WEBVTT

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 Welcome to this video, which I've titled
 Controller-based SDN architectures.

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 We have three main topics that we're
 going to go through fairly quickly

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 in this video. We're going to talk
 about what is an underlay network,

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 what is an overlay network, and what
 is this thing called an SDN fabric.

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 So let's talk about these from a standpoint
 of architectural elements.

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 We're going to talk about underlay
 networks, overlay networks, and the

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 SDN fabric. So what are they?

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 Let's start with the first
 one, the underlay network.

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 So the underlay network is basically,
 like it says here, the protocols

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 and features established to have full IP
 reachability between your endpoints.

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 So, fancy way of saying, you've physically
 connected your routers and

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 switches together, so those routers and
 switches, they're physical links,

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 they're their NIC cards, that's
 part of the underlay network.

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 That's all your layer one and your
 layer two if we're talking the OSI

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 model here. Then your underlay network
 is also responsible for giving

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 IP reachability end to end.

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 So whatever that is, whether you're configuring
 EIGRP, OSPF, maybe static

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 routes, RIP, so you've got some routing
 protocol now running on all your

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 layer three switches and your routers
 to provide end-for-end reachability.

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 So once that is done and from any point
 on the network you can ping any

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 other point, you have now created
 your underlay network.

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 So from a software-defined networking,
 so I'm an SDN perspective, an underlay

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 network typically has certain
 characteristics.

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 What are those characteristics?

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 Well, number one, all links typically
 configured as layer three point

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 to point. So if you studied switching,
 you've probably learned that you've

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 probably seen a lot of topology diagrams,
 they have three, four, five

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 switches and they're
 all layer two.

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 All those switches are maybe in some
 VLANs with VLAN trunks and ether

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 channels and maybe there's really not
 any layer three in those switches,

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 maybe the first layer three
 is a router up here.

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 So you've got four or five switches
 all doing layer two and this is the

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 very first doing routing.

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 Those links connecting switch one to
 switch two, those would not be VLAN

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 trunk links, those would be layer
 three point to point routed links.

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 So every single link between a router
 and a router or router and switch

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 or a switch and a switch would be a
 layer three routed point to point

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 link. That is a typical design consideration
 when doing an underlay network

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 in software-defined networking.

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 You would use some common industry
 standard routing protocols.

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 Most underlay networks utilize either
 OSPF or ISIS because they're very

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 well known, pretty much any vendor's
 equipment supports them.

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 We're not really going to look at proprietary
 routing protocols here when

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 building our underlay network.

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 So the reality is you as a network
 engineer, you've been building and

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 maintaining underlay
 networks for years.

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 You probably just didn't call it that
 but that's what an underlay network

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 is. Now what's this thing
 called an overlay network?

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 An overlay network is a virtual network
 that you're creating on top of

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 or over your underlay network.

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 So your underlay network is providing your
 physical connectivity, is providing

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 the basic IP reachability.

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 An overlay network is on top of that
 and is practically implemented via

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 VRFs, MPLS VPNs, VXLAN
 or something else.

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 So let's just draw something here real
 quick just to demonstrate this

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 and hopefully bring this home.

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 Let's say that this is
 our network right here.

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 So maybe these boxes are
 layer three switches.

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 And here we have some routers and here
 we have a couple of other layer

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 three switches. So what I'm drawing
 right now is my underlay network.

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 And typically in an underlay network we
 want as much redundancy as possible.

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 Multiple redundant links
 between devices.

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 So something like this and
 we'll say R1, R2 and R3.

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 And let's just put one more layer
 three switch right here.

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 Okay. Now as part of building out the
 underlay network we said that every

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 single link is a point
-to-point routed link.

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 So this is going to have most
 likely a slash 30 subnet.

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 This right here is going to
 have a slash 30 subnet.

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 Every single link is going to be
 underlay network is finished.

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 Now I've got full IP reachability from
 every point to every other point

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 on this network.

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 Okay. Now what is the
 overlay network?

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 All right. So let's say that
 we have this design.

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 Okay. Let's say that in the future
 I'm just going to put this here as

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 a payroll server one.

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 And this over here might
 be payroll server two.

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 I'm sort of anticipating that those payroll
 servers are going to be right

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 there. And what I want to have happen
 is the moment those payroll servers

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 actually exist and connect they
 are going to be authenticated.

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 So they're going to have to prove themselves
 to the network by offering

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 digital certificates or credentials
 or something.

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 So we know who and what they are that
 their payroll servers as in this

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 example. And once we know their payroll
 servers the network are SDN controller

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 I should say will already have a policy
 in place that says hey whenever

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 payroll talks to each other
 they should take this path.

