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[AUDIO LOGO]

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So next up, let's take a look
at summarizing external routes.

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Now, truth be told, this
is very similar in process

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to summarizing the
internal routes.

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But, of course, it has to
happen on the ASBR, which

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is the one bringing the routes
into OSPF, to begin with.

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And the command is a
little bit different.

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Not only that, it's a little
bit different between iOS XE

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and iOS XR as well.

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So we will look
at both of those.

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We'll start with iOS XR.

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So let's take a look
at our topology,

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see where these routes
are coming from,

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where we're going
to summarize them,

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and then we'll jump
into the command line

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and see external
summarization in action.

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So let's start by taking
a look at that topology.

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So in this
demonstration, we will

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be focusing on those four
external routes coming

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from router 9.

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These are the routes
that begin with

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200.200.something.something,
but it's really the third octet

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where they're different.

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So remember that these
four routes are being sent

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through EIGRP up to router 2.

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Router 2, in this
case, is the ASBR.

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So that, of course,
means that this

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is the router redistributing
these routes into OSPF.

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That means that this is
where we will have to do

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our external summarization.

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Now keep in mind that
router 2 is running iOS XR,

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so that's how we'll
be configuring it.

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We'll also take a
look at the command

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just on one of our
iOS XE devices.

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So, as usual, let's
jump in and start

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by taking a look at
these routes on router 7.

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Looking at this sort of as
our looking-glass router.

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We'll see what these routes
look like, to begin with.

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And then we'll implement
our summarization.

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So let's jump over to
that command line on R7.

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So, once again, we'll do
our show IP route OSPF here

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on router 7.

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But this time, we will
filter for external type

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2's because that's what these
200.200 routes are coming

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in as.

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So, as we can see as we begin,
we have four subnets coming

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from router 2, which are
in the 200.200 range.

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And, similarly, to what we
did with our internal routes,

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these are simply subnets
200, 210, 220, and 230.

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So now let's jump
over to router 2

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and take a look at
how to summarize

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these external prefixes.

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So as usual, of course, we
go to Configuration mode

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for router OSPF 1.

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And the command here on
iOS XR is summary prefix.

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And the way we
enter this on iOS XR

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is in the form of
the address/the mask.

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So, in this case, it
would be 200.200.0.0/16.

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We're going to summarize to
a /16 just like we did with

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the internals.

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If we take a look, we again
have some options on this.

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We can set a route
tag if we want to.

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And we're not going to
demonstrate it again

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because it works the same way
as it did with the internals,

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but notice we also have that
option to not advertise.

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So we can, again, use this
as a form of filtering.

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In our case, we're just
going to hit Enter.

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Of course, we'll
commit our changes.

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And let's go back and
verify this on router 7.

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So we'll just hit our up
arrow, repeat the last command.

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And we see that, of course,
now we have that /16.

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Just for completeness,
let's take a look

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at what happens when
one of these routes

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flaps just like we did
with the internals.

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So, once again, we'll do a debug
IP OSPF SPF here on router 7.

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Then we'll go over
to router 9, which

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is where these prefixes are
actually originating from.

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And we'll take down
one of our interfaces.

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Similarly to what we did
with the internal routes,

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let's take down the 210 network.

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Once again, we'll just
shut the interface.

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Just to verify that it's
gone, let's hop over

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to router 2 real quick.

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Let's do a show IP route EIGRP.

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And we can see that 210 is,
in fact, gone now on the ASBR.

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We, of course, do still have
the other three prefixes.

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We can also see that we're
generating the OSPF route.

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Then we do a show IP
OSPF database external

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for the 200.200.0.0.

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So we can see that we are, in
fact, generating this summary

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route.

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So that means that if
we go over to router 7,

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we can see that absolutely
nothing has happened

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on our debug
because, again, it's

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hiding the flapping of that
network behind the summary.

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And, of course, that means if
we look at the route again,

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it's going to look exactly
the same because, again,

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that failure is being hidden.

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So as we can see here, the
summarization of the externals

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is almost identical to the
summarization of the internals

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except it's, of course,
a different command.

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Now we saw it on iOS XR.

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Let's take a look at that
same command here on iOS XE.

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We don't really have any place
to summarize routes on iOS XE

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right now.

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But we can just look at the
command right here on router 7.

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So if we just go into
Configuration mode for router

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OSPF 1, it's actually
summary address on iOS XE

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rather than summary prefix.

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We can also see that the way it
wants us to put in the address

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is just by the IP address.

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So we'll just pick
on something here.

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We'll say something
like 10.1.1.0,

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even though we're not
actually summarizing that.

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And then it wants the summary
mask in decimal format.

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So we could say something
like 255.255.255.0.

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And then if we hit the question
mark at the end of this,

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we can see that we have
those similar options

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of setting a tag on the route
or not advertising the route.

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On iOS XE here, it has
an additional feature

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that we can limit this
summary to just go into NSSA

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or not-so-stubby areas.

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So in this video,
we looked at how

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to summarize external
routes in OSPF.

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Some of the key
takeaways here, remember,

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we can only do this on an ASBR.

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It has to be on
the router that's

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bringing the routes into OSPF.

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Like internal
summarization, it can also

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be used as a filter using
that not advertised option.

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And, keep in mind, that it
still functions in the same way.

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That it hides flapping
of prefixes and failures

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on those remote
networks that are

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being hidden by the summary.

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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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