[AUDIO LOGO]
Next, let's talk about prefix suppression.
The idea behind prefix suppression
is we want to take the networks or prefixes on the transit
links between our routers and keep them out
of the routing table.
The reason for this is in most environmental environments,
all we care about is getting to end-device networks--
in other words, where our hosts are or printers or servers.
Those are the destinations we care about.
Traffic is not destined to a link between two routers.
That's never a destination.
That's just a transit link.
So there's really no need for all those transit links
to be in our routing table.
Now, that said, there is a little bit
of a debate on this topic among engineers.
Some engineers actually like to have the transit
links in their routing table.
They think it makes the traces and stuff like that
easier to read.
And if that's what you prefer, then certainly,
there's no need to turn on this feature.
However, if you prefer to keep those prefixes out
of your routing table, this is an easy way to do it.
Now, keep in mind, a future skill in this course
is going to cover filtering.
And we can, of course, filter all these routes out manually
if we wanted to.
But that would be a massive administrative overhead
nightmare.
This allows us to do it very quick, very easy.
All we have to do is know the rules.
And that's what we're going to be talking about in this video.
What does it suppress?
What gets advertised?
So we'll take a look at those rules.
They're fairly simple.
And then we'll jump into the command line
and actually see prefix suppression in action.
So again, let's take a quick look at those rules
and then jump into the CLI.
So although we may not have said it directly,
I think we pretty much implied that prefix suppression is used
to reduce the size of both the link-state database
and the routing table in our core network.
And again, the way this functions
is by automatically suppressing transit links.
However, it still allows the advertisement
of things like edge networks, things like BGP next hops,
which are quite commonly loopback interfaces.
So we as administrators are going
to control which prefixes get advertised and which ones get
suppressed.
But we're going to do it in a very easy way,
not creating prefix lists and matching
on actual prefixes and such.
We're going to do it based on the interface.
This is either done under the interface itself or, of course,
under the routing process.
But either way, it's configured per interface, not per prefix.
So let's take a look at our topology
and get an idea on what we're going
to expect to see suppressed once we start enabling this feature
and configuring it.
So again, we're still on IS-IS Topology 1.
And here's what we'll be looking for.
If we go to any of these routers right now
and we just do a show ip route isis,
we're going to see all of these transit networks
in our routing table.
We have 50's.
We have 172.16's.
They're all over.
And what we're going to be expecting
to see is when we turn on prefix suppression,
these will disappear out of our routing tables.
And ultimately, what we're going to be
looking for is just the loopback interfaces
in our routing tables.
That's going to be our goal in this section.
So now let's jump over to the command line and configure
prefix suppression.
So let's just start off here on router 7 with a show ip
route isis.
And what we see is that we currently
have all the device's loopbacks as well as some transit links.
We see some 50's.
We see some 172.16's.
So what we want to do is remove these with prefix suppression.
So let's start by doing this on router 1.
And we can see how this is done on IOS XR.
So configuring this feature is fairly easy.
That's one of its advantages.
If it was complicated, we wouldn't really
have a need for the feature.
So we go into address Family ipv4.
And all we have to say is advertise passive-only.
As the name implies, that means that we're
going to be only advertising passive interfaces.
So we'll go ahead and commit our configuration.
And then let's go back over to router 7
and see what's happened.
So if we go to router 7 and we look at the exact same thing,
we're going to notice that not much changed at this point,
because, well, honestly, all the other routers are still
advertising transit links.
But something actually sort of important did happen.
Router 1's loopback is no longer here in our routing table.
And this is because we do not currently
have our passive interfaces configured.
Notice that the feature here is actually called
advertise passive-only.
It's not exactly called prefix suppression,
although if you look through the documentation,
that is what they call this feature.
I want to compare and contrast this with OSPF in case
you're used to that feature.
So just for a reference here, OSPF prefix suppression
sort of does the same thing.
But here's where it's different.
In OSPF, prefix suppression will still advertise passive
interfaces, /32 host routes, usually because it's a loopback
interface, and secondary addresses on interfaces.
As you can see here, IS-IS does not behave the same way.
We've lost the loopback on router 1 because it's not
passive, even though it may be a /32.
So in IS-IS, we do have to configure the interfaces
properly to be passive, or we'll lose them.
So clearly, we need to go back over
to router 1 and fix this issue.
So all we have to do--
go back into isis.
Go back into our address family.
Go into our interface loopback 0.
And set it to passive.
If we commit this change and again go back to router 7,
once again, look at our routing table,
of course, what we see is that router 1's loopback is right
back here where it belongs.
This is what we're looking for.
Now, again, we're not going to see a massive change here
until we configure the rest of the routers.
So let's begin by going through the rest of our IOS XR routers.
Since they're going to be configured exactly the same
as router 1, we just need to turn
on the advertise passive-only and turn on passive
on the loopback.
So since these are all going to be the same,
we will simply copy and paste the configuration.
So first, on router 2, we'll simply
paste the configuration in.
As you can see, all we're doing is
turning on the advertise passive-only,
going into the loopback, and setting it to be passive.
We'll do the same thing on router 3.
So we'll just paste it in here.
That takes care of router 3.
And finally, router 4.
And router 4 is configured as well.
So now let's go ahead and see how to do this on IOS XE.
So let's move on to router 5.
And to be honest, this is almost the same configuration.
We still go into router isis.
Now, we don't have to go into the address family here,
since IPv4 is the default address family.
So we can just, right here, say advertise passive-only.
Then we can say to pass loopback 0.
Again, this doesn't get configured under the interface,
so to speak here.
It's just under the process.
And we can exit configuration.
Once again, this will be exactly the same
on all the other routers.
So we'll just go ahead and paste this in.
So router 6, router 7, router 8, and router 9.
Router 10 is not running IS-IS.
So it doesn't need the command.
Now that this is configured on all the routers, now
we can see what it actually does.
Let's go back over here to router 7,
since this is what we've been using
as our looking-glass router.
And once again, we'll just hit Up
here, look at the routing table for isis.
And as we can see, exactly what we wanted,
only the loopback interfaces are now being advertised.
As a quick note here, notice that router 5 and router
8's loopback interfaces are both showing up here as /24's.
Now, this is simply because we summarized them
in a previous skill.
However, note that that's not having anything
to do with prefix suppression here in IS-IS.
As long as that interface is passive, well, it's good to go.
And IS-IS will then advertise it.
So in this video, we took a look at prefix suppression
in IS-IS, how we can use it to remove transit networks
from our routing table.
And again, sometimes this is preferable.
Some people don't like this feature.
It really is a personal choice as far as your environment,
whether you want these routes in your routing table or not.
But if you don't want them, this is a nice, quick, easy way
to get rid of them.
As you can see, with some very, very minor configuration,
all of those transit links are gone out of our routing table.
I hope this has been informative for you,
and I'd like to thank you for viewing.