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[MUSIC PLAYING]

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So now that we've looked at
both internal and external route

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summarization in OSPF,
let's take a look

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at some of the limitations
of that summarization.

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Now we have spoken about these
things as we've gone through

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those sections as far as which
routers you can summarize

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on and which ones you can't.

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The main purpose of
this video is going

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to be to demonstrate that.

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We're going to try to summarize
places where we're really

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not allowed to.

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And we'll see the result.
Now the result, of course,

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is going to be that
it doesn't work.

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But the main thing is
to understand that.

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And really just prove that
those are the rules in OSPF.

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So let's start off by taking a
quick look at our topology just

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to discuss what we're
going to be trying to do.

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And then, of course, we'll
jump into the command line,

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set up some things,
and see that these are,

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in fact, the rules for OSPF.

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So here's what we're
going to do in this video.

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We're going to take the routes
that are coming from router 10

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through router 6, which are, of
course, the 100.100 networks.

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And they are currently a /16.

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We're going to take a
look at these on router 5

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this time as it's going
into another area, area 100.

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We'll see that it's a /16.

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And then what we'll do is we'll
attempt to summarize it again

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here on router 1 to a /8.

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So, in other words,
we will attempt

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to summarize our summary.

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This cannot be done in OSPF.

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Again, we're going to
do this to prove that.

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Then we're going to move
on to our externals.

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With the externals,
we're going to do,

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essentially, the same thing.

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These routes are currently
being brought in on router 2.

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These are the 200.200 networks.

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Again, currently
coming in as a /16.

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And what we'll do with these
routes is we'll attempt

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to summarize them on router 6
coming up into area 200, again,

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to a /8.

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As we've stated, these
aren't going to work.

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But our main point is just to
show that and demonstrate that

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you cannot summarize
a summary in OSPF.

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So let's jump into
the command line.

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We'll start with
the internal routes.

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And, therefore, we'll start
by looking at router 5.

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So looking at these
routes here on router 5,

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we'll, again, just do
a show IP route OSPF.

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And here we'll look
at the IA routes.

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Again, the internal
route being summarized.

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And what we're looking for
is that we do, in fact,

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have this 100.100.0.0/16 route.

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This is what we're expecting
because of the summarization

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we did previously.

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So now let's see
what happens if we

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try to summarize this on
the ABRs for area 100 so,

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in other words,
router 1 and router 3.

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We'll start on router 1.

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Now this is our first time
configuring the area range

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command on iOS XR, but
it's very, very similar.

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The only real difference
is you say area 0.

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And then we put range
on a separate line.

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The other slight
difference is, of course,

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the way we enter the
address and the prefix.

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Rather than putting the prefix
in decimal format or the mask,

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we do /notation here on iOS XR.

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So, again, we're going to try
to summarize this to a /8,

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so it would be 100.0.0.0/8.

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Notice we have the
same basic options here

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on iOS XR that we did in iOS XE.

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Again, advertises the
default. And we're

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not trying to filter right
now, so let's just hit Enter.

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We'll commit our changes.

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And we'll go do the
same thing on router 3.

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So now let's go check router 5.

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We simply repeat the
command on router 5.

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What we notice is that our
100.100.0.0 is still a /16.

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So as we've said numerous times,
you cannot use the area range

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command on what is
already a Type 3 LSA.

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It can only be done on the
originating ABR that's bringing

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these routes into OSPF.

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They have to be an O route or
learn from a Type 1 or a Type 2

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LSA before we can use
the area range command.

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Not only can we now
summarize a summary,

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but we can't filter
that summary either.

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Let's go back over to router 1.

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And we'll make a change here.

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What we're going to
do is under area 0,

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well say no range 100.0.0.0/8.

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Again, we already know
that's not working.

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But what if we do this?

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What if we say range
100.100.0.0/16.

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So we're not trying
to summarize it again,

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we're matching on the existing,
but we add the not advertise.

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So we'll commit these changes.

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Of course, we have to do the
same filtering on router 3

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if we're going to see it work.

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But we'll notice when we
go back over router five,

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look at our routing
table again, we notice

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that still nothing's changed.

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So not only can we not use
this to summarize further

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or to summarize a
summary, we can't

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use this for our filtering
between areas either.

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So this is going to be
totally ineffective on routes

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that are already summarized.

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Now let's take a look
at the same thing

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but with external routes.

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So to look at the externals,
let's jump up to router 10.

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And we'll do a show
IP router OSPF.

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And this time, we'll
look at the E2 routes.

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And we see that we
have this summary.

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The 200.200.0.0/16 coming from
all the way over on the EIGRP

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side coming through router 2.

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So let's try to summarize
this on router 6.

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Because if we were going to be
able to summarize it somewhere,

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it would probably be on the ABR.

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So let's go there.

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So going to OSPF 1.

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Now since these are
external, clearly,

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we would have to use the
summary-address command.

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And we would try to summarize
them to say 200.0.0.0.

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And we'll make these a /8.

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But we'll see that
if we go up here

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to router 10,
repeat our command,

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it's done absolutely nothing.

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We still have the
200.200.0.0/16 as the summary.

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Once again, we can also show
that the filtering doesn't

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work.

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If we go back up to
router 6, let's attempt

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to match on the network as it
currently is and do the not

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advertise option.

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So attempting to filter it.

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We'll, again, go back to
router 10, hit the up arrow.

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And once again, we see that
this has had no effect.

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So, in this video, we
looked at the limitations

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of summarization in OSPF.

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To summarize, once
again, you can only

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do route summarization
for internal routes

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on the ABR that's
originating the routes

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into the other areas, the
ones that are being learned

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from Type 1 or Type 2 LSAs.

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You cannot summarize or filter
using the area range command

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after the route is
already a Type 3 LSA.

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Likewise, on the
external side, you

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can only do this when you
are originating the route

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on the originating ASBR.

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Once it's a Type 5 LSA, then
the summary-address command,

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or summary-prefix command,
has no effect on it.

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We can't filter it.

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We can't summarize it again.

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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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