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[AUDIO LOGO]

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So if we do make it
through cost with a tie

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between multiple routes, we
do have a few more tiebreakers

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that we have to deal with.

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Now, these don't
happen all that often,

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but it is definitely
be worth exploring.

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So we're going to actually look
at two of those in this video

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because they're both
relatively small topics.

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The first is the
forwarding metric.

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And we'll see that
that's going to work

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sort of like a type
1 external, and we're

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going to prefer the lowest.

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And then we're going to take a
look at the forwarding address.

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This is a very simple rule.

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We actually prefer routes
that do not have a forwarding

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address over those that do.

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Now, in order for us to really
see what's going on here,

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we are going to
have to start off

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with two external routes
that are currently a tie.

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They're making it all
the way through cost,

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and we're currently
using both of them.

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So let's jump to
our topology just

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to get a quick look
at what's going on.

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And then we'll see you
on the command line.

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So for this demonstration,
we have a loopback interface

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on router 10 as
well as on router 9,

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both of these having the /24
address 100.100.100.0/24.

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So they're both going
to be advertising

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essentially the same route.

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Now, in the case of
router 10, router 10

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is doing a redistribution
of connected into OSPF.

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And in the case of router
9, since this is actually

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running EIGRP 100 up
here, it's actually

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router 2 that's doing
the redistribution.

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So if we were to go right
now and look specifically

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at router 7 at this route,
what we're going to see

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is two equal routes because
these gigabit links in here

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are going to be
counting as the same,

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as well as this
one and this one.

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So with these links
being equal on each side,

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we're going to start off
with two equal-cost routes.

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So that's how this
is going to start.

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Then we're going to
change the forwarding

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metric on one of these.

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So essentially, we'll change the
cost on one of these interfaces

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to affect which way we
want our traffic to go.

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And then of course, we'll
take a look at what happens.

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We will then set it back to a
tie, essentially start over,

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and then we'll change
the forwarding address.

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Now, in this case,
we'll be specifically

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having router 2 set the
forwarding address when

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it does its
redistribution over here

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because that's going to be
the easiest place to do it.

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And then we'll
see that it begins

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to prefer the route that doesn't
have the forwarding address.

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So as usual let's start
off on router 7 looking

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at our existing equal routes.

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So we'll start with our show
ip route for the 100.100.100.0

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network.

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And we see that it is currently
doing equal-cost multipathing,

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headed towards our ASPR of
router 10 versus router 2.

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So currently load sharing.

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Also, notice that
this is being listed

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with a forward metric of 2000.

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If we say show ip
ospf border, we

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can see that the cost
to get to both router

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2 and 10, our ASBRs in this
case, are both a cost of 2000.

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That's where the forwarding
metric comes from.

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So this is what
we're going to need

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to change to break this tie.

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Let's go ahead and
change it on router 6,

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and that will cause us to
prefer the path to router 2.

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So on router 6, we'll simply go
into its Gigabit 2 interface.

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And we'll change the cost.

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We'll just make it 1001 since
it's defaulting to 1000.

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All we have to do
is break the tie.

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Let's go back over to router 7.

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We can look at
the borders again.

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And we can see that
to get to router 10

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is now 2001 as expected.

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And if we look at
our route, we'll

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see that we're no longer doing
equal-cost multipathing thing.

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We're instead preferring
to go to router 1

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towards router 2 for our path.

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And again, this is coming down
completely to the forwarding

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metric being lower.

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So hopefully, you can
see here that by looking

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at the forwarding metric
for an external type 2,

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this is actually acting
very similar to the behavior

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that we would have with
an external type 1.

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The main difference is
with an external type

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1, the forwarding metric
and the route metric

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are combined together
as the total metric

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in the routing table.

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And as we can see here, in
the case of type 2 externals,

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the metric stays the same.

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And then it comes
down to the forwarding

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metric as the tiebreaker.

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So it's simply making the
same decision at a later rule.

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It just makes it easier to
control the path selection

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with the cost itself in the
case of an external type 2.

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So now that we've seen how that
works, let's change this back.

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We'll simply take our cost off.

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Go back to router 7.

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And we should be doing
load sharing again.

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And we can see that we are.

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So now let's see what happens
if we have one route that has

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a forwarding address and
another one that doesn't.

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Let's start by looking
at the OSPF database.

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And what we see is neither
one of these entries

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currently have a
forwarding address.

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So this is not being
used as a tiebreaker.

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But let's go over to
router 2 and get it

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to set the forwarding address.

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Now, the easiest
way to do this is

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to simply have router 2 run
OSPF on its egress interface.

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So we'll simply go into
router ospf 1 for area 100,

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and we'll bring in
interface gig 0/0/0/2.

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Commit the changes.

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And we'll look at our
self-originated type 5 LSAs,

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specifically for
that 100.100.100.0.

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And we can see here
that that forwarding

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address is in fact now set.

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So let's see what
happens on router 7.

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First, if we look at our
database entries again,

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we see that this entry
coming from router 2

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now has the forwarding address,
and the one from router 10

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does not.

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Well, how has that
affected our routing?

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Notice we're now preferring
the one from router 10, the one

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without the forwarding address.

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And that's simply what this
route selection criteria says--

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prefer a route without
a forwarding address.

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And we can see that that's
exactly what it does.

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In this video, we took a look
at both the forwarding metric

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and the forwarding address
on external type 2 LSAs

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to see how that affects OSPF's
route selection criteria.

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What we saw, of course, is the
lowest forwarding address wins.

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And if that's a tie,
then it'll come down

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to preferring routes without a
forwarding address over those

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that have a forwarding address.

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I hope this has been
informative for you,

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and I'd like to thank
you for viewing.

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