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So now let's talk about leaking routes between IS-IS levels.
We're going to take a look at the automatic process.
What does IS-IS do automatically without us intervening,
and we'll take a look at what we can control.
We can control a little bit of that automatic process,
as well as doing some completely separate leaking
that the automatic process doesn't do at all.
So we'll take a look at leaking from level one
to level two, level two to level one, and so on.
But let's go ahead and jump in first
and talk about what happens automatically
in IS-IS without us having to do anything.
So what IS-IS is going to do automatically or by default
is step one, is it's going to leak routes from level one
to level two.
So again, this happens automatically,
we're going to take all of the prefixes
that we learn from level one send them into level two.
The second thing we do is we're going
to block routes going the other direction.
We don't take any of the routes from level two
and send them into level one.
This is just simply not done.
And number three, since we're not sending those routes
into level one, we're instead going to inject a default route
into level one.
Otherwise, of course, nobody in level one
would be able to get out of that area.
So this is done to allow us to forward the traffic when
we're not sending all the routes in the level one.
Just to put this in a little bit of perspective
and compare it to something in OSPF,
this is a bit like an OSPF totally stubby area
or a total stub.
We're not going to send any routes from the other areas
into this level one adjacency, instead
we just give it a default, which is exactly how a totally
stubby area works.
Now, again, this is simply the default behavior.
We can modify this by manually configuring route leaking.
Now what we can control is this behavior in both directions.
What does that mean?
Well, first overriding rule two up at the top,
we can actually leak some or all of the routes from level two
to level one.
Which of course, the default here as we mentioned above
is we would get none of these, but we can control this.
We can also limit what's happening in rule number one.
So again, we say limit here because this is happening
by default. Again, like rule number
one says, we are leaking routes from level one to level two.
But here, we can limit either some
or all of those from level two.
So we would not be sending them into level two any longer.
So these are the things that we can control with route leaking.
Of course, the best thing is to see this happen.
So let's take a look at our topology
discuss where we're going to do this, and then
jump into the command line and do some route leaking.
So first, we'll look at filtering some routes coming
into router five.
Remember that router five is only a level one adjacency.
So this would represent leaking routes from level two
to level one.
And then we'll also do some filtering over
here between router eight and router seven,
where we'll filter a route going into level two.
So we'll take a look at both of these locations.
Let's start off by looking at the routing table
as it currently is on router five.
So let's do a show IP route here on router five.
And as we can see, all we're getting from IS-IS
right now are the two loopbacks for router two and router
four, which are of course, level one routes within this area.
And of course, that level one default
that's being generated by both router two and router four
to this level one adjacency.
So now let's take a look at actually leaking a route.
We'll do this on router two, and what we'll do
is we'll just leak router seven's loopback in here
to router five.
So the first thing we're going to need is a prefix set.
This is going to be the match on what we want to leak,
so we'll simply call this leak routes.
This will match on router seven's loopback
because that's what we said we wanted to leak.
This could, of course, be anything
you want to do leak here, we'll end the set,
then we'll write our policy.
We'll simply call this RP underscore ISIS leak.
For our logic, we will say if destination is in our prefix
set, then we're done.
So that's going to cause it to leak whatever
matches that prefix set.
Then we'll say, else drop.
Because we're going to want to drop everything else.
We don't want to leak all the other prefixes.
We'll end our statement, we'll end our policy,
then we go into router IS-IS one,
under the address family IPv4.
And we'll say, propagate level two--
this is where the router coming from--
into level one but going through the route policy
RP underscore IS-IS leak.
If we've done everything correctly,
we should be able to commit our changes, and our configuration,
and go check out and see if we have our new route on router
five.
On router five will simply repeat the last command.
And we see that just like that, we
have an i a, which is IS-IS inter-area,
so we have an IS-IS inter-area route
to 10.7.7.7 router 7 loopback, which was exactly our design.
Now notice that prefix is not multipathing
sort of like our default is, we only have the single path.
And that's because we only leaked it on router two.
We could, if we wanted to go leak this on router four,
and then of course, we would have multipathing.
But this particular video is not really about multipathing, it's
more about the leaking.
And now we see how to do the leaking,
and that's the important part.
So we'll leave it like this, and we'll
jump over to routers seven and eight
and do some filtering over there for prefixes going
from level one into level two.
So let's start here on router seven,
we'll just do a show IP route ISIS
and we see here that we have router 8 loopback being
learned here on router seven.
We see that it's a /24 with a high metric.
This is from stuff we did in previous skills.
However, it is here and let's take a look
at how we can filter it.
So let's go over to router eight.
So the first thing we're going to need is a prefix list.
So we'll call this PL underscore L1 filter and we're going to be
denying this time the 10.8.8.0/24, then of course,
we'll permit everything else, so that will permit everything
else.
Then we'll write a route map.
And let's call this our M underscore L1 filter.
And in this route map we will match on the IP address
based on the prefix list, PL underscore L1 filter.
Will go into our routing process.
Now here it's not propagate, it's redistribute.
So it's going to be redistribute IS-IS for IP level
one, in the level two, and go through the route map RM
underscore L1 filter.
If we've configured everything correctly,
we should be able to go back over
to router seven and router eight loopback should be missing.
So as we can see here, router eight loopback is now gone.
It's no longer being shown, it's filtered out of IS-IS.
In this video, we took a look at IS-IS route leaking.
Now we started off by looking at the default behavior of IS-IS.
Remember that we take all level one routes
and send them into level two.
But level two to level one, everything gets blocked,
and instead we generate a default.
Then we saw how we can override that behavior really
in both directions, either by filtering the level one
routes that are sent into level two,
or by allowing routes to be leaked from level two
into level one like we did on the router two, router five
side.
And as you can see, we can filter or allow
whatever routes we want through manual control of IS-IS
leaking.
I hope this has been informative for you.
And I'd like to thank you for viewing.