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[AUDIO LOGO]

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Next, let's talk about
prefix suppression.

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The idea behind
prefix suppression

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is we want to take the networks
or prefixes on the transit

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links between our
routers and keep them out

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of the routing table.

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The reason for this is in most
environmental environments,

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all we care about is getting
to end-device networks--

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in other words, where our hosts
are or printers or servers.

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Those are the destinations
we care about.

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Traffic is not destined to
a link between two routers.

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That's never a destination.

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That's just a transit link.

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So there's really no need
for all those transit links

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to be in our routing table.

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Now, that said,
there is a little bit

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of a debate on this
topic among engineers.

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Some engineers actually
like to have the transit

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links in their routing table.

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They think it makes the
traces and stuff like that

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easier to read.

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And if that's what you
prefer, then certainly,

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there's no need to
turn on this feature.

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However, if you prefer to
keep those prefixes out

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of your routing table, this
is an easy way to do it.

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Now, keep in mind, a
future skill in this course

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is going to cover filtering.

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And we can, of course, filter
all these routes out manually

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if we wanted to.

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But that would be a massive
administrative overhead

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nightmare.

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This allows us to do it
very quick, very easy.

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All we have to do
is know the rules.

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And that's what we're going to
be talking about in this video.

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What does it suppress?

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What gets advertised?

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So we'll take a
look at those rules.

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They're fairly simple.

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And then we'll jump
into the command line

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and actually see prefix
suppression in action.

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So again, let's take a
quick look at those rules

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and then jump into the CLI.

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So although we may not
have said it directly,

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I think we pretty much implied
that prefix suppression is used

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to reduce the size of both
the link-state database

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and the routing table
in our core network.

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And again, the
way this functions

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is by automatically
suppressing transit links.

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However, it still
allows the advertisement

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of things like edge networks,
things like BGP next hops,

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which are quite commonly
loopback interfaces.

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So we as administrators
are going

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to control which prefixes get
advertised and which ones get

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suppressed.

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But we're going to do
it in a very easy way,

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not creating prefix
lists and matching

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on actual prefixes and such.

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We're going to do it
based on the interface.

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This is either done under the
interface itself or, of course,

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under the routing process.

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But either way, it's configured
per interface, not per prefix.

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So let's take a
look at our topology

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and get an idea on
what we're going

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to expect to see suppressed once
we start enabling this feature

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and configuring it.

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So again, we're still
on IS-IS Topology 1.

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And here's what
we'll be looking for.

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If we go to any of
these routers right now

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and we just do a
show ip route isis,

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we're going to see all
of these transit networks

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in our routing table.

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We have 50's.

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We have 172.16's.

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They're all over.

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And what we're going
to be expecting

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to see is when we turn
on prefix suppression,

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these will disappear out
of our routing tables.

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And ultimately, what
we're going to be

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looking for is just
the loopback interfaces

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in our routing tables.

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That's going to be our
goal in this section.

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So now let's jump over to the
command line and configure

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prefix suppression.

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So let's just start off here
on router 7 with a show ip

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route isis.

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And what we see is
that we currently

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have all the device's loopbacks
as well as some transit links.

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We see some 50's.

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We see some 172.16's.

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So what we want to do is remove
these with prefix suppression.

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So let's start by
doing this on router 1.

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And we can see how
this is done on IOS XR.

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So configuring this
feature is fairly easy.

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That's one of its advantages.

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If it was complicated,
we wouldn't really

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have a need for the feature.

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So we go into
address Family ipv4.

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And all we have to say is
advertise passive-only.

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As the name implies,
that means that we're

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going to be only advertising
passive interfaces.

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So we'll go ahead and
commit our configuration.

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And then let's go
back over to router 7

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and see what's happened.

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So if we go to router 7 and we
look at the exact same thing,

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we're going to notice that not
much changed at this point,

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because, well, honestly, all
the other routers are still

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advertising transit links.

