
and we'll do that for you in a moment in the live topology,
we'll use the area range command, on the ABR,
and we'll use the summary-address command on the ASBR.
Now let's go out to this topology
and configure route summarization,
both at the ABR and at the ASBR.
In this topology, we've got a couple of OSPF areas.
We've got area one that connects
into an ERGRP autonomous system.
So that means that, R3 on that topology
that's our ASBR and R2, that's our ABR,
our area border router,
because it's sitting at the boundary
of areas zero and one.
So let's go over to R1 right now
and see what routes it currently knows about.
I've already got OSPF
and EIGRP route redistribution configured.
So R1 should know about all of these networks.
Let's do a show, IP route command and verify that.
Yeah, sure does.
Notice R1 has learned about those area one routes
that begin with 172.17, we've got 172.17.0.1.1.1.2.1.3.1
Those are the IP addresses of loopback interfaces
on router R2.
We've also learned about networks,
in the EIGRP autonomous system.
They began with 172.18.
And because they seem to be contiguous networks,
they are good candidates to be summarized.
Now, these networks that begin with 172.17
they're in another area.
So we going to do summarization for those on the ABR,
the 172.18 networks,
they're in a different autonomous system.
So we need to go to the ASBR to do summarization for them.
But first, let's consider the 172.17 networks.
How many bits do they have in common?
Well, just like we saw a moment ago
when we have a zero, one, two and three in that third octet,
we've got the first 22-bits in common.
So what I could do is send a summary advertisement
for 172.17.0.0/22 and suppress the sending
of those four individual routes.
Let's configure that on our ABR, router R2,
let's go into router configuration mode
for OSPF process ID one.
And here we're going to use the area range command.
I'm going to say area, and I give the area
containing these networks and that's area one,
and I'm going to say, range
and I'll specify the summary advertisement
that I wish to send.
It's going to be 172.17.0.0 with a 22-bit subnet mask,
because each of those networks
have the first 22-bits in common.
And that's going to be 255.255.252.0
And we're done on R2.
Let's go back over to R1 and confirm that
those 172.17 networks are now summarized.
What's once again issue the command that show IP route.
Great news, that 172.17.0.0/22 network.
That is a summary advertisement,
and we no longer see the individual routes.
Now let's turn our attention to the 172.18 networks
that live in EIGRP.
We need to go to the ASBR to set that up,
and we going to use the summary-address command there.
The subnet mask is going to be the same.
These networks still have the first 22-bits in common.
We want to send a summary advertisement of 172.18.0.0
with a 22-bit subnet mask.
So let's go into router configuration mode
for OSPF process ID one, on the ASBR.
And instead of using the area range command,
I'm going to say summary-address.
And I give the address summary that I want
to advertise instead of those individual networks.
And that's going to be 172.18.0.0 with a 22- bit subnet mask,
which in dotted decimal is 255.255.252.0
and we're done on R3.
Let's go back over to R1 and confirm,
that we're now summarizing these four networks.
Let's do another show IP route,
and look at this, 172.18.0.0/22
We see that summary advertisement
instead of those four individual routes.
So what we've done here in this video
is we've seen how to significantly, in some cases
reduce the size of our routing table,
which might make it easier for us to do verification
and troubleshooting, by summarizing inter-area routes
with the area range command,
and summarizing external autonomous system routes
with the summary-address command.