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So what exactly are these switched virtual interfaces anyways?

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Well, if you're like me, you probably refer to these as simply VLAN interfaces.

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And if you're like me, you're probably also wondering why is there such a

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technical term for it like SVIs.

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And I don't have a great answer for that, but I can tell you that Cisco loves

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to call them SVIs.

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And yet if we want to configure them, we're simply going to get onto an iOSX-E

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device.

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And we're going to type in interface VLAN and then some kind of ID.
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And we're going to type in interface VLAN and then some kind of ID.

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So VLAN 10, VLAN 20.

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And what this is, is it's a layer three interface that is directly bound to a V

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LAN.

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So there are certain situations where I can create layer three interfaces like

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loop X that are part of the layer three domain.

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But in our case, this layer three interface is going to be part of a layer two

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domain.

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It's going to be part of a specific VLAN.
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It's going to be part of a specific VLAN.

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What exactly does that mean?

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Well, I like to refer to this as a point of presence or a pop.

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We'll call it that.

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But it's a point of presence on a VLAN for the switching question.

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So what are the uses for this?

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Well, very commonly, we're going to rely on these as our default gateways.

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So VLAN 10 is going to have a bunch of hosts on it.

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I'm going to create a SBI for VLAN 10, give it an IP address, and all of my
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I'm going to create a SBI for VLAN 10, give it an IP address, and all of my

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hosts will use this SBI as their default gateway.

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It's their way off of the subnet, which we saw at the end of the last video,

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when we were trying to communicate with a host on a different VLAN.

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But we can use SBIs for other reasons beyond default gateways.

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For example, it can be used as a point of management on the switch.

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If I want to log into the switch, I need to have layer three IP address to

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connect to.
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connect to.

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And even if it's a layer two switch, I'm going to have to log into that switch

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somehow.

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And using an SBI as a management interface is pretty common.

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And lastly, and this is often overlooked, I can tell you I've used this a lot

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of situations.

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It is a wonderful way to troubleshoot when we're having routing and switching

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issues.

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So maybe we're not able to get a ping across the network.

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Well, I've got switches in the path that I could install or create an SBI on
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Well, I've got switches in the path that I could install or create an SBI on

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and use that to run troubleshooting.

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And the more we become one with layer two and layer three, well, the more we're

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going to be able to look at a situation and say,

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"Oh, yeah, I can troubleshoot this just by creating a new SVI."

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So let's look at that blended topology once again.

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If I'm looking at a switch and I know that I've got this VLAN, let's just again

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call it VLAN 10,

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and I want to connect VLAN 10 to this layer two domain, well, I might have a
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and I want to connect VLAN 10 to this layer two domain, well, I might have a

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bunch of switches all communicating with one another on VLAN 10 in some

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capacity,

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and then I've got a bunch of hosts down here as well.

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Now notice this blended diagram doesn't tell us how these switches are

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physically connected.

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We just know that they have presence on VLAN 10's network.

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But here's the question, can these hosts communicate with one another without
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But here's the question, can these hosts communicate with one another without

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there being an SVI on these switches?

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And the answer is yes.

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An SVI is not part of the traffic flow, it's very important.

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In this situation, I don't even have SVI's configured, and these hosts are able

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to communicate, we can see that using the logical side of this diagram.

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But what if I do create SVI's?

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What if I have an SVI here on the left, an SVI here on the right, and again,

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when I create it, as we've said, it gets bound to that VLAN.

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So this little connecting line right here, that happens automatically, the
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So this little connecting line right here, that happens automatically, the

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switch says, okay, as part of this layer two domain, I'm going to attach an SVI

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.

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And so at this point, do I need to go through the SVI, and the diagram didn't

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really change that much.

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I mean, we can see that I can communicate directly with a host on the same VLAN

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.

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But can I communicate with a host on a different subnet?

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What if I've got this subnet over here, which is kind of what we saw, I'll go

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through there, there we go.

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What we saw at the end of the last video, and the answer is no, I can't just
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What we saw at the end of the last video, and the answer is no, I can't just

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hop from one line to the other like this.

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I'm going to have to go up and get routed.

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The question is, what am I getting routed to?

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In this situation, I have nowhere to go.

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From a layer three perspective, I can't jump from this SVI onto this VLAN.

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I don't have that path available.

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And so this is why this type of diagram can be so useful for troubleshooting,

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because this host might not be able to ping this host, and when we diagram this

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all out, we realize that there's no routing path.
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all out, we realize that there's no routing path.

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I don't have a way to route from one VLAN to the other.

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So what's the solution? Well, in this case, I need to create an SVI for VLAN 20

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, and this will allow the switch to route to another layer three interface, and

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then get handed off from a layer two perspective, thus enabling the ping

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between those hosts.

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Another way of looking at this when we zoom in just on a single switch is to

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recognize that I might have, now we can draw this VLAN, let's say VLAN 10,
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recognize that I might have, now we can draw this VLAN, let's say VLAN 10,

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inside of the switch, since we're just within a single switch.

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So now I've got multiple hosts hanging off of VLAN 10 here, and they can all

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communicate.

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And then I say, OK, but I want this switch to have an SVI for VLAN 10, and so

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this allows me to communicate and ping these hosts from this SVI.

