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So how does a router

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make its routing decisions?

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Well, it uses a routing table.

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Just like our switches

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used a Mac address table or a CAM table,

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we're going to use routing tables

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to decide where our packets need to go

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inside and outside of our networks.

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Now these routing decisions are going to contain

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layer three information,

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and they're going to do a layer three to layer two map.

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The router is going to use an arp cache to map an IP address

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to a given Mac address,

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and this way it knows inside its local area network,

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which IPs are going to relate to which Mac addresses.

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Now, each packet forwarding decision

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is then going to be based on its internal routing table.

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Those internal routing tables

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are really focused on the logical address side of things,

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or that IP address.

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So, let's dig a little bit deeper into these routing tables

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to really see how they work.

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These tables are kept by the router,

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and they help them determine which route

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is best fit for them as they're trying to route the traffic

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throughout the different networks.

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Now, a route entry has a prefix,

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and the longer the prefix is

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the more specific that network is.

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So if we look here at this chart,

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I have three different networks showing up.

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There's 125.0.0.0,

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I have 161.5.0.0,

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and I have 134.7.0.0.

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Now, which of these would have the longest prefix?

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Well, it's the bottom two,

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because they're the most specific.

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These are the ones that are most specific

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because they have the first two octets specified,

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whereas the first one, with the 125.0.0.0,

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only has the first octet specified,

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so it is the least specific.

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So if I'm dealing with something like 10.1.1.0/24

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that is really, really specific,

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because I have the first three octets specified,

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the 10.1.1.

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That means I only have 256 possible IPs left

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because of that .0 at the end.

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Now instead, if I use something like 10.0.0.0/8

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this means I only have one octet specified,

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and the other three are really wild cards.

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So I can have up to 16 million IP addresses

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because of those three sets of zeros.

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So you can see here the longer the prefects we have

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or the higher the CIDR notation,

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the more specific that route becomes.

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Now when we look at these routing tables

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and we look at all the routes in the table,

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those routes are going to tell us

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what the destination network is

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and which router it should go to to get to that network.

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It'll also tell us which port on the router

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is going to be used to send traffic out

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and the cost of that route,

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which is basically like switching,

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where it's dependent upon numbers of different factors

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like the link speed and other factors like that.

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Now we're going to talk specifically

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about those different factors in another lecture

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when we start diving into routing protocols,

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but for now, just realize that every link in a route

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does have a cost associated with it.

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So what are some of the sources

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of routing information that we can have?

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Well, there are really three different sources

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that routers are going to use.

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The first one is called a directly connected route.

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Now, a directly connected route

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is learned by a physical connection between two routers.

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So, if you look on the screen here,

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I have three different routers,

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router 1, router 2, and router 3.

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Now, router 2 knows where router 1 is

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and router 3 are

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because it has direct cabling between those.

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And this is shown by those zigzag lines,

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or that lightning bolt looking line,

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that's here on my diagram.

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This denotes that it is a serial connection.

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Next, we have static routes,

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and this is the second way that routers know

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how things are connected to them.

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Now, a static route are routes that are configured

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by an administrator manually.

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So, for example, router 1 knows how to get to router 2

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because it's directly connected,

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but it doesn't know how to get to router 3 yet.

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So for me to tell it how to get to router 3,

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I can put a route into my routing table

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that says every time you want to send something

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to router 3,

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just go ahead and send it through router 2.

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That would become a static route.

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Now, there is always one default static route

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in every router, and it's a special one.

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It's known as 0.0.0.0/0.

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Basically it says to the router,

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if you don't know where to go, just go here.

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It's kind of like your default gateway.

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And so for router 1, it might have a default route

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of 0.0.0.0/0 that says, go to router 2

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anytime you don't know how to get someplace.

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And then it becomes router 2's problem

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to figure out what to do with that traffic.

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Now, the third way that routers can do this

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is by using what's known as dynamic routing.

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And this is done by using dynamic routing protocols.

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These protocols are learned by exchanging information

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between routers automatically based on the protocols.