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 They should go across here.

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 They should go across
 here, here, and here.

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 So payroll should not be allowed to
 go on any other links beside that.

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 Furthermore if we have another department
 that comes online at some point.

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 Maybe HR. So here's HR right here.


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 Maybe HR right here.

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 Well if they ever connect once they're
 authenticated and we verify who

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 they are. Well their traffic
 should take this path.

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 So what we're building out here is
 what's called the overlay network.

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 In other words once payroll connects the
 software defined networking controller

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 is going to send down some policies to
 these devices to this switch right

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 here, this switch right here, this guy,
 this guy, and this guy ensuring

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 that those links exist, ensuring those
 paths exist, and also putting in

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 maybe some access lists.

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 Saying okay when P1 and P2 talk to
 each other these are the only links

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 they're allowed to take.

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 They're not allowed to
 take any other path.

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 So we have just built a path and the
 same thing is going to happen when

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 HR connects. When HR connects we're
 going to build path that's going to

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 be enforced via access lists.

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 Maybe certain quality of service will
 be implemented on that path to ensure

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 certain bandwidth and delay
 requirements are met.

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 And now any packets from HR will
 have to go across this path.

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 This is considered
 overlay networks.

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 We now have two overlay networks.

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 We have the green overlay network for
 payroll and we have the red overlay

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 network for human resources.

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 Now those overlay networks couldn't even
 happen unless the underlay network

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 was there to support it.

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 If those routers and switches weren't
 already there, if they weren't already

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 cabled up, if they didn't already have
 routing protocols to give end to

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 end IP reachability, these red and
 green paths would be meaningless.

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 There wouldn't be anything
 to carry them.

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 So the overlay network is a dynamic
 path that we program on top of our

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 supporting structure which
 is the underlay network.

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 And these overlay networks
 can grow and shrink.

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 They can change just depending on what
 our policy is at any given time.

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 And this is the primary job of the now
 there's one other term I want you

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 to be familiar with.

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 So we just talked about underlay and
 overlay which is something called

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 the SDN fabric. Now this is just a fancy
 term for the physical infrastructure

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 that's used to build
 the underlay network.

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 So the fabric is composed of the actual
 switches, routers, cables, and

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 internal switching paths used
 to build the underlay network.

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 It's typically used to describe only
 those devices in the underlay network

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 that can be programmed or controlled
 by the SDN controller.

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 Now if you've watched any other videos,
 you've gotten a taste of the fact

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 that when the SDN controller is speaking
 to my networking devices and

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 controlling them, there's a variety
 of languages used to support that.

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 It might be something like simply in
 a CLI session over SSH which pretty

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 much any device supports or might be
 more programmatic like net conf yang

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 or rest conf yang which only certain
 router switches support.

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 So whatever the mechanism, whatever
 the language or protocol that the

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 SDN controller and our automation tools
 are using to build those overlay

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 networks, well our underlay devices
 have to support that.

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 Now that's not always possible.

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 Sometimes we have an existing network
 that's already in place that when

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 we get our controller in there, if our
 controller is speaking for example,

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 net conf yang as a native language, maybe
 our existing devices don't support

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 that. And so instead what we decide to
 do is well we're going to buy some

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 new devices at the edge that do support
 net conf yang that natively speak

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 that language but our sort of interior
 of our existing network doesn't

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 support that. Well a lot of times this
 term the fabric will be relating

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 to those new devices we just bought
 that speak the same language as the

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 controller. So the controller doesn't
 really have any direct influence

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 or impact over those middle devices
 necessarily but it does on the outer

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 devices. Now in an ideal world every single
 device you have, every firewall,

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 every router, every switch would support
 the exact same language as the

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 controller. So the controller could
 reach out and command and control

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 all of those devices.

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 That's not always possible but
 that is an ideal situation.

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 So a lot of times in documents and papers
 when they have a little device

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 can be controlled by
 your SDN controller.

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 And the last thing I want to mention
 here is that in an ideal world your

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 fabric would be fully meshed.

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 There'd be multiple connections to
 get everywhere, there'd be multiple

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 equal cost paths to get between destinations
 so you could do, you could

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 take advantage of load balancing.

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 That's not always practically possible
 but this term the fabric a lot

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 of times assumes that.

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 So the term, an SDN fabric a lot of times
 assumes that you've got a network

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 where every single device supports whatever
 the language is that the controller

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 is using to control them and that we have
 multiple equal cost paths between

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 destinations. Even if that's not necessarily
 true that's a lot of the

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 assumption that goes behind this.

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 So that brings us to the
 end of this video.

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 Thank you for watching.