WEBVTT

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 So now we're going to look
 at how our routes selected.

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 And what I mean by that is if a router
 learns of a route to a destination,

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 either it's a connected route or a static
 router, he dynamically learned

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 it, if that route is good, if it meets
 all of our criteria, it's got a

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 next hop, it's got a subnet mask,
 all that stuff we talked about.

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 So if that is the only path he has,
 he'll put that in the routing table.

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 It's selected because he doesn't have
 anything to choose from, that's

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 the one and only one.

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 But what if he gets two or more routes
 to that same destination network?

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 I talked about earlier that he
 wants to choose the best one.

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 So how is he going to do that?

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 And that's what we're going to
 look at here in this section.

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 So the first question a router asks
 itself when a routing update or when

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 a route comes in or maybe you statically
 configure a router, maybe he's

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 directly connected.

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 Basically the router says, look, I've
 just learned of a network that I

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 already know about that's already
 in my routing table.

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 The first question he asks himself
 is, is the way I learned it, is the

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 method by which I learned it the same
 as the method I've already got it

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 in my routing table or is
 it a different method?

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 If it is a different method, then he
 says, okay, of all the ways I could

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 learn about a route, either because I'm
 sitting on it, I'm directly connected

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 to it, somebody statically typed it
 in as a static route, or I learned

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 about it via some routing protocol of
 all those methods, each one of them

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 I'm going to prioritize them from the
 absolute best out of all of them

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 to the absolute worst
 out of all of them.

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 Each one of these is going to be prioritized
 with a number called administrative

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 distance. So with administrative distance
 like it says, it defines the

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 trustworthiness or the preferably
 of a routing protocol.

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 And when I say a routing protocol,
 I mean any method or route could be

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 learned, including static routes
 and connected routes.

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 Those also have a number called
 administrative distance.

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 It's an 8-bit number, as we
 can see here, from 0 to 255.

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 And the lower the
 value, the better.

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 So the lower the administrative distance,
 the more preferable that is.

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 So we can see here this chart.

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 And if you're preparing for your CCNA,
 yes, unfortunately, this is something

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 you're going to want to memorize all
 these administrative distance values

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 that you can see right here.

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 So let's look at it.

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 So if the router has a routing table
 and it got there because he's sitting

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 on that interface, he is
 a host in that network.

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 He is directly connected to it.

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 You can't get any
 better than that.

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 If I'm sitting in a room, if I'm sitting
 in the kitchen, and all of a

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 sudden somebody shouts in through the
 door, hey, Keith, if you follow

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 me, I can tell you how
 to get to the kitchen.

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 I'm not going to listen to him.

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 I live here. I am in the kitchen.

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 I'm directly connected to it.

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 Nothing beats that.

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 So that's why I directly connected route
 has an administrative distance

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 value of zero, the lowest
 best number you can get.

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 And that you cannot change.

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 You can never change
 that behavior.

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 So if I try typing in a static route
 for something he's connected to,

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 the static route won't even show up in
 my routing table because his connected

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 route beats everything.

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 Now, let's say he's not
 connected to something.

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 If I statically give him a static route,
 the IP route command, that is

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 going to beat out any of
 these routing protocols.

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 Static routes have an administrative
 distance of one which is lower than

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 any of the routing protocols.

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 Now everything from here to here, everything
 in this box is changeable.

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 So there are commands you can use to
 actually make a static route less

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 preferable. You could say, well, I actually
 want RIP to be preferred over

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 static routes. Or I want ISIS to
 be more preferred than EIGRP.

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 You can do that by modifying these
 administrative distance values.

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 Now, at the CCNA level, they do not
 expect you to know how to do that.

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 The only administrative distance value
 that you're expected to know how

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 to change at the CCNA level is the
 administrative distance value for a

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 static route. And
 that's real easy.

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 You just configure your
 IP route statement.

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 However, it's going
 to be IP route.

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 And you give it whatever
 your route's going to be.

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 And then you give it your mask.

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 And then you give the next top
 address, whatever that is.

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 And then the very last thing you type in
 is a number, administrative distance

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 value. For example, I could
 type in the number 100.

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 And by typing in the number 100, that
 means this particular static route

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 no longer has administrative
 distance of 1.

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 It now has an administrative
 distance of 100.

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 Which means if I learn of that same
 route as an EIGRP internal route,

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 the EIGRP internal route will win.


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 That will be the one that gets into the
 routing table because it is better

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 than administrative
 distance of 100.

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 These other ones here you also can change,
 but that is at the CCNP level

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 where you learn how to
 change those values.

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 So if I learn of a route and it's a
 route I already knew in my routing

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 table and the method
 was different.

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 I learned about it via OSPF, but
 now here it comes in via RIP.

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 Okay. Compare the administrative distance
 values between OSPF and RIP,

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 which one's better.

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 Oh, I learned about it via BGP,
 but it comes in via OSPF.

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 Which one is better?

