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So next up, let's take a look at summarizing external routes.
Now, truth be told, this is very similar in process
to summarizing the internal routes.
But, of course, it has to happen on the ASBR, which
is the one bringing the routes into OSPF, to begin with.
And the command is a little bit different.
Not only that, it's a little bit different between iOS XE
and iOS XR as well.
So we will look at both of those.
We'll start with iOS XR.
So let's take a look at our topology,
see where these routes are coming from,
where we're going to summarize them,
and then we'll jump into the command line
and see external summarization in action.
So let's start by taking a look at that topology.
So in this demonstration, we will
be focusing on those four external routes coming
from router 9.
These are the routes that begin with
200.200.something.something, but it's really the third octet
where they're different.
So remember that these four routes are being sent
through EIGRP up to router 2.
Router 2, in this case, is the ASBR.
So that, of course, means that this
is the router redistributing these routes into OSPF.
That means that this is where we will have to do
our external summarization.
Now keep in mind that router 2 is running iOS XR,
so that's how we'll be configuring it.
We'll also take a look at the command
just on one of our iOS XE devices.
So, as usual, let's jump in and start
by taking a look at these routes on router 7.
Looking at this sort of as our looking-glass router.
We'll see what these routes look like, to begin with.
And then we'll implement our summarization.
So let's jump over to that command line on R7.
So, once again, we'll do our show IP route OSPF here
on router 7.
But this time, we will filter for external type
2's because that's what these 200.200 routes are coming
in as.
So, as we can see as we begin, we have four subnets coming
from router 2, which are in the 200.200 range.
And, similarly, to what we did with our internal routes,
these are simply subnets 200, 210, 220, and 230.
So now let's jump over to router 2
and take a look at how to summarize
these external prefixes.
So as usual, of course, we go to Configuration mode
for router OSPF 1.
And the command here on iOS XR is summary prefix.
And the way we enter this on iOS XR
is in the form of the address/the mask.
So, in this case, it would be 200.200.0.0/16.
We're going to summarize to a /16 just like we did with
the internals.
If we take a look, we again have some options on this.
We can set a route tag if we want to.
And we're not going to demonstrate it again
because it works the same way as it did with the internals,
but notice we also have that option to not advertise.
So we can, again, use this as a form of filtering.
In our case, we're just going to hit Enter.
Of course, we'll commit our changes.
And let's go back and verify this on router 7.
So we'll just hit our up arrow, repeat the last command.
And we see that, of course, now we have that /16.
Just for completeness, let's take a look
at what happens when one of these routes
flaps just like we did with the internals.
So, once again, we'll do a debug IP OSPF SPF here on router 7.
Then we'll go over to router 9, which
is where these prefixes are actually originating from.
And we'll take down one of our interfaces.
Similarly to what we did with the internal routes,
let's take down the 210 network.
Once again, we'll just shut the interface.
Just to verify that it's gone, let's hop over
to router 2 real quick.
Let's do a show IP route EIGRP.
And we can see that 210 is, in fact, gone now on the ASBR.
We, of course, do still have the other three prefixes.
We can also see that we're generating the OSPF route.
Then we do a show IP OSPF database external
for the 200.200.0.0.
So we can see that we are, in fact, generating this summary
route.
So that means that if we go over to router 7,
we can see that absolutely nothing has happened
on our debug because, again, it's
hiding the flapping of that network behind the summary.
And, of course, that means if we look at the route again,
it's going to look exactly the same because, again,
that failure is being hidden.
So as we can see here, the summarization of the externals
is almost identical to the summarization of the internals
except it's, of course, a different command.
Now we saw it on iOS XR.
Let's take a look at that same command here on iOS XE.
We don't really have any place to summarize routes on iOS XE
right now.
But we can just look at the command right here on router 7.
So if we just go into Configuration mode for router
OSPF 1, it's actually summary address on iOS XE
rather than summary prefix.
We can also see that the way it wants us to put in the address
is just by the IP address.
So we'll just pick on something here.
We'll say something like 10.1.1.0,
even though we're not actually summarizing that.
And then it wants the summary mask in decimal format.
So we could say something like 255.255.255.0.
And then if we hit the question mark at the end of this,
we can see that we have those similar options
of setting a tag on the route or not advertising the route.
On iOS XE here, it has an additional feature
that we can limit this summary to just go into NSSA
or not-so-stubby areas.
So in this video, we looked at how
to summarize external routes in OSPF.
Some of the key takeaways here, remember,
we can only do this on an ASBR.
It has to be on the router that's
bringing the routes into OSPF.
Like internal summarization, it can also
be used as a filter using that not advertised option.
And, keep in mind, that it still functions in the same way.
That it hides flapping of prefixes and failures
on those remote networks that are
being hidden by the summary.
I hope this has been informative for you.
And I'd like to thank you for viewing.