So we still have a problem that we have to address.
In a traditional network, we typically have a pair of distribution switches and
these switches
are going to run SVIs.
These SVIs serve as our default gateways for all of the hosts that reside on a
given
subnet.
And so logically speaking, we have hosts down here and they connect up to one
of these
SVIs as the default gateway.
We run a first hop redundancy protocol between these to make sure we have WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
We run a first hop redundancy protocol between these to make sure we have
redundancy in
case one of the distributions, which is goes down.
And then every other subnet has something similar where we have a SVI on each
one of
these switches and they connect into a layer 2 domain.
And the hosts off of these are serviced by those SVIs.
Physically, we are talking about access switches that come down here.
Maybe we have a port channel somehow.
We were running VSS or a stackwise virtual, but either way, we have a layer 2
connection
down to an access switch and the hosts connect here on a particular VLAN, like WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
down to an access switch and the hosts connect here on a particular VLAN, like
VLAN 10, and
then the trunk carries them up to the distribution layer.
So this is the physical representation of what's happening.
This is the logical representation, but either way, we have a redundant way for
our hosts
to connect to their default gateways.
But now we have a very different story.
We have a fabric where we have multiple edge nodes, potentially lots of edge WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
We have a fabric where we have multiple edge nodes, potentially lots of edge
nodes here.
Let's just draw three for now.
And we've said because of VXLAN, we can say that each one of these is
connecting to the
same VLAN.
Let's say VLAN 10 in this case.
And so we can kind of draw dotted lines in here, which indicates that this is a
stretched
subnet.
And we've got VXLAN tunnels happening here on the back end to extend this out.
And so I've got a host here on VLAN 10.
I've got a host here on VLAN 10.
A host over here.
And these are all on the 10.10.0.0/24 subnet. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And these are all on the 10.10.0.0/24 subnet.
So here's the question.
Where do the SVIs and the default gateways live?
Remember, this is a layer 3 fabric up here.
We do have VXLAN tunnels, so we have layer 2 access among all of the switches.
But when we're talking about the default gateway, where are we going to put it?
I might put it here on this switch, and that's great for this host because it's
physically
attached to it or through it either way. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
attached to it or through it either way.
It has a direct access to its default gateway.
But what about these hosts over here?
So let's say this is the .1 IP address, or maybe .254 if we want to.
And so we might say, OK, well, all of these are going to have to go through the
VXLAN tunnel
just to get to their default gateways.
Well, what if we want to communicate with another VLAN here?
Let's say that these are all connecting in.
Actually, let's just draw it here, hanging off of this one switch, VLAN 20, and WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
Actually, let's just draw it here, hanging off of this one switch, VLAN 20, and
a host
that is connecting in off the same switch.
Two different VLANs require going through the default gateway.
So in order for these two devices to communicate, the one on the left and VLAN
10, the one
on the right on VLAN 20, they're going to have to go over here to the default
gateway and
get switched over.
And that even assumes that the VLAN 20 SVI exists over on that switch and can
get routed
between the two. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
between the two.
So we can tell that we've got a lot of problems if the SVI resides on a single
switch.
And again, specifically the default gateway.
So what are we going to do here?
Well, we're going to do away with this old model.
We're going to get away from thinking about a single SVI on each switch and
then we're
running default gateways through them and FHRP's for redundancy.
We're going to leave all of that behind and embrace a completely new
methodology known WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
methodology known
as the anycast gateway.
Anycast is a very interesting traffic type that we don't talk about quite as
often as all
the other casts, meaning we have Unicast and broadcast that we talk a whole lot
about.
And then we study multicast as well.
And so we're either talking to one device or all devices or some devices.
And you'd think that maybe that covers everything except we also have any cast. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And you'd think that maybe that covers everything except we also have any cast.
And anycast says not one specific device, not all devices or some devices, but
any eligible
device.
In other words, I don't care as long as somebody answers.
So typically from a networking perspective, this is going to find the nearest
resource that
has a particular IP address, which has an implication.
