WEBVTT

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 So, up until this point, we've talked
 about why do we need routing?

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 We've talked about what happens to
 a packet when it gets routed, what

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 changes occur to it.

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 We've talked about how can routes get
 put into the routing table, connected,

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 static, dynamic routes.

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 We've talked about if I learn about
 the same network via multiple next

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 hops, how do I select
 the best path?

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 So, now we're going to start focusing in
 on either routing protocols themselves.

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 Now, we're going to focus exclusively
 on dynamic routing protocols.

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 And now, if you are in the position
 where you have to choose a routing

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 protocol, most likely if you are taking
 a job as a network engineer or

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 network admin for some company, they
 probably already have some routing

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 protocol in place.

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 But let's say at some point in your
 career, you actually are designing

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 a network and it's in your hands to
 decide what kind of routing protocol

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 to select for that network.

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 Because you've got about six choices
 to choose from, it helps to be able

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 to sort of contrast and compare them against
 each other to make an informed

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 choice. And this last section of video
 is going to help guide you down

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 that path. So, let's start
 with IGP's and EGP's.

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 So, in order to understand these acronyms
 of IGP's and EGP's, we first

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 have to understand what
 an autonomous system is.

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 So, an autonomous system, as it says
 here, is a collection of networks

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 all under one administrative
 authority.

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 What does that mean?

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 Okay. Let me give you two
 contrasting examples.

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 Think of your network
 at your house.

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 It's probably pretty small.

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 You probably have like a cable
 modem or a DSL modem.

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 And you might just have a single
 laptop connected to that.

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 Or maybe you've got a switch and a couple
 laptops and a printer connected

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 to that. However big or small your
 home network is, who controls it?

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 You do. You are the central administrative
 authority for your network

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 at home. So, that network you have at
 home technically is considered an

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 autonomous system.

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 You are in charge of it.

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 It's managed by you.

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 Now, let's go to the opposite
 end of the extreme.

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 Think about a company like Cisco Systems
 that has close to 100,000 employees

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 almost. They've got, in their San Jose
 campus, they've got over 40 buildings,

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 40 buildings in their
 San Jose campus.

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 So, their network is massive.

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 And yet, that huge network they have
 out there is all managed by their

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 IT department. Which is pretty big, but
 there's one central administrative

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 authority which is Cisco's IT department
 which is managing that huge network.

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 That is also an example
 of an autonomous system.

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 So, what does that have to
 do with IGP's or EGP's?

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 Routing protocols at a very high level
 when you think about what problem

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 were they designed to solve.

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 Why were they invented
 in the first place?

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 One of the first things you have to
 ask yourself is, was the intention

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 of this routing protocol, was its purpose
 to be able to exchange routes

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 with routers that are in the same autonomous
 system within the same company

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 or within routers that are in
 different autonomous systems?

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 And IGP, interior gateway protocol,
 was designed to provide reachability

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 for information within
 an autonomous system.

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 Pretty much all your routing protocols
 but one fall into this category.

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 So, RIP, EIGRP, OSPF, ISIS, they are
 all examples of routing protocols

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 that are interior
 gateway protocols.

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 So, when I'm a router and I'm running
 RIP or EIGRP or OSPF for ISIS and

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 I hear from another routing protocol
 that's speaking my same language,

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 I trust that router.

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 I assume that router is running in
 the same autonomous system as me.

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 He belongs to the same company
 as me, so I trust him.

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 Now, EIGRP actually
 sort of checks that.

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 When you configure EIGRP you actually
 have to tell it what autonomous

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 system number it's in and it will only
 speak to other routers that are

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 in the same autonomous system as itself,
 because EIGRP is an interior

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 gateway protocol.

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 So, IGP's, their main responsibility
 is to speak to routers in the same

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 AS's themselves and to very quickly
 transfer information.

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 IGP's main goal is to have
 very quick convergence.

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 Now, in case you're not familiar with
 that term convergence, all convergence

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 means is that if you think about a
 network, however big or how small,

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 let's say one part of that
 network goes down.

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 Convergence is the length of time it
 takes for the entire network to be

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 aware that that portion
 has gone down.

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 That's what we mean
 by convergence.

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 So, we want quick convergence and this
 is what IGP's were designed for.

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 IGP's were designed for
 quick convergence.

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 As soon as a new network comes up, let's
 let everybody know about it as

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 quickly as possible.

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 If some network changes or go down,
 let's let everybody know about it

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 as quickly as possible.

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 We want fast convergence.

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 So, we want to learn everything
 and we want to learn it quickly.

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 That is the primary goal of an
 interior gateway protocol.

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 An exterior gateway protocol, and there's
 only one, BGP, the border gateway

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 protocol, is our one and only example
 of an exterior gateway protocol.

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 This is designed to provide prefix reachability
 between different autonomous

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 systems. For example, Internet service
 providers, Time Warner Cable, Comcast,

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 Sprint, they use BGP so their routers
 can talk to each other.

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 Now, when you're talking about one
 service provider talking to another

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 service provider, you're talking about
 them exchanging possibly hundreds

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 of thousands of routes, hundreds
 of thousands of routes.

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 So, the primary responsibility of an
 exterior gateway protocol is to be

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 secure. You don't trust
 routers blindly.

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 You only talk to routers that
 you're supposed to talk to.

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 And to be able to handle massive
 quantities of routes.

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 An exterior gateway protocol is not
 concerned as much with convergence.

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 In other words, an exterior gateway
 protocol, if it learns that a new

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 network has come up or some network has
 gone down, it will tell its neighbors

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 about that. But its primary job is not
 to convey that information super,

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 super fast. The primary job of an exterior
 gateway protocol is to be able

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 to handle mass bulk transfers of routing
 information, tens or hundreds

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 of thousands of routes.

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 That's what it's optimized for, for
 scale, for grandness, not for being

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 quick. So, if you are designing a routing
 protocol in your network, chances

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 are you're going to select an
 interior gateway protocol.

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 Now, if you happen to be a network engineer
 for an Internet service provider,

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 and your job is to design a routing
 protocol that will talk to another

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 router that's owned by a different service
 provider, then your car probably

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 going to want an exterior
 gateway protocol.

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 In the next sections of videos, we're
 going to now start drilling down

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 a little bit more and see what categorizes
 our various interior gateway

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 protocols from each other and sort of
 contrast that against our one and

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 only exterior gateway protocol.