One of the goals of diving deeper into the world of routing and switching is to
become
one with layer two and layer three.
It's a little phrase I say because it just sounds funny to say one, two, three
like that,
but we have to become one with layer two and layer three and understand how
they come
together.
So, our focus right now is on layer two, but VLANs are absolutely intertwined
with layer
three technologies.
Our VLAN interface is default, gateways, subnets, all of this really could fall
into the layer WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
into the layer
three category, but it's built on layer two and VLANs.
And so when we start to draw topologies, usually we're talking about drawing
physical
switches and their connections.
Maybe this is an ether channel between these switches and they connect down to
this other
switch and we'll draw plenty of these types of diagrams when we're looking at
spanning
tree and the like, but this is a physical drawing that shows me which links are
connected WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
connected
to which devices and which devices are connected to other devices and how they
're connected.
But this doesn't tell me anything about layer two, it doesn't tell me anything
about layer
three at least natively.
And so this is where I like to try to inject a little bit more information as
far as how
to read a diagram here.
For example, are these links layer two or are these links layer three?
And we can kind of start to get around some of that if we put more details into WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And we can kind of start to get around some of that if we put more details into
our drawings.
For example, whenever I draw topologies, I like to use a dot to represent a
layer three
interface.
This is something that I've done pretty much ever since I got my CCNP and I
started doing
additional studying to go deeper was recognizing that some of these interfaces
might be layer
two, but some of them might be layer three.
And so I'm going to start using these dots as my indicator that we have a layer
three WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
three
interface.
So if these are layer three connections, then I could put a dot where these
lines intersect
and this tells me that oh, these devices are all connected at layer three,
which is very
useful and very important when you're looking at a physical diagram and trying
to tell what's
going on.
But the reality is that we might actually see layer three interfaces that aren
't physical
either.
And so that creates an interesting situation.
What if I have a couple of switches like this and it's more of a layer two top WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
What if I have a couple of switches like this and it's more of a layer two top
ology.
So once again, I've got an ether channel between these two and then I've got a
switch down
here and I connect all these together.
And so far, all of this is layer two and we know that because there's no dots.
Now just to be abundantly clear, when I'm talking about these dots and layer
three interfaces
versus layer two, this is very much a Jeff thing.
This is not an industry standard concept.
So when you're looking at other people's diagrams, don't assume it's layer two WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So when you're looking at other people's diagrams, don't assume it's layer two
because
there's no dots.
I'm teaching this from the perspective that as you dive into topologies, if you
're drawing
your own topologies, if you want to really be able to know what exactly you're
drawing
and look at it later and understand the topology or maybe you're doing a lab
and you just want
to draw it up real quick, this is a great way that you can do this.
And so whether you choose to do it or not, it's perfectly up to you.
But also understand that as I'm drawing things, that's how I'm representing
information. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
information.
So as we're drawing this out, we might say, okay, this is all a layer two
environment
because there's no dots here.
But where are the dots?
Well, let's say that this is running a bunch of VLANs.
Maybe I'm just running VLANs 10 and 20.
And these are all trunk links, so I'm carrying the VLANs on all of these
connections.
Well VLANs 10 and 20 are going to need SVIs.
They're going to need those switched virtual interfaces, which are VLAN
interfaces.
And so I can see these represented by dots here on the switches. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And so I can see these represented by dots here on the switches.
And notice that they don't belong to any physical port, they're just sort of
floating
inside the switches.
And so now I can see that I've got a VLAN 10, SVI, VLAN 20, SVI 10 and 20.
And this shows me that there are layer three interfaces in the network.
Furthermore, maybe I've got a router upstream and this router comes in from a WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
Furthermore, maybe I've got a router upstream and this router comes in from a
layer three
perspective to these two switches and maybe we're running a routing protocol up
to that
router.
And now we can start to see the layer three world coming together and also
understand that
while I might need to do layer two switching to go from one layer to interface
to another
layer to interface or something along those lines, if I'm routing out to maybe
through
a default gateway, now I see that this switch is able to route among these
different layer
three interfaces. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
three interfaces.
But also notice that an SVI is really no different from a physical interface,
they're all treated
the same from a layer three perspective.
