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Alright, so we've got a list set of terminology that we're going to have to get

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familiar with.

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So at this point, I'm going to stop numbering routers here, but we are going to

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recognize

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that this here is our list environment.

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Meaning we could have a bunch of other routers inside of this space, kind of

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the drawing we

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had before there, a bunch of routers, but we do have some router specifically

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at the

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edge that are going to have subnets attached, probably more than one subnet per

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router, but
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router, but

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we get the idea here.

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So in general, we're going to have subnets, so again subnet A, B, and C. And as

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we start

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to build our terminology over here, we're going to, first of all, review a

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couple of important

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ones.

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So here's the terminology.

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And the first one is simply the endpoint identifier or the EID.

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The endpoint identifier, which again, stands for endpoint here, and point
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The endpoint identifier, which again, stands for endpoint here, and point

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identifier equates

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to the subnet.

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So even though we call it an endpoint identifier, we might say, okay, well,

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that must be a

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host on the subnet.

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Well, from a list perspective, it's talking about the entire subnet.

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That subnet could be a slash 24 or slash 28, a slash 22, doesn't matter.

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It is a subnet that gets represented in the list, borrowed as an EID.

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We then have the R lock, which stands for the routing locator.
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We then have the R lock, which stands for the routing locator.

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And the R lock is going to represent the location address, which again,

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typically is going

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to be something like loopback zero or something like that, a loopback interface

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that is reachable

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by everybody else.

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These loopback addresses are going to be part of the underlay.

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All of the routers inside of this space, knowing where each other are, this in
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All of the routers inside of this space, knowing where each other are, this in

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here is the underlay.

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Meanwhile, these routes that are all sort of part of the outer edge where they

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can just

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go from one subnet to another, that would be the overlay in this example.

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So I'll just write it over here as overlaying kind of point to these different

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subnets here.

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But we can start to make sense of the fact that I as router a here, this edge

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router, I

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know where this router is, this I know, this down here, I don't know because it
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know where this router is, this I know, this down here, I don't know because it

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's part

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of the overlay, not the underlay.

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So let's start giving some terms to these routers.

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If a packet is coming into one of these routers, then this makes this router

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the ITR, the

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ITR stands for the Ingress Tunneling Router.
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ITR stands for the Ingress Tunneling Router.

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Meanwhile we're bound to go out another router somewhere out here.

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So let's say we're going to subnet B, this turns this router into the ETR.

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And as we might imagine, from the letters, that stands for the Egress Tunneling

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Router.

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Now ingress and Egress are just English words that mean in and out.

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We're coming into the list domain via the ITR.

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We're exiting the list domain via the ETR.
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We're exiting the list domain via the ETR.

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Funny enough, these roles can be flipped.

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What happens now if we have return traffic?

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Well I'm coming in via this bottom router, which turns it into an ITR, and it

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exits the

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list domain over here via the ETR.

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So these are always going to be relative to the traffic flow.

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Meaning that I might be an ETR with one packet and an ITR on the next packet,
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Meaning that I might be an ETR with one packet and an ITR on the next packet,

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which is

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why we sometimes just call these XTRs because they really could serve either

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role.

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Now next let's talk about that database.

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Remember, we said that we needed a database that has all of the EID to our lock

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mappings.

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And this is a mission critical part of the list control plane.

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So this actually gets split into two different functions.
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So this actually gets split into two different functions.

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We have the mapper's allver, as we've already mentioned, and that would be the

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MR.

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And it also gets a map server or MS role as well.

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We're going to define these in a little bit better formly coming up here

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because it's

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important for us to understand what the difference is.

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The reality is that in smaller environments, we simply have one device that is
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The reality is that in smaller environments, we simply have one device that is

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going to

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do both.

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We have the MS MR or maybe the MRMS depending on who's writing it out.

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But either way, we have one device that can do both in a lot of situations.

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This is going to be how we do it with software defined access.

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On the internet, if we have dozens or hundreds of routers running Lisp, then

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this separation

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is for the sake of scaling and I can create multiple of these databases to make

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sure

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that I scale out so that I don't get overwhelmed.
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that I scale out so that I don't get overwhelmed.

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And lastly, Cisco doesn't actually believe that Lisp is going to take over the

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entire internet.

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It's going to take over pockets of the internet.

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So I mentioned, maybe we've got dozens of routers in this space.

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But if we only have dozens of routers in the space, that means we don't have

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the rest of

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the internet.

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So we're going to have this outside world that isn't running Lisp.

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In order for me to connect to the outside world, I'm going to need a router
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In order for me to connect to the outside world, I'm going to need a router

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that has that

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connection.

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And this is going to take the form of a proxy and we'll just say XTR.

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Meaning that we have this router operating either as a PITR for when traffic is

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coming into

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the Lisp domain.

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And we have a proxy egress tunneling router or a peter, I guess we could call

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it.

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But as the traffic is leaving the Lisp domain, this kind of gives us that thing
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But as the traffic is leaving the Lisp domain, this kind of gives us that thing

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we said we

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can't have a sort of default route.

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So if I bring a packet in and I'm destined for subnet W, I might check the

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database and

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it says, hey, I don't actually have an entry for W as a subnet as an endpoint

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identifier.

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So what do I do?

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Do I just drop it at that point?

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Well, no, that's exactly what the, in this case, the PTR is going to be all
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Well, no, that's exactly what the, in this case, the PTR is going to be all

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about.

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I forward the traffic, through the underlay, to get to the peter.

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And then the peter is going to send it out into the outside world.

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And likewise, again, when it comes back in, it'll take on the PITR form of

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operation.

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So these proxy XTRs, they're very important because they can connect us to this

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outside

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world, again, specifically it's not part of our Lisp domain.
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world, again, specifically it's not part of our Lisp domain.

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So we don't know where something is.

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We're going to assume it exists out there and we're going to send it to the

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nearest proxy

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external tunnel router.

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So just some terminology, we probably need to make sure that we have coming out

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of this

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conversation.

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And as we drill even deeper into Lisp in the next video, we're going to see how

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a lot of

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these concepts really start to come together.

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But again, let's make sure that we have this terminology down and we know what

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we're talking

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about when we make a reference to the ETR or the ITR or the MR or the MS
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about when we make a reference to the ETR or the ITR or the MR or the MS

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because that's

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going to be critical for us when we're talking about Lisp.