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 So that's how you answer that question,
 Ministry of Distance.

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 But what if the way that you learned
 it is the same, right?

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 What if I learned about a network and
 I say, okay, it came in via EIGRP,

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 but I already had it via EIGRP.

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 I already knew about it via EIGRP.


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 Then a Ministry of Distance is not going
 to be able to help you because

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 the methods are exactly identical.


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 So in order now to figure out which
 is the best route, now we look at

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 something called the routing metric, which
 is used for the best path selection

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 process. So all of your, with static
 routes there is no routing metric.

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 So in other words, if I, I could create
 seven static routes to the same

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 destination prefix, IP route 9.9 blah,
 blah, blah, blah, IP route 9.9,

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 blah, blah, blah, blah, IP route
 9.9, I could keep doing that.

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 They would all be
 seen as the same.

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 So all of those static routes would
 be put into my routing table.

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 And my router could now load balance
 between those different next hops

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 to get to the 9.9 network.

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 All of your routing protocols though,
 when I'm a router and your router,

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 my favorite terminology, I'm
 a router, your router.

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 And we're speaking a routing
 protocol to each other.

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 When I send you a prefix, when I send
 you a network, in addition to sending

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 you the network, the subnet mask and
 saying, hey, you can use me, I'm

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 your next hop, let me give
 you my IP address.

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 In addition to that, I give you a value,
 a descriptive characteristic

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 of that route that gives you some indications
 far as what it takes me

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 to get there, what it
 costs me to get there.

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 That is called the metric,
 the routing metric.

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 And different routing protocols describe
 a routing metric in different

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 ways. Whatever the way it is,
 it's going to be a number.

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 It's a numerical number
 and lower is preferred.

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 Just like with administrative
 distance, lower was preferred.

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 If I learn about the route via the
 same method, let's say they're both

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 learned via RIP or they're
 both learned via OSPF.

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 I'm going to say, okay, what
 is the metric that OSPF uses?

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 Which one has the
 lower OSPF metric?

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 What's the metric that RIP uses?

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 Which one has the
 lower RIP metric?

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 And depending on the routing protocol
 we're talking about, depends on

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 the different metric.

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 So this is also something you definitely
 need to know for the CCNA exam.

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 What the metrics are called and how
 they're derived for the different

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 routing protocols.

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 For example, let's just take
 the easiest one, RIP.

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 The metric of RIP is Hopcount.

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 In other words, when I send you, let's
 say you and I are both speaking

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 RIP and I send you a RIP update, I send
 you a RIP message that says, hey,

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 I'm your neighbor and you can get
 to the 70 network through me.

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 And the Hopcount is four.

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 What that means to you is you know that
 to get to the 70 network you have

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 to hop through four routers.

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 That when you're going to send a packet
 to the 70 network, if you use

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 me, if you select my path as the best
 path, you'll be hopping through

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 four routers to get there.

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 Whereas if another RIP router says,
 hey, I can also get you to the 70

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 network with a Hopcount of two, well,
 now you're going to choose him because

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 lower is better, right?

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 Two hops is closer to
 you than four hops.

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 So the metric RIP
 uses is Hopcount.

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 OSPF uses something
 called bandwidth.

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 OSPF says, okay, what is the
 bandwidth of the path?

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 Let me put that bandwidth into a formula
 and I'll come up with a metric

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 called cost. I should actually
 put that in here.

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 The actual technical term is called
 a cost, which is based on bandwidth.

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 So when I send you an OSPF link state
 advertisement that has a route inside

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 of it, I'll say, and by the way,
 the cost is, let's say one.

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 Well, as it turns out, if I have a fast
 ethernet interface, 100 million

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 bits per second, that's the bandwidth,
 100 million bits per second.

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 Well, if I take that 100 million bits
 per second, plug it into the formula

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 that OSPF uses, it will come
 up with a cost of one.

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 If somebody else says, hey, I can get
 you also the 70 network with an

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 OSPF cost of five, lower is
 better, you'll prefer me.

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 An OSPF cost of one is better
 than OSPF cost of five.

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 Now, you can start to see how if I sent
 you the 70 network via OSPF with

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 a cost of one, and he sent you the
 70 network with a RIP, Hopcount of

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 one, you couldn't compare
 those metrics.

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 It's like comparing
 apples to oranges.

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 You can't compare a cost
 against Hopcount.

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 That's why we have administrative
 distance.

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 When the protocols are different,
 we use administrative distance.

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 When the protocols are the same, we
 use the metric that protocol uses

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 to figure out the best one.

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 And the EIRP metric
 is fairly complex.

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 It actually computes a lot of different
 factors into its formula when

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 it comes up with its metric.

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 And its metric is called distance.


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 OSPF metric is called distance.

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 So that concludes this video on
 figuring out the best route.

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 In the next video, we're going to start
 our section on comparing and contrasting

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 routing protocols from a high level to
 see what makes them different from