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But again, notice the traffic pattern here.

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If I want this host to communicate to this host, I just need to hit the layer
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If I want this host to communicate to this host, I just need to hit the layer

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two network and come immediately down.

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I do not pass through the SVI, and the SVI is not part of that traffic flow.

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Conversely, now, let's say that we've got a bunch of switches, and so this is

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just one of the switches in the path, and I've got this VLAN that I've got some

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hosts that connect down off of VLAN 20, let's say this is VLAN 20 in this case.

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And the question is, OK, how do I manage this switch?
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And the question is, OK, how do I manage this switch?

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This switch doesn't have a layer three interface, I can't ping it, and I can't

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log into it.

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Now, the question that we might have to ask is, is this working?

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Is this an online switch?

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And the answer is yes.

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It's working just fine, it's working at layer two.

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These hosts can communicate with one another, even though I can't ping the

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switch.

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How can I ping the switch?

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Well, as long as we're talking about being available via VLAN 20, I can again
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Well, as long as we're talking about being available via VLAN 20, I can again

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create that SVI.

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Or what if this switch has managed another way?

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Maybe it's got an out of band management port on the back, and it goes off to a

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management network.

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And so now I'm trying to ping, I'm trying to make things work, I can't figure

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out, these hosts aren't working very well, or something along the lines of

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network connectivity issues.

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How could I troubleshoot?

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Well, once again, I could create an SVI in VLAN 20.

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This is not going to be used as a default gateway, it's not going to be used as
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This is not going to be used as a default gateway, it's not going to be used as

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a way to manage the switch.

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But what I can do is, as mentioned, I can start pinging these other hosts.

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Or I could ping from this switch out to another switch via VLAN 20.

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It starts to give me that point of presence, again, the point of presence, on a

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VLAN.

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And so that's a great way for us to start to think about how these SVI's can be

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used beyond just the most common ways that we deploy them.
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used beyond just the most common ways that we deploy them.

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Now, lastly, we've mentioned a couple of times that we like there to be a one-

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to-one relationship between our VLAN and our subnets.

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Well, let's say like this, our subnets and our VLANs.

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But it doesn't have to be this way. We could actually have a many-to-one

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relationship between the two.

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And the way this works is now I can take all of this that I've applied and

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apply now multiple subnets to my SVI.
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apply now multiple subnets to my SVI.

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How do I do that? I do it using the concept of secondary IP addresses.

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This is a concept that was used more early on in networks, especially as we had

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to evolve from network supporting multiple subnets to trying to split them all

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out.

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And secondary IP addresses can be very useful from the perspective of migrating

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or merging networks or maybe demerging networks or whatever the situation is,
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or merging networks or maybe demerging networks or whatever the situation is,

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where we just have to support multiple addresses.

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For example, if I'm converting, if I'm doing a conversion from one subnet to

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another, well, then I probably need to support both of those subnets at the

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same time within the same VLAN and a secondary is a great way to do it.

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The way this works is when I get onto that interface for VLAN, let's say 10, I

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can configure a primary IP address, let's say it's 10.0.10.1, and then I give
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can configure a primary IP address, let's say it's 10.0.10.1, and then I give

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it the subnet mask.

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We'll just give it a /24 in this case.

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So I configure an IP address and that's great, but now I can configure another

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IP address, so maybe a different subnet, let's say it's 20.1, and same thing

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with a subnet mask, and I tack on this word, secondary.

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And what's interesting about this is I can actually add as many secondary
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And what's interesting about this is I can actually add as many secondary

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addresses as I want. I can say IP address and then 10.0.30.1, same subnet mask,

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and secondary, and both of these commands will remain on the switch

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configuration.

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So at that point, I support three different subnets all on a single domain. But

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if we look at our topology diagrams here, there's no problem with that. We

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haven't layered subnets into this conversation.

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We've simply pointed out where there's layer two and layer three interfaces. So

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now we might actually see where we have a split.
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now we might actually see where we have a split.

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Maybe, for example, as we look, let's say this topology right here, maybe these

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two hosts are in that 10.0.10.0/24. But maybe we've got a couple of hosts over

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here.

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And these hosts are actually in one of these secondaries. Let's say it's the 10

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.0.20.0/24. So is this okay? Is this topology where two different subnets are
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.0.20.0/24. So is this okay? Is this topology where two different subnets are

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running on the same VLAN? Is that going to work?

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And the answer is it's going to work. And these two hosts will be able to

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communicate to one another. These two hosts will be able to communicate with

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one another. No problem. So what is the problem?

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Well, remember, VLAN is a broadcast domain. And because of that, a broadcast

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that is sent by this host on the left is going to be received by the host that

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's in the same subnet.

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And it's going to be received by the hosts that are in the other subnets
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And it's going to be received by the hosts that are in the other subnets

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because they're all part of the same VLAN and all part of the same broadcast

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domain.

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And yeah, they'll just have to discard that traffic, but they still have to

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receive it and process it. So this is why we don't like to use secondary

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addresses.

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But as mentioned, for IP address conversions, for network migrations, this can

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be a powerful tool in our belt and one that we might just have to use.