In this lesson,
we're going to talk about route selection.
Now, when we talk about routers,
we have to think about how is traffic
going to go across these routers
and determine which path they're going to take.
And that's what we're talking about
when we talk about route selection.
Now, when it comes to route selection,
the first we have to consider
is the believability of a route.
Now, what exactly does that mean
when we talk about the believability of a route?
Well, let's say I have a network
that's using more than one routing protocol,
because my routers can support multiple
protocols all at once.
For example, I might be running RIP and I might have OSPF,
and both of them are being used
on the same router in the same network.
So how do I know which one we're going to go with
when a packet enters the router
and has to be moved around the network?
Well, these routing protocols
are going to be considered more or less believable
based on some different factors that we have.
For example, RIP is an older protocol,
and it's considered to be less believable than OSPF,
which was a newer protocol.
Now, why is that?
Well, this is because routers use an index of believability,
and we like to call this the AD or administrative distance.
Now, if a route has a lower administrative distance,
it's going to be considered more believable
or more trustworthy,
and that means it's going to have a better or lower number,
just like in golf.
So the lower the number,
the better it is in terms of believability.
Now here's a chart for you
with all the administrative distances
for the different protocols that we talked about.
For the exam, you do not need to memorize this chart,
but you should at least have a sense
of which ones are the higher numbers
and which ones are the lower numbers.
Now, when we look at this
and you're dealing with a directly connected route,
we're going to have this as our most believable.
Now, why is that?
Well, it's because the router
is directly connected to another router,
so it knows itself
and it trusts its own opinion
that it's connected to that router.
And this makes a lot of sense
because if I know you personally
and we're holding hands,
we're both connected and therefore we have trust,
and that trust would have
a very, very low administrative distance
because we trust it highly.
That's the idea of a directly connected route.
It's a fully trusted route
because the router itself determined it.
Now, the next one we have is going to have an AD of 1,
and this is called a statically-configured network.
Now, this is the next most believable
because routers are designed to do what they're told.
So as a router,
I'm going to trust myself first,
and then I'm going to trust my programmer,
and then I'm going to trust all the dynamic protocols.
That's kind of the order of these things.
So if you as a network administrator
go in and configure a static route in the router,
you told the router,
"I want you to believe this
"above everything else you know,
"unless you're directly connected to somebody else
"'cause then you know better.
"Beyond that, you're going to trust me
"because I'm the network administrator
"and I told you that's the route I want you to use."
All right, the next one we have
after our directly-connected routes
and our static routes
is going to be EIGRP.
Now, EIGRP is a very trustworthy protocol
because it's one of the newer protocols we have.
After that, we're going to go backwards in time a little bit
and go to OSPF,
and then we'll go backwards again to RIP, or R-I-P.
After that, we're going to go into an external protocol
known as external EIGRP,
and then we go all the way to the bottom of our chart,
and you'll see the worst believability we have,
which is unknown or unbelievable,
which means we're going to have a rate of 255
for its administrative distance.
Now, the reason we set this at 255
is basically we're saying,
"We can't find this network anymore.
"Do not trust it at all.
"Don't send data that way.
"It is totally unbelievable."
So as I said, for the exam,
you do not need to memorize all these numbers,
but you need to have an idea
of the order of these things
in terms of their believability or trustworthiness.
Remember, directly connected is the most believable,
then static, because you as a human
told it what to do
will be the next believable.
After that, we go to the more modern protocols like EIGRP,
and then we start working backwards in time
to OSPF and then RIP.
In fact, if you think about this,
this makes a lot of sense
because RIP was one of our earliest protocols,
so it is going to be one of our least believable.
Then we had OSPF was an improvement to RIP,
so it's a little bit more believable.
And then we had EIGRP,
which was an improvement on OSPF.
So, again, it's more believable.
This is the kind of memory I like to use
when I'm trying to remember
which ones are most or least believable
as I'm going through my charts.
Lastly, we have the metrics
associated with each of these routes.
And when we look at these metrics
and we look at these routes,
there's going to be a routing protocol
that's able to choose which path it's going to go on.
And we do this based on those metrics.
These are things like hop count,
believability, reliability,
bandwidth, delay, cost, and other metrics like that.
Each protocol is going to use a different metric
based on its programming
to determine which route it should be using
and which one is going to be considered faster or slower.
Depending on which route it is,
this is going to determine which metric you're going to use.
And so when you're dealing with these metrics,
it's always going to be thinking about the fact
that our lower numbers are considered better
and our higher numbers are considered worse
whenever we're dealing with routing.
So, again, I like to think about this as golf rules.
You want the lowest score possible
to be the fastest you can be.
Now, anytime we talk about hop count,
we want the least number of hops
because that gives us less distance to cover
and it means we'll be able to get things done quicker.
If we start talking about bandwidth,
we want the lowest cost of bandwidth,
which means we're going to have
the highest amount of bandwidth for us to use,
and so our lower cost becomes better.
If we're dealing with delay,
we want the least amount of delay.
We talk about believability,
we want the lowest number
because a zero is directly connected and most believable.
Now, this is the way we start thinking about these things.
Lower numbers are always going to be better
when we're talking about route selection.
Now, the last thing I want to talk to you about
is this summary slide here.
Now, this is one of those things
that I like to write down in my notes
so you can have all your routing protocols in one place,
and it summarizes everything really nicely,
from RIP to OSPF to EIGRP to IS-IS and BGP,
and it shows you which ones are interior,
which ones are exterior,
which ones are distance vectors,
and which ones are linked states.
I hope this helps you
as you're studying for your exam
and you're trying to remember
all your different routing protocols
and some of the key facts and figures for them
before you take your exam.