- [Narrator] In this video we're going to see
how to configure rout summarization with OSPF.
First of all let's address what
rout summarization can do for us.
It can help us take multiple entries,
multiple network advertisements that
would be entries in our IP routing table,
and consolidate them down into a summary advertisement.
For example, we've got four networks coming into router R1,
192.168.0.0/24, .1.0, 2.0, 3.0.
Those addresses look like they have a lot in common,
in fact if we were to break this
down into binary we would see,
well we can obviously see that the first 16 bits
are in common, the first two octets.
But as we examine that first octet,
and if we start to count the number
of bits we have in common,
we actually have six bits in that third octet that are
in common among all four of these individual routes.
That means instead of advertising
the four routes by themselves,
we could send a summary advertisement.
And that's going to reduce the number
of entries in our IP routing table.
It can help with processor utilization on the router.
Maybe make it easier to troubleshoot as well.
So here's how we do this, we say I've got 22 bits in common.
So instead of advertising each of these /24 networks,
here's what I'm going to do.
I'm going to figure out what my subnet mask is,
22 binary one's followed by 10 binary zero's,
and dotted decimal, that's going to be a 255.255.252.0.
And the network's specifically that I'm going to be
advertising is what we get if we put 10 zero's,
the host bits, after all of those
22 bits that we have in common.
We make the last 10 bits zero's,
and that's going to give us the network that we advertise.
Which is going to be 192.168.0.0.
Specifically the route with the
subnet mask that we advertise,
the summarized rout, is 192.168.0.0/22.
And router R1 can send that single summarized route
as opposed to sending all those individual routes.
But with OSPF we have a couple of options
for doing this route summarization.
We might be sending routes from
one antonymous system into another antonymous system.
Like here we have router R3 acting
as an antonymous system boundary router,
sitting between the EIGRP antonymous system,
and the OSPF antonymous system.
There's one way to summarize routes
as we go between antonymous systems,
or we might want to summarize routes
as we're going from area one into area zero.
And we would do that on the area border router,
on the ABR, that's router R2.
Here are those two different options,
and the syntax we're going to be using in this video.
We can summarize at the ABR,
the area border router in this case,
for networks being advertised from
one area into another area.
That's where we can do the summarization,
as we're coming into the area.
But if we're coming in from another antonymous system,
like router R3 on screen.
We're coming in from that EIGRP antonymous system,
we could use the summary hyphen address command.
Now lets go out to a live interface,
and we're going to configure this topology,
where we're going to be summarizing
the loop back interfaces on R3, the 172.18 networks.
We're going to summarize those,
that EIGRP right now, as they're being injected into OSPF,
we're going to summarize those.
And also on router R2 we're going to summarize the 172.17
networks as they're about to be injected into area zero.
Now, let's go out to a live interface and check it out.
In this demonstration we want to take a look at a
couple of ways at doing that summarization.
As we said, there's one way to do it as we're going from
one antonymous system into OSPF.
There's another way of doing it
if we're going between OSPF areas.
We want to summarize the 172.18 networks on router R3,
those are router R3's loop back interfaces,
we want to summarize those as they're coming into OSPF.
And on router R2 we want to summarize
those loop back interfaces, the 172.17 networks,
as they're being injected into area zero.
Let's begin though, before we do anything, by going to R1
and just seeing what networks do we know about right now?
If I do a show IP route,
it looks like we know about all of
the individual routes, all the 172.17 routes,
and all the 172.18 routes.
Nothing is currently being summarized.
All right, let's fix that.
First let's summarize those 172.18 networks assigned
to router R3's loop back interfaces.
And doing the binary math as we did just a few moments ago,
we're going to see that we get the first
22 bits in common among all of those networks.
In other words, I want to send a summary address
of 172.18.0.0/22 into the OSPF antonymous system.
So lets do that,
we're going to do that on the ASVR, router R3.
And I'm going into global configuration mode,
and I'll say router OSPF process ID one,
that's already configured,
and here's how I do the summarization.
I'm going to say summary hyphen address
172.18.0.0 and we give a /22, which a dotted decimal is,
255.255.255.0. And that's it.
We're now sending a summary of
those four individual networks,
instead of those networks individually into OSPF.
Let's confirm that.
Let's go over to router R1,
where just moments ago we all of
the individual 172.18 networks,
lets see what we have now.
Let's do another show IP route, and look at this.
Instead of the individual networks showing up,
we've got a summarized network, 172.18.0.0/22, great news.
Now let's go to router R2 and summarize those 172.17
networks that are the loop back interfaces on R2
as they're being injected into area one into area zero.
Let's summarize those into 172.17.0.0/22.
Let's go over to router R2,
and let's go into OSPF router configuration mode
for process ID one.
And I'll say, area one range 172.17.0.0,
and a subnet mask is going to be 255.255.252.0.
Let's use some context sensitive help here,
oh and something I want you to note is,
this number right here, this area one range,
a lot of students have confusion
about what area number to put in here.
They wonder is it the area that I'm summarizing from,
or is this the area that I'm summarizing into?
Well it's the area that we're summarizing from.
Think of it as the area where things exist right now,
we're trying to change it,
but right now we've got those four
individual networks present in area one,
we want to summarize that,
so that's the network we're injecting from.
And also notice that one of the
things we could specify here is
the cost of a route as we're injecting it into another area.
If I don't specify the cost, what's going to be used?
Well here's what Cisco IOS does,
Cisco IOS is going to use the
best metric of all subordinate routes,
it's going to pick the one with the best metric.
In other words the lowest cost.
And it's going to assign that cost to the summarized route.
Or if I wanted to set it to something else,
I could set the cost here.
Oh and something else,
and this works the same as it did on the ASVR.
If I say, hey here's a summary route to send into this area,
if I don't have any subordinate
routes for that summarized route,
I'm not going to advertise the summarized route.
I've got to have something that actually falls into that
summarized address space or else we're not going to
advertise the summarized route.
Now let's see if this works, let's go ahead and press enter,
I'll just take the default cost,
and let's go over the router R1,
and once again we'll do a show IP route
and see if we have summarized those 172.17 networks.
Look at this, yes we have.
Instead of seeing those four individual networks,
now we're looking at a summary route of 172.17.0.0/22.
And that's a look at a couple of ways of doing OSPF route
summarization using the summary
hyphen address command on an ASVR,
and using the area range command on an ABR.