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Now, instead of me having to go into router 1

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and tell it how to get to router 3,

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instead, I can just let router 2 do it.

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Now the idea here is that when router 1

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and router 2 become directly connected,

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they start sharing their routing tables.

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So router 1 is going to say,

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"Hey, router 2, do you know how to talk to some people?"

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And router 2 will say, "Oh yeah,

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"I'm connected to router 3.

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"So anytime you got something for router 3,

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"just send it to me and I'll pass it on for you."

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And they do this all by themself automatically

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using these dynamic routing protocols.

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This is really great, especially in large networks,

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because we don't have to manually configure everything.

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So, for example, think about if I took your router at home

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and I had to put down every single route for you

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to be able to find every website

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you wanted to get to on the internet,

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you would not be able to do it.

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It was just too many things out there, right?

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So instead, we use dynamic routing to accomplish this.

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When you connect to the internet, you connect to your ISP,

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and they know that they are now your default route.

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Anything you want to get to that's not inside your network,

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you send it to your ISP,

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and then your ISP will send it to the next router,

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and it keeps going up that chain that way.

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So that's a great overview

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of the three different types of routes.

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We have directly connected routes,

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static routes, and dynamic routes.

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So, let's go ahead and dig a little bit deeper

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into each of these three types

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so you can understand them a little bit more.

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First, let's take a look at a directly connected route.

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When we look at a directly connected route,

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it's going to look something like

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what you see here on my screen.

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Now, as you can see, router 1 and router 2

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are directly connected with that zigzag line.

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That's that serial connection.

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Now, these routers both know how to get

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to the other switches

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because they're directly connected to them, right?

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So if I look at router 1's table,

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it says I'm to 10.0.1.0/24,

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which is the network that it owns on switch one.

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It's also connected to 192.1.1.0,

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because this is the serial connection

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between the two routers.

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Now, notice that router 1 does not know how to get

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to the 10.0.2.0 network,

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because this is on switch two,

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and that's hanging off of router 2.

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It has no information on this

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because it only knows things that are directly cabled to it.

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Now, if we go further into static routing,

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I can actually write that down as part of my default route,

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or 0.0.0.0 for router 1.

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Now this tells me that anytime I don't know an IP address,

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I'm simply going to send it out port serial 1/1,

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which is going to push it over to router 2.

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And so in this case, if PC1 tries to get to PC2,

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it would get as far as router 1,

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and then router 1's going to say,

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"Ugh, I don't know how to get to 10.0.2.2,

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but maybe router 2 does,

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and it's going to push that over to them

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over that wide area network connection,

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that serial connection that's directly connected.

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That's how this manual configuration works

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with a static route. All right, so I think you understand

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how direct connections work and how static routing works.

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Let's take a look at the dynamic routing protocols instead.

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Now, dynamic routing protocols can have more than one route

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for an existing network.

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If I have a more complex network diagram,

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like these five routers that you see here,

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and I wanted to get from one to five,

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I can go a couple of different ways.

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I can go one, two, three, four, five.

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And I really just start zigzagging all over the network.

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Or I might go one, two, four, five,

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or I might go one, three, four, five.

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There's lots of different ways for me to go,

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because all the different connections that are there.

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So how does my router know which one is best?

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Well, the dynamic routing is going to get negotiated for us

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based on the number of hops that are there,

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which is the number of times I have to go

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through a different router,

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the number of link bandwidth that's available,

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so how fast it is.

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I might go through the ones that are faster or slower.

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And other criteria that are out there.

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All of these dynamic routing protocols

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can be able to be supported

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depending on what routers you're using

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and different criteria of how you want to set 'em up,

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depending on how we configure them.

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Now we're going to spend an entire lesson

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on the different types of dynamic routing protocols,

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so we can really dig into each of them

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and you can really understand,

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because you're going to get test questions on them,

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come test day.

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Now the last thing I want to talk about in this lesson

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is preventing routing loops.

