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So, in this scenario, we have a packet arriving on Switch1 towards a particular

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destination,

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and we need to know what exactly SD Access does with this packet and how

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exactly it knows

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where to send it and if it doesn't know where to send it, what does it do with

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it?

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So, looking through these questions, what type of message is first sent by

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Switch1?

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Well, it's going to send a request to the control plane node because, again, we

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need

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to pull down the information.

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We need to fix this pull-based methodology we really need to start to get used
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We need to fix this pull-based methodology we really need to start to get used

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to because

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we're no longer pushing routes everywhere.

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Furthermore, when we talk about routes, we are talking about those endpoint

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identifiers

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that LISP manages.

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Now, we might think of those as routes like going to a /24, but in the case of

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SDA, we're

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talking about individual /32s.

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We need to know where this specific host lives because the subnet is everywhere

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.

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I mean, we've probably got a host over here that's 10.100.1.51.
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I mean, we've probably got a host over here that's 10.100.1.51.

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I mean, they could be adjacent from an IP address perspective, so it doesn't do

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me any good

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to know where the subnet lives because the subnet kind of lives everywhere.

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I need to know where this specific host lives, and so that's the responsibility

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of the control

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plane node.

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As soon as Switch2 sees a packet from this host, we're going to register that

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with the

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control plane node.

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And now, when we go and we make a request of the control plane node, we're

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going to pull
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going to pull

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that information down.

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Yeah, we could push this information everywhere, and then everybody knows where

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every single

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host in the network is.

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It doesn't scale particularly well, and keep in mind also that we can cache

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this information.

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I don't have to do this every single time.

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I can know that, hey, .50 goes to Switch2, and I can remember this over the

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course of

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time.

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And yeah, eventually I'll clear that out if traffic stops, but as long as

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traffic is

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flowing to that host, I'm going to keep this information in the cache, and I
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flowing to that host, I'm going to keep this information in the cache, and I

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don't have

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to go all the way up to the control plane node in order to get that information

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again.

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So that's how we go about doing this.

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The first step is to send that request up to the control plane node, and then

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what happens

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if it has an entry?

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Well, we respond.

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We send a response back to Switch1, and it encapsulates this into VXLAN and

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sends it

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across to Switch2.

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And we use VXLAN because it maintains the entirety of the Layer 2 frame,

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including the

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original Ethernet headers.
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original Ethernet headers.

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And this allows us to, I really like drawing out, make the dotted line

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connection here so

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that if we have VLAN 100 on one side, we can have that same Layer 2 VLAN over

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on the other

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side, despite the fact that our fabric is built on Layer 3 for the sake of

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scaling and

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an eliminating spanning tree protocol.

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It's a beautiful thing.

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So, what happens if the control plane node doesn't have an entry though?

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What if it says, "Hey, I don't know where .50 is."

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In fact, let's just change the scenario here ever so slightly.
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In fact, let's just change the scenario here ever so slightly.

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Now let's say a new packet arrives and it's destined for .65.

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And so we send our pull request and we don't know where .65 is.

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Maybe it's part of a completely different subnet.

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Either way, we're going to let Switch1 know we don't know this, and therefore

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Switch1

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is going to do the only thing it knows to do, which is to send it in VXLAN over

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to the

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border node.

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The border node is going to have to decide what to do with this.
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The border node is going to have to decide what to do with this.

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And maybe it has a route and it knows to go out towards the core of the network

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.

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But the reality is that if we don't know where it is, it's probably just going

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to follow

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the default route, which ultimately takes us towards the internet.

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And that's the goal here because we do still need to have a default route path

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in the fabric,

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and that would be handled by the border node.

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Remember that there are internal border nodes, right up here.

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There's internal border nodes that handle internal connections.
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There's internal border nodes that handle internal connections.

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There's external border nodes that handle internet connections, and they can

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certainly

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have both roles on a single device.

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Now lastly, how is the packet encapsulated and where is it sent?

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We kind of answered that throughout this whole process.

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The big thing is it's going to use VXLAN in order to maintain the layer two

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information.

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So that brings us to the end of this review on the software defined access and

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software

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defined when lots of information to unpack.

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As a set of the very start, a lot of review like this to help you identify your
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As a set of the very start, a lot of review like this to help you identify your

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areas

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of weakness.

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If you have anything or identified any particular concerns, then go back and

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watch those videos

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in those skills in order to make sure we've got it all down before moving on.

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Otherwise, I hope this has been informative for you, and I'd like to thank you

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for viewing.