[MUSIC PLAYING]
So now that we've looked at both internal and external route
summarization in OSPF, let's take a look
at some of the limitations of that summarization.
Now we have spoken about these things as we've gone through
those sections as far as which routers you can summarize
on and which ones you can't.
The main purpose of this video is going
to be to demonstrate that.
We're going to try to summarize places where we're really
not allowed to.
And we'll see the result. Now the result, of course,
is going to be that it doesn't work.
But the main thing is to understand that.
And really just prove that those are the rules in OSPF.
So let's start off by taking a quick look at our topology just
to discuss what we're going to be trying to do.
And then, of course, we'll jump into the command line,
set up some things, and see that these are,
in fact, the rules for OSPF.
So here's what we're going to do in this video.
We're going to take the routes that are coming from router 10
through router 6, which are, of course, the 100.100 networks.
And they are currently a /16.
We're going to take a look at these on router 5
this time as it's going into another area, area 100.
We'll see that it's a /16.
And then what we'll do is we'll attempt to summarize it again
here on router 1 to a /8.
So, in other words, we will attempt
to summarize our summary.
This cannot be done in OSPF.
Again, we're going to do this to prove that.
Then we're going to move on to our externals.
With the externals, we're going to do,
essentially, the same thing.
These routes are currently being brought in on router 2.
These are the 200.200 networks.
Again, currently coming in as a /16.
And what we'll do with these routes is we'll attempt
to summarize them on router 6 coming up into area 200, again,
to a /8.
As we've stated, these aren't going to work.
But our main point is just to show that and demonstrate that
you cannot summarize a summary in OSPF.
So let's jump into the command line.
We'll start with the internal routes.
And, therefore, we'll start by looking at router 5.
So looking at these routes here on router 5,
we'll, again, just do a show IP route OSPF.
And here we'll look at the IA routes.
Again, the internal route being summarized.
And what we're looking for is that we do, in fact,
have this 100.100.0.0/16 route.
This is what we're expecting because of the summarization
we did previously.
So now let's see what happens if we
try to summarize this on the ABRs for area 100 so,
in other words, router 1 and router 3.
We'll start on router 1.
Now this is our first time configuring the area range
command on iOS XR, but it's very, very similar.
The only real difference is you say area 0.
And then we put range on a separate line.
The other slight difference is, of course,
the way we enter the address and the prefix.
Rather than putting the prefix in decimal format or the mask,
we do /notation here on iOS XR.
So, again, we're going to try to summarize this to a /8,
so it would be 100.0.0.0/8.
Notice we have the same basic options here
on iOS XR that we did in iOS XE.
Again, advertises the default. And we're
not trying to filter right now, so let's just hit Enter.
We'll commit our changes.
And we'll go do the same thing on router 3.
So now let's go check router 5.
We simply repeat the command on router 5.
What we notice is that our 100.100.0.0 is still a /16.
So as we've said numerous times, you cannot use the area range
command on what is already a Type 3 LSA.
It can only be done on the originating ABR that's bringing
these routes into OSPF.
They have to be an O route or learn from a Type 1 or a Type 2
LSA before we can use the area range command.
Not only can we now summarize a summary,
but we can't filter that summary either.
Let's go back over to router 1.
And we'll make a change here.
What we're going to do is under area 0,
well say no range 100.0.0.0/8.
Again, we already know that's not working.
But what if we do this?
What if we say range 100.100.0.0/16.
So we're not trying to summarize it again,
we're matching on the existing, but we add the not advertise.
So we'll commit these changes.
Of course, we have to do the same filtering on router 3
if we're going to see it work.
But we'll notice when we go back over router five,
look at our routing table again, we notice
that still nothing's changed.
So not only can we not use this to summarize further
or to summarize a summary, we can't
use this for our filtering between areas either.
So this is going to be totally ineffective on routes
that are already summarized.
Now let's take a look at the same thing
but with external routes.
So to look at the externals, let's jump up to router 10.
And we'll do a show IP router OSPF.
And this time, we'll look at the E2 routes.
And we see that we have this summary.
The 200.200.0.0/16 coming from all the way over on the EIGRP
side coming through router 2.
So let's try to summarize this on router 6.
Because if we were going to be able to summarize it somewhere,
it would probably be on the ABR.
So let's go there.
So going to OSPF 1.
Now since these are external, clearly,
we would have to use the summary-address command.
And we would try to summarize them to say 200.0.0.0.
And we'll make these a /8.
But we'll see that if we go up here
to router 10, repeat our command,
it's done absolutely nothing.
We still have the 200.200.0.0/16 as the summary.
Once again, we can also show that the filtering doesn't
work.
If we go back up to router 6, let's attempt
to match on the network as it currently is and do the not
advertise option.
So attempting to filter it.
We'll, again, go back to router 10, hit the up arrow.
And once again, we see that this has had no effect.
So, in this video, we looked at the limitations
of summarization in OSPF.
To summarize, once again, you can only
do route summarization for internal routes
on the ABR that's originating the routes
into the other areas, the ones that are being learned
from Type 1 or Type 2 LSAs.
You cannot summarize or filter using the area range command
after the route is already a Type 3 LSA.
Likewise, on the external side, you
can only do this when you are originating the route
on the originating ASBR.
Once it's a Type 5 LSA, then the summary-address command,
or summary-prefix command, has no effect on it.
We can't filter it.
We can't summarize it again.
I hope this has been informative for you.
And I'd like to thank you for viewing.