Well, switches are well known for running VLANs and trunks and everything like
that,
but what about routers?
ROUDERS are typically known for running Layer 3 interfaces.
However, they are also able to run Layer 2 trunks.
Kind of an interesting concept, so what are we talking about here?
Well, the idea is that I might have a scenario where I have a Layer 2 switch
and I'm running
a bunch of VLANs and I can now trunk this up to a router and allow this router WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
a bunch of VLANs and I can now trunk this up to a router and allow this router
to participate
in all of those VLANs.
This allows me to configure this truly as a trunk between the two and I'm going
to carry,
let's say VLANs 1 through 10 on this trunk link and extend those VLANs up to
the router.
Why would I go about doing this and how does the router support this?
Well, this is actually a very famous and what we might consider to be a legacy
topology
at this point.
This is known as a router on a stick. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
This is known as a router on a stick.
And yes, I laughed the first time I heard that and I still got to laugh when I
hear
it today.
It's meant to be a little bit of a tongue-in-cheek type of description of what
's happening here
and we see why it's called a router on a stick.
It looks like the switch is holding a router up on a stick.
But the big part of this is understanding what we're trying to accomplish.
See, back when networks were new and fresh, we would have these large layer two
networks
and they'd all consist of layer two switches. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
and they'd all consist of layer two switches.
So the question is, where do the gateways live?
Where's the default gateways for VLANs 1 and 5 and 6 and 10?
Well, they can't live on these layer two switches.
They're not capable of running multiple SVIs and running routing protocols
because they're
true layer two switches.
So what we do is we deploy a layer three device up here and the layer three
device, in our
case a router, is going to serve as the default gateway on a per VLAN basis and WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
case a router, is going to serve as the default gateway on a per VLAN basis and
it's going
to form the routing via upstream connections.
So whatever layer three world lives upstream, whether that's connecting to the
internet
or going off into a wide area network space, either way, we are forming routing
adjacencies
into this router and allowing it to serve as the default gateway on a per VLAN
basis.
This is why we have to trunk all of these VLANs up to the router. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
This is why we have to trunk all of these VLANs up to the router.
But in order to really understand what's happening here, let's go ahead and use
our
dot notation to take a more detailed look.
Because what we're really doing here is we're extending many different VLANs up
to this router,
but inside of the router, we're going to take those VLANs and split them off.
And we're going to have an interface, a layer three interface, which of course
we're going
to represent as a dot.
And I'll draw a few of these here.
I won't draw 10 of them. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
I won't draw 10 of them.
Let's just draw five.
So one more.
So now we've got five of these interfaces.
There are three interfaces, so these are layer three.
But they're no longer an interface.
This physical interface is an interface, but they're also not VLAN interfaces
because
those exist on switches.
So instead, we're going to call these sub interfaces.
And these sub interfaces are going to get mapped each one to a VLAN ID, meaning WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And these sub interfaces are going to get mapped each one to a VLAN ID, meaning
that
this sub interface over here might get mapped to, let's say, VLAN 5.
And then because it's a layer three interface, I can apply an IP address to it.
And whatever the gateway address is for VLAN 5, and then this one here, I can
do the same
thing but for VLAN 4 with an IP address.
So the sub interface concept being used on a router and a stick topology is WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So the sub interface concept being used on a router and a stick topology is
what a lot
of networks thrived on for a long time when they had a pure layer two switching
network
that couldn't handle the ability to deploy multiple SVIs.
And at that point, we required a layer three device.
Now eventually, the solution led us to multi-layer switches or layer three
switches depending
on how we want to call them.
But with multi-layer switches, we were able to say, you know what?
You don't need a router on a stick anymore because now I can do what we've been WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
You don't need a router on a stick anymore because now I can do what we've been
drawing,
which is to have sub, not sub interfaces, but SVIs for VLANs 4 and 5 and allow
those
to connect in to our different layer two VLANs as we've sketched out a few
times.
This drastically improved the traffic flow in a network because if we think
about it,
if I have a host hanging off of this layer two switch that's in VLAN 4 and a
host over
here that's in VLAN 5, how do these devices communicate with one another? WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
here that's in VLAN 5, how do these devices communicate with one another?
Well, as we think about the topology here, this host is connected to VLAN 4
which is
carried up on this trunk over here on this side.
And then we'll just make VLAN 5 here green.
And so same thing here except it's in a different connection, a different layer
two domain that
goes off to the right.
And then this one comes up here.
So as we color these in a little bit, we start to see how exactly this traffic WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So as we color these in a little bit, we start to see how exactly this traffic
has to flow.
It has to flow up the trunk to the sub interface, get routed over to the other
sub interface
and then get sent back down the trunk out to the other device.
So this is known as tromboning of traffic, named after the instrument that sort
of has
the big long loop like that at the end.
Well, that's exactly where we're doing.
We have a big long loop where I go up to a device and then right back down as WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
We have a big long loop where I go up to a device and then right back down as
we clear
up this drawing a little bit more.
And we see that's very inefficient.
I'd really rather just send that traffic right from one host to the other even
if it
involves routing and that's exactly what multi-layer switches do.
Now I can get from this host on the left in VLAN 4 to the host in VLAN 5.
Yes, I do have to go up and get routed over, but it stays on the same device.
I don't have to worry about sending it off to some externally connected layer
three router WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
three router
on a stick.
So this is how router on a stick architectures worked, how sub interfaces
worked.
And we still see sub interfaces in use today every now and again, but generally
speaking
in real world networks, we don't have to worry about sub interfaces and trunks
out to routers
because we don't have to worry about router in a stick anymore thanks to our
multi-layer
switches.