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[AUDIO LOGO]

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So for our final
topic in this skill,

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let's go ahead and talk about
the IS-IS best path selection

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criteria.

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That's just a big
fancy way of saying

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if IS-IS has multiple paths
to get somewhere, how does it

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choose which one is the best.

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Now, overall, it's
much like OSPF

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in that it uses lowest cost
for most of its decisions.

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However, also like OSPF,
there's a little bit more

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to it than that.

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We have to consider if the
route is going to leave the area

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or stay in the area, and so on.

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So let's go ahead
and just jump in

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and talk about
this in more detail

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so we can understand more
completely how the IS-IS path

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selection process works.

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So as we mentioned
earlier, Cisco

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does set the default IS-IS
cost on, unfortunately,

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every interface to 10.

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And as we mentioned,
if that cost is 10,

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it really sort of
means that this

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is acting like a hop count,
to a certain extent, making it

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not much better than RIP
as far as choosing the best

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path through the environment.

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So it is definitely recommended
for you as the administrator

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to basically choose a cost.

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So basically, look
at the interfaces

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that you have in
your environment

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and choose what
cost you want to use

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for each different
type of interface,

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then apply those costs
manually to each specific type

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of interface.

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Once all of that is
properly configured,

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IS-IS will then
use the lowest cost

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path to choose the best path
within the following logical

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framework.

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What does that mean?

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Well, we have to
go over some rules

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that sort of override or
work with lowest cost.

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That will make more sense
as we go through this.

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So the first thing is if
it's a Level 1 router.

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Two things we have to consider.

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First, the destination
is in a different area.

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Well, that means we have
to go to a Level 2 router.

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So it's going to deliver
this to the closest Level 1

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Level 2 router.

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This would be, again, the
OSPF equivalent to an ABR,

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the closest router that
can deliver the traffic out

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of the area.

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If, however, the destination
is in the same area,

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it's simply going
to use the Level 1

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database to choose the lowest
cost to the other routers'

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system ID.

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So this is where the
lowest cost would come in.

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Now, if it's a Level 1-2
router, so this is the ABR,

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we're still going to look at
the area because remember,

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the areas are not crossing
on the routers here in IS-IS.

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Remember, they
cross on the links,

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so I still have to
look to see if this

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goes to the same area that
I'm in or a different area.

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And again, if the destination
is in a different area,

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well, then we're going to use
the Level 2 database to route

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to the correct area ID.

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If the destination, however,
is in the same area,

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well, we're going to do the
same thing that the Level 1

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router does.

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We're going to use the
Level 1 database to route

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to that system ID.

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So once again, we would
be using the lowest cost.

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And we would be using the lowest
cost to get to this area ID

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as well.

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We would just be using the Level
2 database for that rather than

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the Level 1 database.

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Once that best path
has been chosen,

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that path is then offered
to the routing table,

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which follows normal
Cisco routing table rules.

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Meaning?

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First, we look at the lowest
administrative distance.

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So if we have two
identical routes,

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say one is coming from OSPF,
and one is coming from IS-IS,

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we look to the admin
distance or the AD.

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Remember that in the case of
AD, lowest wins-- keyword there.

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IS-IS defaults to 115,
which places it basically

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between RIP and OSPF.

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RIP defaults to 120.

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OSPF defaults to 110.

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So we see what's happening
here is it will actually

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believe OSPF before IS-IS
but IS-IS before RIP,

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so it falls between
those two protocols.

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This, of course, can be changed.

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These are just the defaults.

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Like always, we use the
longest prefix match

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to any destination.

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So, for example, if I had
a route to 10.1.0.0/16

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and a route to 10.1.1.0/24
and then a packet shows up

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that's going to 10.1.1.30,
it would, of course,

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use this route because that's
the longest prefix match.

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On the other hand, if a packet
came up going to 10.1.3.10,

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well, then it would use
this route because only

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the first two octets match.

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So that top route would
become the longest prefix

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match for that destination.

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So that's what this rule means.

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And just keep in mind,
this is not an IS-IS thing.

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This is just how
Cisco does routing.

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And not even just Cisco.

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This is just how
routing is done.

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We always follow the
longest prefix match.

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So this is certainly not
an IS-IS-specific thing.

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And this last thing here is by
no means a rule or anything.

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It's really just sort of
a piece of information

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that when we do look at
the routing table, which

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we will do here
in just a moment,

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this will indicate whether it's
a Level 1 or a Level 2 route.

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So that said, let's go ahead
and jump into the command line

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and look at this.

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We'll look at our
topology real quickly.

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We see in the topology, we just
have a very, very simple IS-IS

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topology.

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Everything is in Area
1, so we're not actually

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going to have any
Level 2 routes.

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But let's just hop in
real quick on router 1,

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look at the IS-IS routing
table, and will at least

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be able to see that it
does, in fact, indicate

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which type of route it is.

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So again, we'll just
do a show ip route isis

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and a couple of things
we want to look at.

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First, we do see that these
are all Level 1 routes.

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This would, of
course, say Level 2

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if they were Level
2 learned routes.

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And notice that the admin
distance on all of these

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is the default of 115.

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Another interesting
thing to note

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is that, even in our simple
topology here, all of the costs

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are multiples of 10.

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That's because all of the
interfaces default to 10.

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They've not been changed
in this environment.

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So depending on how many
interfaces it's going through,

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it's 10, 20, 30,
40, 50, and so on.

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This is what we mean that it's
acting more like a hop count.

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In this video, we jumped
in, and we took a look

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at the IS-IS best path
selection process.

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We saw that how a
Level 1 router will

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try to leave the area
through the closest exit

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if the destination is
outside of the area,

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but it simply uses lowest cost
if the route stays in the area.

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We saw that a Level
1/Level 2 router

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chooses which database to
look things up in based

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on where the destination is.

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Again, if it's going
to a different area,

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it uses the Level 2 database.

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And if it's staying
in the same area,

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it uses the Level 1 database.

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We also talked about how it
uses admin distance, just

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like every other
routing protocol,

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when putting routes
into the routing table,

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and it defaults to 115,
and that every interface

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is set to a cost of 10 unless
you, as the administrator,

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go in and change that to
something that makes a little

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more sense in your environment.

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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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