So, let's get back to the campus.
We know that in a campus environment we're going to have a distribution layer
and the
distribution layer or distribution block is going to have a pair of switches
and then
we're going to bring those connections into the access layer in some capacity.
So the access layer as we talked about earlier might be a single chassis, a
single stack
of switches.
It could be a loop of switches that's hanging off of here.
We could even end up potentially looping our network through and up to the WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
We could even end up potentially looping our network through and up to the
distribution
layer.
But either way, we know that we're going to come down to the access closet and
one of
these connections is going to get blocked by a spanning tree protocol.
Now the distribution switch is going to have to maintain spanning tree root
status up here
in order to make sure that the access layer is actually blocking our links
because the
last thing we'd want is for the root to show up down here in one of our many
access layer
closets. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
closets.
And now the distribution layer has to block links and it gets really awkward at
that point
when we're trying to troubleshoot network issues if the root isn't where we
think it
is.
So we need to make sure that the spanning tree root exists up here at the
distribution
layer.
We also know that we're going to deploy first-top redundancy protocols.
And so the first-top redundancy protocols are going to result in us having
active and
standby connections.
So these are some considerations that we have to make with our distribution
layer configuration. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
layer configuration.
But the question is how do we go about deploying these configurations to create
an efficient
network?
For example, the classic scenario is that I make this left switch the root for
all VLANs.
If it's the root for all the VLANs, then we're going to block this link and use
this link
for all of our connections.
And so our topology is going to look like this.
Meanwhile, this gig or 10-gig connection here is completely idle. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
Meanwhile, this gig or 10-gig connection here is completely idle.
And we want to prevent this if at all possible.
We've spent a lot of money for that 10-gig connection and those 10-gig optics
and the fiber
that's required.
And so how can we go about making it so that we at least use that 10-gig link
as often
as we can?
Now this is going to lead us to the conversation of Cisco's per VLAN spanning
tree and ultimately
our ability to deploy policies per VLAN.
This is going to apply on a basis of spanning tree, whether we're running Cisco WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
This is going to apply on a basis of spanning tree, whether we're running Cisco
's PVST+ or
rapid PVST+ and even the industry standard multiple spanning tree.
We can deploy per VLAN topologies with STP and therefore I could create a root
switch over
here for another VLAN.
So let's say the left root is for VLAN 10 and the right root is for VLAN 20.
Well now VLAN 20 will block this link on the left and will allow traffic to WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
Well now VLAN 20 will block this link on the left and will allow traffic to
flow like
this.
So now we have two different topologies.
One looks like this.
The other looks like, well, this.
There we go.
And as a result, we are using both of those links.
We're just making it so VLAN 10 can't use the link on the right and VLAN 20 can
't use
the link on the left that prevents loops and it prevents the concept of one of WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
the link on the left that prevents loops and it prevents the concept of one of
the links
being idle, which is our primary goal.
But the per VLAN concept also applies to our first top redundancy protocols.
So whether we're talking about HSRP or VRRP, especially since GLBP will low
balance, we're
going to be setting our active and our standby according to the VLAN.
I can make it so that on VLAN 10, let's just go back to our example on the left
that for
VLAN 10 we're also the HSRP active. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
VLAN 10 we're also the HSRP active.
But for VLAN 20, the device on the right is HSRP active.
This way, we can stay very efficient with our design.
Think back to what we just said about VPCs with efficient traffic forwarding.
Imagine for a moment that it's not the right switch that's active for VLAN 20,
but the left
switch is also active for VLAN 20.
So what happens with the VLAN 20 when a packet arrives here that's destined for WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
So what happens with the VLAN 20 when a packet arrives here that's destined for
the gateway?
Well, it's going to be forwarded up here to the root because this link is
blocked.
And then it's going to have to get forwarded across the link to gain access to
the SVI over
there because that's where the HSRP active is.
We want to avoid having that inefficient communication path.
And so instead, we are going to apply the active configuration to wherever the
root is WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
root is
on a per VLAN basis.
So that is the primary takeaway from this conversation.
