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Well, switches are well known for running VLANs and trunks and everything like

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that,

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but what about routers?

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ROUDERS are typically known for running Layer 3 interfaces.

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However, they are also able to run Layer 2 trunks.

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Kind of an interesting concept, so what are we talking about here?

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Well, the idea is that I might have a scenario where I have a Layer 2 switch

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and I'm running

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a bunch of VLANs and I can now trunk this up to a router and allow this router
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a bunch of VLANs and I can now trunk this up to a router and allow this router

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to participate

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in all of those VLANs.

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This allows me to configure this truly as a trunk between the two and I'm going

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to carry,

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let's say VLANs 1 through 10 on this trunk link and extend those VLANs up to

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the router.

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Why would I go about doing this and how does the router support this?

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Well, this is actually a very famous and what we might consider to be a legacy

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topology

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at this point.

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This is known as a router on a stick.
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This is known as a router on a stick.

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And yes, I laughed the first time I heard that and I still got to laugh when I

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hear

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it today.

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It's meant to be a little bit of a tongue-in-cheek type of description of what

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's happening here

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and we see why it's called a router on a stick.

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It looks like the switch is holding a router up on a stick.

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But the big part of this is understanding what we're trying to accomplish.

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See, back when networks were new and fresh, we would have these large layer two

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networks

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and they'd all consist of layer two switches.
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and they'd all consist of layer two switches.

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So the question is, where do the gateways live?

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Where's the default gateways for VLANs 1 and 5 and 6 and 10?

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Well, they can't live on these layer two switches.

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They're not capable of running multiple SVIs and running routing protocols

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because they're

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true layer two switches.

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So what we do is we deploy a layer three device up here and the layer three

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device, in our

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case a router, is going to serve as the default gateway on a per VLAN basis and
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case a router, is going to serve as the default gateway on a per VLAN basis and

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it's going

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to form the routing via upstream connections.

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So whatever layer three world lives upstream, whether that's connecting to the

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internet

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or going off into a wide area network space, either way, we are forming routing

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adjacencies

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into this router and allowing it to serve as the default gateway on a per VLAN

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basis.

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This is why we have to trunk all of these VLANs up to the router.
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This is why we have to trunk all of these VLANs up to the router.

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But in order to really understand what's happening here, let's go ahead and use

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our

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dot notation to take a more detailed look.

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Because what we're really doing here is we're extending many different VLANs up

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to this router,

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but inside of the router, we're going to take those VLANs and split them off.

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And we're going to have an interface, a layer three interface, which of course

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we're going

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to represent as a dot.

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And I'll draw a few of these here.

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I won't draw 10 of them.
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I won't draw 10 of them.

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Let's just draw five.

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So one more.

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So now we've got five of these interfaces.

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There are three interfaces, so these are layer three.

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But they're no longer an interface.

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This physical interface is an interface, but they're also not VLAN interfaces

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because

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those exist on switches.

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So instead, we're going to call these sub interfaces.

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And these sub interfaces are going to get mapped each one to a VLAN ID, meaning
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And these sub interfaces are going to get mapped each one to a VLAN ID, meaning

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that

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this sub interface over here might get mapped to, let's say, VLAN 5.

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And then because it's a layer three interface, I can apply an IP address to it.

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And whatever the gateway address is for VLAN 5, and then this one here, I can

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do the same

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thing but for VLAN 4 with an IP address.

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So the sub interface concept being used on a router and a stick topology is
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So the sub interface concept being used on a router and a stick topology is

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what a lot

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of networks thrived on for a long time when they had a pure layer two switching

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network

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that couldn't handle the ability to deploy multiple SVIs.

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And at that point, we required a layer three device.

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Now eventually, the solution led us to multi-layer switches or layer three

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switches depending

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on how we want to call them.

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But with multi-layer switches, we were able to say, you know what?

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You don't need a router on a stick anymore because now I can do what we've been
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You don't need a router on a stick anymore because now I can do what we've been

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drawing,

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which is to have sub, not sub interfaces, but SVIs for VLANs 4 and 5 and allow

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those

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to connect in to our different layer two VLANs as we've sketched out a few

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times.

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This drastically improved the traffic flow in a network because if we think

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about it,

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if I have a host hanging off of this layer two switch that's in VLAN 4 and a

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host over

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here that's in VLAN 5, how do these devices communicate with one another?
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here that's in VLAN 5, how do these devices communicate with one another?

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Well, as we think about the topology here, this host is connected to VLAN 4

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which is

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carried up on this trunk over here on this side.

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And then we'll just make VLAN 5 here green.

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And so same thing here except it's in a different connection, a different layer

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two domain that

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goes off to the right.

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And then this one comes up here.

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So as we color these in a little bit, we start to see how exactly this traffic
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So as we color these in a little bit, we start to see how exactly this traffic

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has to flow.

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It has to flow up the trunk to the sub interface, get routed over to the other

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sub interface

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and then get sent back down the trunk out to the other device.

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So this is known as tromboning of traffic, named after the instrument that sort

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of has

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the big long loop like that at the end.

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Well, that's exactly where we're doing.

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We have a big long loop where I go up to a device and then right back down as
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We have a big long loop where I go up to a device and then right back down as

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we clear

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up this drawing a little bit more.

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And we see that's very inefficient.

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I'd really rather just send that traffic right from one host to the other even

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if it

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involves routing and that's exactly what multi-layer switches do.

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Now I can get from this host on the left in VLAN 4 to the host in VLAN 5.

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Yes, I do have to go up and get routed over, but it stays on the same device.

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I don't have to worry about sending it off to some externally connected layer

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three router
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three router

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on a stick.

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So this is how router on a stick architectures worked, how sub interfaces

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worked.

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And we still see sub interfaces in use today every now and again, but generally

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speaking

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in real world networks, we don't have to worry about sub interfaces and trunks

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out to routers

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because we don't have to worry about router in a stick anymore thanks to our

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multi-layer

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switches.