If I try to communicate with an anycast IP address like 10.1.1.1, well, if I'm WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
If I try to communicate with an anycast IP address like 10.1.1.1, well, if I'm
saying that
there's a nearest one, that means there are other ones out there as well.
There must be two or three of these.
And so that means that we actually do have and support duplicate IP addresses
out on the
network.
So what does this look like from a default gateway perspective?
Well, what it looks like is now we throw an SVI, not just onto one of these
switches, but
onto every one of these switches for VLAN 10. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
onto every one of these switches for VLAN 10.
And then we could do the same for all the other SVIs.
Furthermore, all of these are going to share the same IP address 10.10.0.1.
We'll say is the default gateway.
So all of these VLAN 10 hosts point to this IP address as their default gateway
.
We say, well, wait a second here.
We have three different IP addresses that are all duplicates of one another.
Which one are we going to talk to when we try to go to our default gateway? WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
Which one are we going to talk to when we try to go to our default gateway?
Well, the answer is right here.
It is the nearest of our gateways or of our IP addresses.
What's the nearest one?
Well, the switch over on the left, this host, its nearest one is its directly
attached
switch.
And this one over here is also its directly attached.
And this one on the right also its directly attached switch.
So we don't have to go very far.
I just ARP, send an ARP for my default gateway and I'm going to get an ARP WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
I just ARP, send an ARP for my default gateway and I'm going to get an ARP
response back
from the appropriate switch.
The namely, the first one that sees my ARP request.
Which means that all three of these hosts are using different default gateways
even
though they all have the same IP address.
So is this a problem?
And the answer is no.
In fact, it's a wonderful solution.
It doesn't cause additional problems. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
It doesn't cause additional problems.
It allows us to find a default gateway on the local edge node.
Whatever my local edge node is, whether I connect to it directly, whether I
connect through
it, maybe I've got a series of switches that connect to this edge node.
And so I'm a host hanging out here.
Well, the first IP address that I find that fits my default gateway IP is going
to be
this switch here that's in the path, or before I hit that fabric and get onto WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
this switch here that's in the path, or before I hit that fabric and get onto
the SDA world.
Furthermore, I'm going to be able to do local switching now, local SVI
switching.
So if I need to communicate with hosts, let's draw it over here.
So I've got two different edge fabric switches.
These are both edge switches.
They both connect into the fabric.
And I have SVI's for VLANs 10 and 20 on both of these. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
And I have SVI's for VLANs 10 and 20 on both of these.
And they're both running any cast.
So the IP address for the SVI for 10 is the same on both of these switches,
same thing for
the 20s.
So if I now have a host on VLAN 10 and we need to communicate to the host on V
LAN 20, well,
I can stay on my local switch.
I hit my default gateway.
It gets layer 3 switched, you know, change colors, layer 3 switched over to the WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
It gets layer 3 switched, you know, change colors, layer 3 switched over to the
SVI for
VLAN 20 and then sent down.
Well, what about this switch over here?
What if I have the exact same thing at the exact same time except now it's two
different hosts
that are connected to the same two VLANs?
Well, same thing here.
I send traffic up.
It hits the default gateway, gets switched over to the local SVI and then
forwarded down
VLAN 20 at layer 2.
Both of these operations are happening at the same time and they stay within
the fabric
edge switch. WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
edge switch.
You have to go through the VXLAN tunnels and trombone traffic back and forth.
That is not required.
We can stay on our local switches and furthermore, if I do need to leave the
subnet and get
routed out somewhere.
Well, at this point, I'm going to hit my local SVI and then we'll enter into
the SDA world
where I'm referencing list information and I'm getting encapsulated into VXLAN.
So either way, my gateways are always going to be local.
I'm going to have the same IP addresses and the same SVI's configured on all of WEBVTT X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:900000
I'm going to have the same IP addresses and the same SVI's configured on all of
my edge
switches, wherever those VLANs exist.
And importantly, we're going to deploy no FHRP's.
Those aren't going to be needed because our redundancy is built using any cast
rather
than having any other kind of sophisticated method for trying to share a
default gateway.