[MUSIC PLAYING]
So now we've been through almost all
of the OSPF path selection criteria.
We just have one thing left.
When we get to the end, if what we have are external routes,
it depends if they're normal external routes coming
from a type 5 LSA, or if they're in NSSA external route coming
from a type 7.
And as we've discussed, it does depend on which
RFC is in effect on the router.
So what we're going to do in this video
is we're going to focus in on router 6
because this is only ever going to happen on an ABR
with an SSH area involved.
Otherwise, there would be no type 7s.
So we're going to do this on router 6.
And we're going to start off with the default RFC 3101.
We'll see how that behaves.
And then we'll switch it to RFC 1587.
And we see how that changes the behavior.
So let's jump in and get started.
We'll, again, look at our topology
just so we have a quick idea of what's going on
and where the routes are coming from.
And then we'll jump into our command line.
So again, for this demonstration,
we will be dealing with the loopback on router 10,
and the loopback on router, which both represent that same
address the 100.100.100.0/24.
These are both going to be arriving
as external routes on router 6.
Now of course, the one coming from EIGRP 100, so the router
2 side, this is going to be coming in as an E, external.
And for this other route, area 200
has been converted to an NSSA.
And therefore, this one will be coming
in as an N route or a type 7.
So we'll be focusing again on router 6, which
is acting as the ABR.
And we'll begin by making sure that those two
routes are equal.
Now at this point, they have been manipulated
so that they should be showing up
is equal entries on router 6.
Now it's not going to be using both.
So we'll have to look at the database
to ensure that they're equal.
Number two here has actually already been done.
We did change area 200 to an NSSA.
So we're already receiving the type 7 LSA on router 6.
And of course, we're going to be looking at the results here.
And specifically what happens as we change the rules on router 6
as far as the RFC.
So let's jump into router 6 and see how this works.
So the first thing we want to do on router 6
is ensure that we are, in fact, getting both LSA types,
the type 5 and the type 7.
So let's start with a show IP OSPF database.
We'll do external first.
And of course, we'll filter for that specific prefix.
What we see here is that we are, in fact, getting the type 5
LSA from the advertising router 10.2.2.2.
And it has a metric of 20.
And it has a forwarding address set.
Also note, if we scroll down, we also
have a type 5 LSA here that's being generated by this router.
The advertising router is 10.6.6.6.
So this indicates that we are, in fact, translating the type
7 to the type 5.
Now that's a pretty good indicator
that we're already getting the type 7.
But of course, we want to verify that.
So let's change this to NSSA.
And here we can see that we are, in fact, getting the type 7.
This type 7 is coming from advertising router 10.10.10.10.
It also has a metric of 20.
And it has the forwarding address set.
NSSA always sets the forwarding address just as a note.
Now let's see which route we're actually using.
Here what we see is that we're actually following the route
to router 10.
That means that we're preferring the N route over the E route.
We're preferring the type 7 over the type 5.
We can actually verify this if we do a show IP OSPF,
and include RFC, we can actually see here
that for NSSA, it is currently being compatible with RFC 3101.
Keep in mind that our router 6 is running iOS XE.
And it's following our newer method.
However, we can change this to be backwards compatible.
We simply go into our OSPF process.
And we say compatible RFC 1587.
If we look at our command now, we
see that it says it's compatible with RFC 1587 now.
And that also means we look at our routing table,
notice we are now following the type 2 external,
in other words, the type 5 LSA.
It has changed the behavior just as we would expect it to.
In this video, we simply looked at how the router handles
an N route or a type 7 LSA, versus an E
route or a type 5 LSA.
If we're following RFC 3101, we prefer the N route.
If we're following RFC 1587 we prefer the E route.
And as we can see on iOS XE, we can switch back and forth
between which one we want to be compatible with.
On iOS XR, keep in mind, at this point,
it will only follow RFC 1587.
Again NSSA, probably not a very high priority
on the iOS XR code chain.
So that may be changed in the future.
But as of right now, it simply follows RFC 1587.
I hope this has been informative for you.
And I'd like to thank you for viewing.