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So if we do make it through cost with a tie
between multiple routes, we do have a few more tiebreakers
that we have to deal with.
Now, these don't happen all that often,
but it is definitely be worth exploring.
So we're going to actually look at two of those in this video
because they're both relatively small topics.
The first is the forwarding metric.
And we'll see that that's going to work
sort of like a type 1 external, and we're
going to prefer the lowest.
And then we're going to take a look at the forwarding address.
This is a very simple rule.
We actually prefer routes that do not have a forwarding
address over those that do.
Now, in order for us to really see what's going on here,
we are going to have to start off
with two external routes that are currently a tie.
They're making it all the way through cost,
and we're currently using both of them.
So let's jump to our topology just
to get a quick look at what's going on.
And then we'll see you on the command line.
So for this demonstration, we have a loopback interface
on router 10 as well as on router 9,
both of these having the /24 address 100.100.100.0/24.
So they're both going to be advertising
essentially the same route.
Now, in the case of router 10, router 10
is doing a redistribution of connected into OSPF.
And in the case of router 9, since this is actually
running EIGRP 100 up here, it's actually
router 2 that's doing the redistribution.
So if we were to go right now and look specifically
at router 7 at this route, what we're going to see
is two equal routes because these gigabit links in here
are going to be counting as the same,
as well as this one and this one.
So with these links being equal on each side,
we're going to start off with two equal-cost routes.
So that's how this is going to start.
Then we're going to change the forwarding
metric on one of these.
So essentially, we'll change the cost on one of these interfaces
to affect which way we want our traffic to go.
And then of course, we'll take a look at what happens.
We will then set it back to a tie, essentially start over,
and then we'll change the forwarding address.
Now, in this case, we'll be specifically
having router 2 set the forwarding address when
it does its redistribution over here
because that's going to be the easiest place to do it.
And then we'll see that it begins
to prefer the route that doesn't have the forwarding address.
So as usual let's start off on router 7 looking
at our existing equal routes.
So we'll start with our show ip route for the 100.100.100.0
network.
And we see that it is currently doing equal-cost multipathing,
headed towards our ASPR of router 10 versus router 2.
So currently load sharing.
Also, notice that this is being listed
with a forward metric of 2000.
If we say show ip ospf border, we
can see that the cost to get to both router
2 and 10, our ASBRs in this case, are both a cost of 2000.
That's where the forwarding metric comes from.
So this is what we're going to need
to change to break this tie.
Let's go ahead and change it on router 6,
and that will cause us to prefer the path to router 2.
So on router 6, we'll simply go into its Gigabit 2 interface.
And we'll change the cost.
We'll just make it 1001 since it's defaulting to 1000.
All we have to do is break the tie.
Let's go back over to router 7.
We can look at the borders again.
And we can see that to get to router 10
is now 2001 as expected.
And if we look at our route, we'll
see that we're no longer doing equal-cost multipathing thing.
We're instead preferring to go to router 1
towards router 2 for our path.
And again, this is coming down completely to the forwarding
metric being lower.
So hopefully, you can see here that by looking
at the forwarding metric for an external type 2,
this is actually acting very similar to the behavior
that we would have with an external type 1.
The main difference is with an external type
1, the forwarding metric and the route metric
are combined together as the total metric
in the routing table.
And as we can see here, in the case of type 2 externals,
the metric stays the same.
And then it comes down to the forwarding
metric as the tiebreaker.
So it's simply making the same decision at a later rule.
It just makes it easier to control the path selection
with the cost itself in the case of an external type 2.
So now that we've seen how that works, let's change this back.
We'll simply take our cost off.
Go back to router 7.
And we should be doing load sharing again.
And we can see that we are.
So now let's see what happens if we have one route that has
a forwarding address and another one that doesn't.
Let's start by looking at the OSPF database.
And what we see is neither one of these entries
currently have a forwarding address.
So this is not being used as a tiebreaker.
But let's go over to router 2 and get it
to set the forwarding address.
Now, the easiest way to do this is
to simply have router 2 run OSPF on its egress interface.
So we'll simply go into router ospf 1 for area 100,
and we'll bring in interface gig 0/0/0/2.
Commit the changes.
And we'll look at our self-originated type 5 LSAs,
specifically for that 100.100.100.0.
And we can see here that that forwarding
address is in fact now set.
So let's see what happens on router 7.
First, if we look at our database entries again,
we see that this entry coming from router 2
now has the forwarding address, and the one from router 10
does not.
Well, how has that affected our routing?
Notice we're now preferring the one from router 10, the one
without the forwarding address.
And that's simply what this route selection criteria says--
prefer a route without a forwarding address.
And we can see that that's exactly what it does.
In this video, we took a look at both the forwarding metric
and the forwarding address on external type 2 LSAs
to see how that affects OSPF's route selection criteria.
What we saw, of course, is the lowest forwarding address wins.
And if that's a tie, then it'll come down
to preferring routes without a forwarding address over those
that have a forwarding address.
I hope this has been informative for you,
and I'd like to thank you for viewing.