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Just like we had an issue with switches

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where you can get loops and broadcast storms,

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you can get issues with routing

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if things start going into a circular manner,

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things will just end up getting lost in cyberspace, right?

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So, to prevent that,

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we have two different techniques we can use.

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These are known as the Split Horizon and the Poison Reverse.

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Now, with a Split Horizon, this is going to prevent a route

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that's learned on one interface

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from being advertised back out that same interface.

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So, in this example, you see router 1

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knows how to get to router 2

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because of that direct connection

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between those two routers.

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Now, it's not going to go and tell router 2

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how to get to router 2 based on that same connection.

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Because guess what?

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It came in that connection,

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so it can't go out the same connection.

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This is essentially what our spanning tree protocol

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did for us in our switching networks.

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But now we can do this inside of routing.

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Now, the second way we do this

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is what's known as a Poison Reverse.

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And a Poison Reverse is going to cause a route

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that's received on one interface

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to be advertised back out that same interface,

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but with a really, really high cost

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so that nobody would ever want to use it.

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Now these are just two different techniques

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for you to stop routing loops.

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You could use one or the other,

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it really doesn't make a difference,

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as long as you're using one

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that's going to prevent that routing loop for you.

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The first one is I'm just not going to repeat anything.

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The second one is I'm going to repeat it,

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but I'm going to tell you it's so expensive

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that you'll never ever want to use it.

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So, let's go ahead and take a look

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at an example of these routing loops

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and how we can make sure we prevent them.

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So, here's a network with no issues at all.

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I have three routers here.

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I have router 1, router 2, and router 3.

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You can see the different networks based on the IPs

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and the routing tables are shown here on the screen.

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Now we're going to look specifically at the routing tables

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for router 2 and router 3 in this example.

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Now, if I started having an issue, like for instance,

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the network connection from router 3

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for the 10.1.4.0 network went down

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because ethernet 0/1 went down.

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What do you think's going to happen?

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Well, router 2 still thinks it can get to that network,

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because it says, "Hey,

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"I can get there through my serial port of 0/1.

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because I have a connection to router 3.

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Now, if router 3 went down, it's going to say,

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"I don't know how to get there anymore."

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And so it's going to ask its friends

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using the dynamic router protocols.

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And so at this point it's going to say,

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"Hey, router 2, do you know how to get to that network?"

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And what happens is router 2 would say,

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"Oh yeah, I know how to get to that network.

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"I can do it in just one hop.

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"So go ahead and send it to me."

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And then router 3 says, "Oh great,

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"that means I know how to get there, too,

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"because I'm connected to you, router 2,

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"and you know how to do it.

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"So that one hop for you becomes two hops for me."

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And it keeps going back and forth.

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And then router 2 goes, "Oh, I know how to get there.

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"Router 3 knows how to get there.

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"And he says he can do it in two hops,

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"so I can do it in three hops."

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And it keeps going back and forth until both of those

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get to be such a high number that neither of those routers

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can actually ever get to that route anymore.

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Now, this is the idea of what a Poison Reverse would do,

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but when you use a Poison Reverse,

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instead of making it go one at a time and keep iterating up,

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it would immediately just say,

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"Hey, I know how to get there,

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"and it would take me a thousand hops.

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"And so if that's such a huge cost,

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"you won't want to send it to me."

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That's the way that router 3 can say,

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"I don't have this route anymore."

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Now, if you don't use Poison Reverse

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and you don't use Split Horizon,

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what ends up happening is you have this loop that happens,

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and two says, I can do it in one,

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three says I can do it in two,

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two says I can do it in three,

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and they keep counting up,

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and that eats up a lot of your resources

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in your network by causing this loop

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and those two sending packets back and forth to each other,

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continually increasing that cost

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until it gets so high that they stop sending traffic.

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Now, if there's no other way to get to that network,

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it's just going to keep going in this infinite loop,

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and that's going to cause a real problem for you.

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This is why it's really important

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to make sure you're setting up your Poison Reverse

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or your Split Horizons.