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In this lesson,

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we're going to talk about route selection.

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Now, when we talk about routers,

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we have to think about how is traffic

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going to go across these routers

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and determine which path they're going to take.

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And that's what we're talking about

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when we talk about route selection.

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Now, when it comes to route selection,

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the first we have to consider

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is the believability of a route.

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Now, what exactly does that mean

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when we talk about the believability of a route?

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Well, let's say I have a network

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that's using more than one routing protocol,

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because my routers can support multiple

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protocols all at once.

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For example, I might be running RIP and I might have OSPF,

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and both of them are being used

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on the same router in the same network.

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So how do I know which one we're going to go with

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when a packet enters the router

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and has to be moved around the network?

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Well, these routing protocols

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are going to be considered more or less believable

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based on some different factors that we have.

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For example, RIP is an older protocol,

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and it's considered to be less believable than OSPF,

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which was a newer protocol.

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Now, why is that?

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Well, this is because routers use an index of believability,

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and we like to call this the AD or administrative distance.

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Now, if a route has a lower administrative distance,

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it's going to be considered more believable

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or more trustworthy,

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and that means it's going to have a better or lower number,

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just like in golf.

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So the lower the number,

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the better it is in terms of believability.

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Now here's a chart for you

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with all the administrative distances

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for the different protocols that we talked about.

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For the exam, you do not need to memorize this chart,

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but you should at least have a sense

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of which ones are the higher numbers

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and which ones are the lower numbers.

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Now, when we look at this

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and you're dealing with a directly connected route,

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we're going to have this as our most believable.

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Now, why is that?

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Well, it's because the router

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is directly connected to another router,

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so it knows itself

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and it trusts its own opinion

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that it's connected to that router.

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And this makes a lot of sense

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because if I know you personally

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and we're holding hands,

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we're both connected and therefore we have trust,

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and that trust would have

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a very, very low administrative distance

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because we trust it highly.

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That's the idea of a directly connected route.

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It's a fully trusted route

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because the router itself determined it.

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Now, the next one we have is going to have an AD of 1,

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and this is called a statically-configured network.

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Now, this is the next most believable

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because routers are designed to do what they're told.

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So as a router,

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I'm going to trust myself first,

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and then I'm going to trust my programmer,

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and then I'm going to trust all the dynamic protocols.

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That's kind of the order of these things.

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So if you as a network administrator

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go in and configure a static route in the router,

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you told the router,

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"I want you to believe this

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"above everything else you know,

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"unless you're directly connected to somebody else

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"'cause then you know better.

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"Beyond that, you're going to trust me

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"because I'm the network administrator

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"and I told you that's the route I want you to use."

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All right, the next one we have

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after our directly-connected routes

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and our static routes

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is going to be EIGRP.

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Now, EIGRP is a very trustworthy protocol

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because it's one of the newer protocols we have.

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After that, we're going to go backwards in time a little bit

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and go to OSPF,

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and then we'll go backwards again to RIP, or R-I-P.

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After that, we're going to go into an external protocol

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known as external EIGRP,

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and then we go all the way to the bottom of our chart,

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and you'll see the worst believability we have,

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which is unknown or unbelievable,

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which means we're going to have a rate of 255

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for its administrative distance.

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Now, the reason we set this at 255

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is basically we're saying,

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"We can't find this network anymore.

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"Do not trust it at all.

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"Don't send data that way.

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"It is totally unbelievable."

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So as I said, for the exam,

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you do not need to memorize all these numbers,

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but you need to have an idea

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of the order of these things

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in terms of their believability or trustworthiness.

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Remember, directly connected is the most believable,

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then static, because you as a human

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told it what to do

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will be the next believable.

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After that, we go to the more modern protocols like EIGRP,

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and then we start working backwards in time

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to OSPF and then RIP.

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In fact, if you think about this,

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this makes a lot of sense

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because RIP was one of our earliest protocols,

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so it is going to be one of our least believable.

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Then we had OSPF was an improvement to RIP,

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so it's a little bit more believable.

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And then we had EIGRP,

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which was an improvement on OSPF.

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So, again, it's more believable.

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This is the kind of memory I like to use

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when I'm trying to remember

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which ones are most or least believable

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as I'm going through my charts.

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Lastly, we have the metrics

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associated with each of these routes.

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And when we look at these metrics

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and we look at these routes,

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there's going to be a routing protocol

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that's able to choose which path it's going to go on.

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And we do this based on those metrics.

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These are things like hop count,

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believability, reliability,

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bandwidth, delay, cost, and other metrics like that.

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Each protocol is going to use a different metric

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based on its programming

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to determine which route it should be using

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and which one is going to be considered faster or slower.

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Depending on which route it is,

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this is going to determine which metric you're going to use.

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And so when you're dealing with these metrics,

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it's always going to be thinking about the fact

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that our lower numbers are considered better

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and our higher numbers are considered worse

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whenever we're dealing with routing.

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So, again, I like to think about this as golf rules.

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You want the lowest score possible

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to be the fastest you can be.

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Now, anytime we talk about hop count,

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we want the least number of hops

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because that gives us less distance to cover

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and it means we'll be able to get things done quicker.

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If we start talking about bandwidth,

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we want the lowest cost of bandwidth,

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which means we're going to have

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the highest amount of bandwidth for us to use,

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and so our lower cost becomes better.

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If we're dealing with delay,

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we want the least amount of delay.

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We talk about believability,

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we want the lowest number

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because a zero is directly connected and most believable.

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Now, this is the way we start thinking about these things.

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Lower numbers are always going to be better

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when we're talking about route selection.

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Now, the last thing I want to talk to you about

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is this summary slide here.

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Now, this is one of those things

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that I like to write down in my notes

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so you can have all your routing protocols in one place,

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and it summarizes everything really nicely,

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from RIP to OSPF to EIGRP to IS-IS and BGP,

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and it shows you which ones are interior,

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which ones are exterior,

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which ones are distance vectors,

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and which ones are linked states.

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I hope this helps you

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as you're studying for your exam

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and you're trying to remember

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all your different routing protocols

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and some of the key facts and figures for them

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before you take your exam.