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But something actually sort
of important did happen.

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Router 1's loopback is no longer
here in our routing table.

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And this is because
we do not currently

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have our passive
interfaces configured.

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Notice that the feature
here is actually called

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advertise passive-only.

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It's not exactly called
prefix suppression,

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although if you look
through the documentation,

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that is what they
call this feature.

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I want to compare and contrast
this with OSPF in case

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you're used to that feature.

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So just for a reference
here, OSPF prefix suppression

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sort of does the same thing.

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But here's where it's different.

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In OSPF, prefix suppression
will still advertise passive

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interfaces, /32 host routes,
usually because it's a loopback

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interface, and secondary
addresses on interfaces.

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As you can see here, IS-IS
does not behave the same way.

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We've lost the loopback on
router 1 because it's not

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passive, even though
it may be a /32.

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So in IS-IS, we do have to
configure the interfaces

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properly to be passive,
or we'll lose them.

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So clearly, we need
to go back over

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to router 1 and fix this issue.

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So all we have to do--

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go back into isis.

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Go back into our address family.

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Go into our
interface loopback 0.

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And set it to passive.

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If we commit this change and
again go back to router 7,

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once again, look at
our routing table,

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of course, what we see is that
router 1's loopback is right

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back here where it belongs.

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This is what we're looking for.

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Now, again, we're not going
to see a massive change here

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until we configure the
rest of the routers.

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So let's begin by going through
the rest of our IOS XR routers.

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Since they're going to be
configured exactly the same

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as router 1, we
just need to turn

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on the advertise passive-only
and turn on passive

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on the loopback.

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So since these are all
going to be the same,

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we will simply copy and
paste the configuration.

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So first, on router
2, we'll simply

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paste the configuration in.

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As you can see,
all we're doing is

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turning on the
advertise passive-only,

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going into the loopback, and
setting it to be passive.

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We'll do the same
thing on router 3.

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So we'll just paste it in here.

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That takes care of router 3.

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And finally, router 4.

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And router 4 is
configured as well.

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So now let's go ahead and
see how to do this on IOS XE.

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So let's move on to router 5.

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And to be honest, this is
almost the same configuration.

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We still go into router isis.

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Now, we don't have to go
into the address family here,

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since IPv4 is the
default address family.

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So we can just, right here,
say advertise passive-only.

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Then we can say to
pass loopback 0.

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Again, this doesn't get
configured under the interface,

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so to speak here.

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It's just under the process.

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And we can exit configuration.

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Once again, this will
be exactly the same

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on all the other routers.

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So we'll just go ahead
and paste this in.

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So router 6, router 7,
router 8, and router 9.

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Router 10 is not running IS-IS.

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So it doesn't need the command.

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Now that this is configured
on all the routers, now

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we can see what
it actually does.

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Let's go back over
here to router 7,

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since this is what
we've been using

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as our looking-glass router.

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And once again,
we'll just hit Up

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here, look at the
routing table for isis.

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And as we can see,
exactly what we wanted,

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only the loopback interfaces
are now being advertised.

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As a quick note here, notice
that router 5 and router

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8's loopback interfaces are
both showing up here as /24's.

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Now, this is simply
because we summarized them

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in a previous skill.

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However, note that that's
not having anything

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to do with prefix
suppression here in IS-IS.

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As long as that interface is
passive, well, it's good to go.

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And IS-IS will
then advertise it.

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So in this video, we took a
look at prefix suppression

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in IS-IS, how we can use it
to remove transit networks

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from our routing table.

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And again, sometimes
this is preferable.

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Some people don't
like this feature.

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It really is a personal choice
as far as your environment,

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whether you want these routes
in your routing table or not.

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But if you don't want them,
this is a nice, quick, easy way

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to get rid of them.

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As you can see, with some
very, very minor configuration,

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all of those transit links are
gone out of our routing table.

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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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