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So in the last video while we
were looking at the area range

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command, you might have
noticed that there was

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an option there not advertise.

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And this gives us an
interesting opportunity

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to do sort of an unusual
form of filtering in OSPF.

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We can actually use the area
range command to filter routes

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from going into a certain area.

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Now, is this something
that's very commonly done?

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Not particularly
because we do have

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other ways of filtering
routes between areas.

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However, it is an
interesting feature

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so we should at least
take a moment here

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to take a look at it
and see how it works.

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Now we're going to be working
with those same routes coming

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from router 10.

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So I don't think we
need to necessarily

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look at the topology
here, we're just

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going to be looking
at those routes

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that we summarized before.

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We'll be taking out the
summarization that we already

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put in there to the /16,
and then we'll be doing some

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filtering to filter
out some of the /24s.

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So let's just start by jumping
right into the command line

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on router 7.

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We'll take a look at
what we already have,

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and then we'll make our
changes over on router 6

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and take a look at how the
not advertised option works

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on the area range command.

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Let's get started.

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So if we do a show IP
route here on router 7,

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what we're going to be looking
for is that 100.100..0.0/16

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which is the summary that
we have left over from

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our previous video.

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Now if we're going to
use this for filtering,

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the first thing we're
going to have to do

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is get rid of that summary.

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So let's do that.

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Hopping over to router
6, well simply go

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into configuration
mode for router OSPF 1.

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And we'll do a no area
200 range 100.100.0.0,

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and we'll put in the mask.

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This should cause the
removal of that summary.

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Let's make sure by
hopping back over

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to router 7 we'll simply
look at our routes again.

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And we see that we have all of
the 100.100 routes back again.

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So we have all four subnets.

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So now let's get rid of one of
these subnets using the area

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range not advertised command.

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We'll go ahead and pick on
this route, the 100.100.110.0.

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So back over to router 6.

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And we'll say area 200 range,
100.100.110.0 with its mask.

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And we'll say not advertise.

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Now an interesting
thing to note here,

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we're not even
actually summarizing.

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This is already a /24, and
we're summarizing it to a /24.

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So we're not really even
summarizing it, are we?

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Instead what we're doing is just
matching on an existing route,

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and saying not advertise.

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Now, understand this feature
will work either way.

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So we could put a range here,
and it would not advertise

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everything in that range.

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In this case, we're just picking
on that one particular route,

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but just be aware that
you don't actually

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have to do summarization
in order for this process

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to kick in.

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So if we jump back
over to router 7 now

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and we look at our routes
again, we see that as expected,

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we have the other three subnets.

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We have the 100, the
120, and the 130.

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But the 110 is,
in fact, missing.

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It is now being
filtered out by the area

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range not advertise command.

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So let's go back
over to router 6.

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The first thing
we'll do is let's

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take that command back out.

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So if we verify on 7 again,
they should all be back.

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And we can see that they are.

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And now let's demonstrate
what I was talking about about

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doing it as an actual range.

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So, again, if we
go back to 6 here,

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but now we change this back to
the range like we did before,

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we'll do the /16 again, this
will prevent all the routes

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in that range from
being advertise.

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Once, again, we'll go back over
to seven, look at our routes

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again.

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And we see that now we're
missing all of the 100.100

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subnets.

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So, again, this can be
used on a single route

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or a range of routes just
like a normal summary.

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So this allows us to see how
we can use this for filtering.

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So now let's change this back
to the original summary again.

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To do this we would have
to go back to router 6.

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We'll take out the existing
area range command.

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This should put all
the routes back.

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We can see here that it has.

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Then we'll put the summary
back in as it originally was.

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So it had a cost of
100, and, of course,

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we take the no off of it.

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We'll verify it one last
time over on router 7,

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and we can see that we're now
back to the 100.100.0.0/16 with

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a cost of 101.

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So in this video,
we just took a look

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how we can use the
area range command,

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along with the not
advertise option

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as an actual way of filtering
routes between areas in OSPF.

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Now keep in mind
one big limitation

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to filtering routes
this way in OSPF

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is, first, we can
only do it on the ABR,

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but that's OK because that's
the only place that we

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can filter in OSPF anyway.

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But we can only do this
on the originating ABR.

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So the ABR that would normally
be doing the summarization,

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but rather in this case
we're doing filtering.

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I hope this has been
informative for you,

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and I'd like to thank
you for viewing.

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