So in the last video while we were looking at the area range
command, you might have noticed that there was
an option there not advertise.
And this gives us an interesting opportunity
to do sort of an unusual form of filtering in OSPF.
We can actually use the area range command to filter routes
from going into a certain area.
Now, is this something that's very commonly done?
Not particularly because we do have
other ways of filtering routes between areas.
However, it is an interesting feature
so we should at least take a moment here
to take a look at it and see how it works.
Now we're going to be working with those same routes coming
from router 10.
So I don't think we need to necessarily
look at the topology here, we're just
going to be looking at those routes
that we summarized before.
We'll be taking out the summarization that we already
put in there to the /16, and then we'll be doing some
filtering to filter out some of the /24s.
So let's just start by jumping right into the command line
on router 7.
We'll take a look at what we already have,
and then we'll make our changes over on router 6
and take a look at how the not advertised option works
on the area range command.
Let's get started.
So if we do a show IP route here on router 7,
what we're going to be looking for is that 100.100..0.0/16
which is the summary that we have left over from
our previous video.
Now if we're going to use this for filtering,
the first thing we're going to have to do
is get rid of that summary.
So let's do that.
Hopping over to router 6, well simply go
into configuration mode for router OSPF 1.
And we'll do a no area 200 range 100.100.0.0,
and we'll put in the mask.
This should cause the removal of that summary.
Let's make sure by hopping back over
to router 7 we'll simply look at our routes again.
And we see that we have all of the 100.100 routes back again.
So we have all four subnets.
So now let's get rid of one of these subnets using the area
range not advertised command.
We'll go ahead and pick on this route, the 100.100.110.0.
So back over to router 6.
And we'll say area 200 range, 100.100.110.0 with its mask.
And we'll say not advertise.
Now an interesting thing to note here,
we're not even actually summarizing.
This is already a /24, and we're summarizing it to a /24.
So we're not really even summarizing it, are we?
Instead what we're doing is just matching on an existing route,
and saying not advertise.
Now, understand this feature will work either way.
So we could put a range here, and it would not advertise
everything in that range.
In this case, we're just picking on that one particular route,
but just be aware that you don't actually
have to do summarization in order for this process
to kick in.
So if we jump back over to router 7 now
and we look at our routes again, we see that as expected,
we have the other three subnets.
We have the 100, the 120, and the 130.
But the 110 is, in fact, missing.
It is now being filtered out by the area
range not advertise command.
So let's go back over to router 6.
The first thing we'll do is let's
take that command back out.
So if we verify on 7 again, they should all be back.
And we can see that they are.
And now let's demonstrate what I was talking about about
doing it as an actual range.
So, again, if we go back to 6 here,
but now we change this back to the range like we did before,
we'll do the /16 again, this will prevent all the routes
in that range from being advertise.
Once, again, we'll go back over to seven, look at our routes
again.
And we see that now we're missing all of the 100.100
subnets.
So, again, this can be used on a single route
or a range of routes just like a normal summary.
So this allows us to see how we can use this for filtering.
So now let's change this back to the original summary again.
To do this we would have to go back to router 6.
We'll take out the existing area range command.
This should put all the routes back.
We can see here that it has.
Then we'll put the summary back in as it originally was.
So it had a cost of 100, and, of course,
we take the no off of it.
We'll verify it one last time over on router 7,
and we can see that we're now back to the 100.100.0.0/16 with
a cost of 101.
So in this video, we just took a look
how we can use the area range command,
along with the not advertise option
as an actual way of filtering routes between areas in OSPF.
Now keep in mind one big limitation
to filtering routes this way in OSPF
is, first, we can only do it on the ABR,
but that's OK because that's the only place that we
can filter in OSPF anyway.
But we can only do this on the originating ABR.
So the ABR that would normally be doing the summarization,
but rather in this case we're doing filtering.
I hope this has been informative for you,
and I'd like to thank you for viewing.