They just happen to be logical on the SVI side and physical on the port side.
So now this is going to lead us to a different type of diagram because we've
been focused
on physical diagrams, but now we can start to draw those logical diagrams.
We're starting to draw the logical diagrams or logical topologies.
Now our focus is going to be on VLANs and subnets, but especially on VLANs WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
Now our focus is going to be on VLANs and subnets, but especially on VLANs
because they
represent those layer two broadcast domains, which is how we really need to be
thinking
about VLANs.
So what we can do now is say I want to draw this topology out, but do it
logically.
What do we have?
Well, we have VLAN 10 interfaces, we have VLAN 20 interfaces, we've got a
switch hanging
off from a layer two perspective. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
off from a layer two perspective.
And so now what we can do is represent each one of those VLANs as one of these
network
segments.
So here we have VLAN 10 and here we have VLAN 20.
So VLAN 10 we see has, you know, through this other switch, maybe let's just
add a couple
of hosts on here.
So now we've got a couple of hosts down here, one's on VLAN 10 and one's on V
LAN 20.
Now I can draw those hosts, which technically do have layer three interfaces,
so we could WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
so we could
put a dot there if we wanted to.
There's an IP address on that host.
And so this host over here, the host in VLAN 20.
Now how do they communicate to the rest of the world?
Well, notice what we have on these switches.
We have SVIs that are in VLAN 10 and VLAN 20.
But also notice, as we count our dots here, that we're missing a few dots.
For example, I've got two SVIs in VLAN 10. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
For example, I've got two SVIs in VLAN 10.
So this would be, we'll just call it switch one and switch two now.
We didn't give them names, but we've got two different switches in that play
here.
So switch one and switch two.
And so these both exist.
I did that wrong.
This should be a 10 here.
There we go.
So 10 and 10, 20 and 20.
So now we see that all three of these layer three interfaces, whether we're
talking about
VLAN 10 or VLAN 20, they should be able to ping one another because they have WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
VLAN 10 or VLAN 20, they should be able to ping one another because they have
connectivity
through this broadcast domain, through VLAN 10 on the left or VLAN 20 on the
right.
Now lastly, we can use sort of a blended approach, if we want to call it that.
Again, this is not a really a standard term per se, but we could blend the
world of layer,
or a physical and logical together to see layer two networks.
So in our situation here, maybe we draw switch one, the switch on the left. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So in our situation here, maybe we draw switch one, the switch on the left.
And now instead of just drawing dots and downstream physical connections, we
can still
draw those dots to say, here's my VLAN interface for VLAN 10.
But this is actually connected to a broadcast segment that switch two is also a
part of.
And with that other host as well.
So this is again VLAN 10.
And then maybe we'll use another color here just to distinguish, but I can also
start to
overlap a little bit with VLAN 20. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
overlap a little bit with VLAN 20.
And we'll see that we've got another VLAN.
So VLAN 20 is at play here, and I connect these dots down.
And then this connects to the host that's in VLAN 20.
And so now we can start to really see the routing path that we need to take if
these
two hosts are to communicate.
Because this host has no direct path to hop from the green VLAN 10 line to the
orange
or red VLAN 20 line, I can't do that. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
or red VLAN 20 line, I can't do that.
And what I need to do is I need to go through my default gateway here, this SVI
get routed
over to VLAN 20.
And then that gets placed down into the layer two domain that gets passed to
the host.
Now does this show the physical path?
Does it show this switch down here?
No, it doesn't.
All three of these types of diagrams are very useful, but they all contain some
pieces
of information without containing other pieces. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
of information without containing other pieces.
To see the full picture, we at least need a physical and logical, and sometimes
we can
blend those together as I've drawn out down here.
And furthermore, once again, if we have additional routing paths, then we can
see that as we
connect up to whatever layer three domain sort of exists, northbound or beyond
these
two switches.
But we could also see that I can route to my SVI and then route out this way to
the rest
of the network if I'm trying to say get out to the internet or something along WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
of the network if I'm trying to say get out to the internet or something along
those lines.
But let's start thinking really truly about how layer two domains come together
, how we
can send layer two traffic, and how we can represent the layer three world with
some of
these dots to tell us where those interfaces really are.