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So for our final topic in this skill,
let's go ahead and talk about the IS-IS best path selection
criteria.
That's just a big fancy way of saying
if IS-IS has multiple paths to get somewhere, how does it
choose which one is the best.
Now, overall, it's much like OSPF
in that it uses lowest cost for most of its decisions.
However, also like OSPF, there's a little bit more
to it than that.
We have to consider if the route is going to leave the area
or stay in the area, and so on.
So let's go ahead and just jump in
and talk about this in more detail
so we can understand more completely how the IS-IS path
selection process works.
So as we mentioned earlier, Cisco
does set the default IS-IS cost on, unfortunately,
every interface to 10.
And as we mentioned, if that cost is 10,
it really sort of means that this
is acting like a hop count, to a certain extent, making it
not much better than RIP as far as choosing the best
path through the environment.
So it is definitely recommended for you as the administrator
to basically choose a cost.
So basically, look at the interfaces
that you have in your environment
and choose what cost you want to use
for each different type of interface,
then apply those costs manually to each specific type
of interface.
Once all of that is properly configured,
IS-IS will then use the lowest cost
path to choose the best path within the following logical
framework.
What does that mean?
Well, we have to go over some rules
that sort of override or work with lowest cost.
That will make more sense as we go through this.
So the first thing is if it's a Level 1 router.
Two things we have to consider.
First, the destination is in a different area.
Well, that means we have to go to a Level 2 router.
So it's going to deliver this to the closest Level 1
Level 2 router.
This would be, again, the OSPF equivalent to an ABR,
the closest router that can deliver the traffic out
of the area.
If, however, the destination is in the same area,
it's simply going to use the Level 1
database to choose the lowest cost to the other routers'
system ID.
So this is where the lowest cost would come in.
Now, if it's a Level 1-2 router, so this is the ABR,
we're still going to look at the area because remember,
the areas are not crossing on the routers here in IS-IS.
Remember, they cross on the links,
so I still have to look to see if this
goes to the same area that I'm in or a different area.
And again, if the destination is in a different area,
well, then we're going to use the Level 2 database to route
to the correct area ID.
If the destination, however, is in the same area,
well, we're going to do the same thing that the Level 1
router does.
We're going to use the Level 1 database to route
to that system ID.
So once again, we would be using the lowest cost.
And we would be using the lowest cost to get to this area ID
as well.
We would just be using the Level 2 database for that rather than
the Level 1 database.
Once that best path has been chosen,
that path is then offered to the routing table,
which follows normal Cisco routing table rules.
Meaning?
First, we look at the lowest administrative distance.
So if we have two identical routes,
say one is coming from OSPF, and one is coming from IS-IS,
we look to the admin distance or the AD.
Remember that in the case of AD, lowest wins-- keyword there.
IS-IS defaults to 115, which places it basically
between RIP and OSPF.
RIP defaults to 120.
OSPF defaults to 110.
So we see what's happening here is it will actually
believe OSPF before IS-IS but IS-IS before RIP,
so it falls between those two protocols.
This, of course, can be changed.
These are just the defaults.
Like always, we use the longest prefix match
to any destination.
So, for example, if I had a route to 10.1.0.0/16
and a route to 10.1.1.0/24 and then a packet shows up
that's going to 10.1.1.30, it would, of course,
use this route because that's the longest prefix match.
On the other hand, if a packet came up going to 10.1.3.10,
well, then it would use this route because only
the first two octets match.
So that top route would become the longest prefix
match for that destination.
So that's what this rule means.
And just keep in mind, this is not an IS-IS thing.
This is just how Cisco does routing.
And not even just Cisco.
This is just how routing is done.
We always follow the longest prefix match.
So this is certainly not an IS-IS-specific thing.
And this last thing here is by no means a rule or anything.
It's really just sort of a piece of information
that when we do look at the routing table, which
we will do here in just a moment,
this will indicate whether it's a Level 1 or a Level 2 route.
So that said, let's go ahead and jump into the command line
and look at this.
We'll look at our topology real quickly.
We see in the topology, we just have a very, very simple IS-IS
topology.
Everything is in Area 1, so we're not actually
going to have any Level 2 routes.
But let's just hop in real quick on router 1,
look at the IS-IS routing table, and will at least
be able to see that it does, in fact, indicate
which type of route it is.
So again, we'll just do a show ip route isis
and a couple of things we want to look at.
First, we do see that these are all Level 1 routes.
This would, of course, say Level 2
if they were Level 2 learned routes.
And notice that the admin distance on all of these
is the default of 115.
Another interesting thing to note
is that, even in our simple topology here, all of the costs
are multiples of 10.
That's because all of the interfaces default to 10.
They've not been changed in this environment.
So depending on how many interfaces it's going through,
it's 10, 20, 30, 40, 50, and so on.
This is what we mean that it's acting more like a hop count.
In this video, we jumped in, and we took a look
at the IS-IS best path selection process.
We saw that how a Level 1 router will
try to leave the area through the closest exit
if the destination is outside of the area,
but it simply uses lowest cost if the route stays in the area.
We saw that a Level 1/Level 2 router
chooses which database to look things up in based
on where the destination is.
Again, if it's going to a different area,
it uses the Level 2 database.
And if it's staying in the same area,
it uses the Level 1 database.
We also talked about how it uses admin distance, just
like every other routing protocol,
when putting routes into the routing table,
and it defaults to 115, and that every interface
is set to a cost of 10 unless you, as the administrator,
go in and change that to something that makes a little
more sense in your environment.
I hope this has been informative for you,
and I'd like to thank you for viewing.