So we can solve all of our layer two problems by deploying layer three to the
access layer.
This is a pretty interesting way to deploy our access layer devices.
So same thing, we start with a pair of distribution switches.
But now we can connect them at layer three to one another because what we're
going to do
is come down to an access closet here.
And let's say we have a stack of switches or chassis switch.
I'm going to extend layer three down to that access closet. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
I'm going to extend layer three down to that access closet.
This means that I am now performing routing.
I'm going to have to have a routing protocol running here.
Maybe it's OSPF or EIGRP.
And by creating this as a layer three link, this means that there is no longer
any spanning
tree protocol needed.
Furthermore, because we still need VLANs and default gateways and everything,
now our
closet switch here is going to be responsible for defining the VLANs and the SV WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
closet switch here is going to be responsible for defining the VLANs and the SV
Is.
So that as our users connect in, they still have access to their default gate
ways.
The default gateways are just going to be here in the access closet at which
point
we get routed upstream to that layer three distribution layer still.
So more if I deploy another closet, I'm most of our networks have many closets.
We're going to have another switch here.
We can bring layer three down.
And again, since everything is layer three, we don't have any blocked links in WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And again, since everything is layer three, we don't have any blocked links in
this environment.
We again, though, have to create local VLANs.
So I might have VLANs 10 through 15 on the left.
But then guess what?
I can have VLANs 10 through 15 on the right as well because the VLAN IDs are
locally significant.
So that's kind of interesting.
The subnet still have to be unique, but the VLAN IDs themselves can be dupl
icated, which
is one less thing that I have to manage.
Now do note that from a hierarchical model perspective, this does change things WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
Now do note that from a hierarchical model perspective, this does change things
a little
bit because now our SVIs, which normally reside on the distribution layer,
truly reside
at the access layer.
So just keep that in mind, it doesn't break the model, but it does slightly
tweak the
way that the model is deployed.
And within the context of a layer three access layer, that's perfectly
acceptable.
So let's talk about the pros and cons of this design.
The pros, absolutely the fact that we have no STP from spanning tree protocol,
that is
a huge win. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
a huge win.
Anytime we can eliminate spanning tree protocol or specifically the need for it
, that
point it's a win.
We shouldn't just get rid of spanning tree protocol on layer two network
because that
wouldn't be a win.
That would create lots of loops.
Here three though, we're good to go.
We also can safely rely on the fact that we can scale this up to any number of
access closets.
I don't have to worry about the size of my broadcast domains anymore because
those are
completely restricted within each one of these closets. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
completely restricted within each one of these closets.
This is a broadcast domain here.
Every single closet is its own little layer to domain.
And so we don't have to worry about broadcast being sent out unnecessarily to
all the other
access closets in the network.
So should we all go with layer three access layer deployments?
Well maybe, but we should also stop and consider that there are some downsides
to this design.
First of all, layer three switches are more expensive. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
First of all, layer three switches are more expensive.
And unfortunately, this is something we have to consider when it comes to our
network
designs.
Most of us don't have infinite budgets.
And so if I have to spend, just making up a number here, but if I have to spend
twice as
much for every single switch I deploy into my access layer and my access layer,
I have
lots of switches.
Oh, that's going to double the size of my access layer budget.
Or if it increases only by 30 or 40%, it still increases the overall budget by WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
Or if it increases only by 30 or 40%, it still increases the overall budget by
that amount.
And so we have to be concerned about the amount of money that is going to cost
and whether
we can cost justify it.
Furthermore, we do lose that flexibility that we talked about.
On day one, I might not have anything that requires layer two connectivity
between my
closets.
So hey, if we're good today, that's really great.
But what happens if in the future I do end up needing to have an application or WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
But what happens if in the future I do end up needing to have an application or
maybe
a set of devices that require layer two connectivity with one another.
So this is potentially a concern.
And the danger here is that maybe I don't need it today, but I might need it
tomorrow.
I have been part of a network project, a deployment issue where on day one, a
network was deployed
at layer three and that was perfectly fine.
But later on, when I came into the mix, we were hired to help resolve this WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
But later on, when I came into the mix, we were hired to help resolve this
problem where
now they needed devices on both sides of these layer three links that required
layer two connectivity.
So we had to go in and say, well, okay, this was a good design.
When it was made, it was layer three at the time.
But now we're going to have to convert this to layer two, which was a headache
that wouldn't
have been needed if we just deployed layer two to begin with.
And so that's something to consider as well.
It is absolutely more complex as well from a routing protocol perspective. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
It is absolutely more complex as well from a routing protocol perspective.
Remember my distribution layer was running maybe a stub version of a routing
protocol,
but it can't do that anymore.
I'm going to have to deploy OSPF or EIGRP everywhere in this environment.
I might have to deal with OSPF areas, make it so that each one of my closets is
a stub
area.
It's going to be more complexity here, not necessarily complexity we can't
handle, but it is
going to be more complex and therefore it's going to be trickier to support WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
going to be more complex and therefore it's going to be trickier to support
issues that
arise.
And lastly, we do need to consider once again that wireless roaming concept.
Even though we have a lot of our wireless deployed today such that we're using
controllers,
if I don't have a wireless LAN controller type of architecture, whether it says
go or
otherwise, this is going to be a major downside because if I have an access
point here and an
access point over here and we know that we're trying to create one large WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
access point over here and we know that we're trying to create one large
wireless network,
well now when I roam from one closet to the other, this is going to be a layer
three
roam.
Now again, if we have controllers that play, we can absorb that and that won't
be a problem,
but if it is a layer three roam, then that's going to cause us a lot of issues.
So at some point we do have to stop and ask, okay, well which one is best then?
And as you might imagine, the answer is going to be, well, it depends. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And as you might imagine, the answer is going to be, well, it depends.
We have a list of pros and cons for both layer two and layer three and we're
going to have
to get together with our team, with our potentially with our managers and
discuss the pros and
cons of each solution to make sure that we have the correct one deployed.
I will say that the layer two deployment is usually considered to be the safest
because
it's nice and cheap.
We all know how layer two works to the axis layer.
We've been doing it for a long time and it's going to again maximize that WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
We've been doing it for a long time and it's going to again maximize that
flexibility
so we don't have to worry about things changing in the future and having to re
vert back to
layer two if we were to go with layer three.
But the fact that layer three gets rid of spanning tree, it's absolutely going
to create
the cleanest environment and therefore it's going to improve supportability
from the
perspective that we should have fewer issues or at least we might expect that.
And so these are broadly speaking why we might want to go with one over the WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And so these are broadly speaking why we might want to go with one over the
other but
at the same time again we have to compare the pros and cons of each solution
and make the
decision that is best for our organization.