When we're looking at aligning the configurations, maybe of the spanning tree
root and the FHRP's,
we want to make sure that on a per VLAN basis that the active and the root are
the same.
And again, that could be the BRRP backup as well. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And again, that could be the BRRP backup as well.
So this is our FHRP configuration.
This is our STP configuration.
And usually we want to split this up evenly across our VLANs.
So maybe I have VLANs 10, 20, 30, and 40 in my environment.
Well, I'm going to take VLANs 10 and 30.
And I'm going to make, let's say, this switch on the left, both the FHRP active WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And I'm going to make, let's say, this switch on the left, both the FHRP active
and the
STP root.
But for VLANs 20 and 40, I'm going to go to the switch on the right.
And I've just divided my network up by VLAN.
Now a lot of networks choose to split us this up by evens and odds or something
along those
lines.
Now all four of those are even numbers.
But if I look at the tens digit here, one in three, these would be my odd VLANs
.
And these would be my even VLANs over on the right. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And these would be my even VLANs over on the right.
If I actually have VLANs 1 through 9, something like that, then I could
actually choose true,
odd, and even values.
But ultimately, in a lot of situations, we're looking more to keep the number
the same.
We want to balance by VLAN count.
And this is just from an organizational perspective, but it does make an
underlying
assumption.
And that is that different VLANs have roughly the same traffic flow. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And that is that different VLANs have roughly the same traffic flow.
This is very common in most scenarios where we have endpoints that are just
going out to
the internet or to data center servers.
Because HTTP traffic just doesn't take up that much, we can look at our 10-gag
links and
realize that most of them are highly underutilized in a campus environment.
But if I do happen to have a heavy load on VLANs 10 and 30, these are very,
very heavy loads
for some reason, and I place them onto the same distribution switch, well, at WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
for some reason, and I place them onto the same distribution switch, well, at
that point,
I'm heavily loading up one of my links.
And so that would be something to consider as well.
But usually in campus environments in the real world, we don't need to think
too hard about
that.
Now, one last caveat to consider, and yes, this is me bringing up the data
center again,
but we just looked at how in the data center we're going to run VPC
environments.
When we have VPC peers, we just got done saying that in a VPC environment, let WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
When we have VPC peers, we just got done saying that in a VPC environment, let
's draw that
out real quick, there we go, that we are going to load balance traffic both
ways.
And we have active active up here on our FHRP.
This completely changes the game. We have active active at the FHRP level, and
we have no blocking
from an STP perspective.
So at this point, we have to ask the question, does it matter which switch is
which from a root WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
which from a root
and FHRP active alignment?
And the answer is it doesn't really matter anymore.
As a result, Cisco changes the recommendations.
Our VPC peers are going to have a primary and secondary role.
That goes beyond the scope of the course, but in this case, we can identify
that this is
the VPC primary, and Cisco recommends that if it's the VPC primary, that we go
ahead
and make it the STP root, and the FHRP active for all VLANs. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
and make it the STP root, and the FHRP active for all VLANs.
We're not fascinating how that recommendation changes.
And the purpose here is just mostly for us to be able to recognize that it's an
arbitrary
configuration, it doesn't really matter how we do it.
If we choose to split it up still, we're not causing any harm, but the reason
why Cisco
does this is so that we can look at our environment and we can know exactly how WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
does this is so that we can look at our environment and we can know exactly how
it's running
from a control plane perspective.
And because the control plane is very lightly loading, we don't need to worry
about overloading
one of the switches, and this way we know exactly how the network will behave
if we lose
one of these devices.
So that's why the recommendations from Cisco change in the data center when we
're running
VPC peers at the aggregation layer.
But outside of that, here at our campus environment, with a true distribution WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
But outside of that, here at our campus environment, with a true distribution
layer, we're
going to want to make sure that we are splitting up the STP, well, I should say
like this, we
are aligning our STP roots with our FHRP active right here, but we're splitting
that up on
a perv LAN basis.
That will be the goal for us when we're performing our campus designs.