[AUDIO LOGO]
So in this video, let's jump in and get an overview of the OSPF
path selection process.
Now, as we said, we know cost is probably the most common rule
at which decisions get made.
But there are plenty before and after cost.
So in this video, we're going to look over the entire list
just so we have an idea where we're going.
And then in the upcoming videos, we'll
look at each one of these one by one.
So let's go ahead and jump in, looking
at the overall list of the OSPF path selection process.
So as we said during the introduction, when we're
thinking about OSPF and path selection,
we are normally going to be thinking about that lowest
cost.
Again, if you ask most engineers out there,
this would be their go-to answer for OSPF path selection.
However, there is quite a bit more to it
than just lowest cost.
So the first thing we want to talk about
is that it actually prefers internal routes
over external routes.
And essentially, what that means is any route
that is either an O or an O IA in your routing table
is going to beat anything that is either an E1 or an E2,
as well as an N1 or an N2.
It's simply going to win regardless
of any other factors.
Number two, we're going to prefer intra-area
over inter-area.
This rule is simply saying that it's going
to prefer the O over the O IA.
The IA, of course, is indicating that it's Inter-Area.
Intra-area would just have the O in the routing table.
Number three, if we have nothing but externals--
because keep in mind, we wouldn't
have gotten to number three if we had internals.
The internals would have already won at rule number one, right?
So the only way to get to rule number three
is if all the routes are external.
And if all of the routes are external,
we're going to always prefer a type 1 over a type 2.
So this would be an E1 or an N1 beating an E2 or an N2.
1's beats 2's.
Now, remember, we're talking about the external type.
We're not talking about the LSA type here.
These are both either LSA type 5,
if we're talking about just regular externals,
or type 7 LSAs if we're talking about NSSA.
So again, don't confuse the external type
versus the actual LSA type.
Here we're talking about the external type.
Next up is-- well, there it is--
the lowest total path cost.
That's the one that most people know
and, again, is really where commonly the routes
do get decided.
And if we're going to start manipulating
routing and getting paths to go a certain way,
this is usually where we would implement changes
in order to make that happen.
We can simply change the cost on an interface
and influence OSPF's decisions.
So again, definitely the most common but also not the end.
It's quite possible that we get all the way down here,
all the way through rule number four, and still have a tie.
So there are still some tiebreakers here.
Number five, if it is an external type 2--
and again, here we're talking about the type
2's that we were mentioning up here--
then it's going to use something called
the lowest forwarding metric.
Now, if you're not familiar with this,
we will of course cover each of these
in more detail on the upcoming videos here in this skill.
So we will get back to this if you're not familiar with it.
Sort of the easiest way to look at this
is this is essentially the closest exit.
We're going to leave the network sooner rather than later.
So it's actually going to look at the cost to get to the exit.
Again, we'll see this in more detail.
This is just going to be for external type 2's.
Next, we have sort of an odd little rule where
we're actually going to prefer routes that do not
have a Forwarding Address, or an FA,
over routes that do have that Forwarding Address, or FA.
So this is just a rule saying that if there's no FA,
prefer that.
This one is going to be a little bit more obscure as far
as when it actually gets used in a production environment.
However, it is still a rule that we want to be aware of.
And finally, we have a rather interesting rule,
rule number seven.
This is the it depends rule.
Now, as we can sort of see here--
and we're gonna go through this, of course, in detail--
it really depends on two different RFCs,
which RFC is actually implemented
on this particular device.
RFC 3101 is considered the newer way
to do this, or the newer rule to follow.
However, not all of our devices do that.
So let's take a look at this in a little bit more detail.
So the first decision here would be RFC 1587.
And of course, this is the older rule.
And this is actually where we prefer
a type 5 LSA over a type 7 LSA.
And this was the rule for quite some time.
And essentially, what this rule means
is that we would prefer an E1 or an E2 over an N1 or an N2.
Now, keep in mind, we would never get here with
a combination of 1's or 2's.
They'll either both be 1's, or they'll
both be 2's because we wouldn't have gotten through rule number
three up here if we had a combination.
So just keep that in mind as we're looking at this.
It's either going to be 1's or 2's.
This is more about the E versus the N,
whether it is a we can say normal external versus an NSSA.
Now, RFC 1587 does actually have an issue
if you're trying to control the translator for the NSSA.
It can actually cause the changing of the translator
or the manual setting of the translator
to, well, not work as you would expect it to.
And that's really the main reason they changed
and they gave us 3101.
But notice here that the IOS XR only supports,
at this time, 1587.
Now, that said, this could of course change at any time.
They could easily change the code to support 3101.
But again, as of right now, that's not the case.
And it's not really the focus of IOS XR to run OSPF in an NSSA.
So it's probably not that big of a priority on that code chain
to actually be updating this to support the newer RFCs related
specifically to NSSA.
So all of that said, again, RFC 3101
is basically the newer way of doing this.
And here we're going to prefer the type 7 LSAs
or the N routes before the type 5's or the E routes.
So in this case, it's going to be N before E.
Now, this is the default on newer versions of IOS XE.
I forget the exact version.
It's somewhere around 15.1 where they changed that.
But if for some reason, you do need to change it back,
there is a command compatible RFC 1587,
which will flip it back to using the older method.
In fact, when we go over this rule in an upcoming video,
we'll actually turn this feature on and off
and actually see it switch between 1587 and 3100.
In this video, we just jumped in and got an introduction
to the full OSPF path selection criteria.
And as you can see here, it's a bit more complicated
than just the lowest path cost wins.
But it's also one of the big rules that we have in here.
And as we said, it is where most of the decisions
happen in most environments.
But we're going to go through and jump into each one of these
in the upcoming videos.
So let's go ahead and move on to that.
I hope this has been informative for you,
and I'd like to thank you for viewing.