Can anyone please explain L3 SVI configuration ,What is SVI and Where

[ameenkhan82h]

I am learning about switching and came across SVI in vlans ,I Googled and watched couple videos and got really confusing Firsly where do we use Svi? and why do we use it,How to configure svi and what are the benefits of using svi, How to determine svi on a cisco or Nexus Switch .please any help is highly appreciated.

[Richard Burts]

You ask a question about some fundamentals of switching. So let me begin by reviewing some fundamental concepts:

- a vlan on a switch is a layer 2 entity and uses layer 2 mac addresses.

- ports on a switch can be assigned to vlans (some switches may have only a single vlan but many switches have several vlans).

- a switch will forward traffic using the layer 2 mac address.

- a switch can forward traffic between 2 devices if they are in the same vlan. But a layer 2 switch can not forward traffic between vlans.

- to forward traffic between vlans the switch needs to operate a layer 3 device (using IP addressing).

- An SVI (Switched Virtual Interface) is how you implement layer 3 operation for the switch.

- a switch will need an SVI for each vlan for which it wants to do layer 3 forwarding.

- you configure an SVI for the vlan and assign an IP address and subnet mask for the SVI and that enables the switch to do layer 3 forwarding (frequently called routing) for that vlan.

So now lets answer your questions:

* Firsly where do we use Svi? You use an SVI on the switch for each vlan for which you want to do layer 3 forwarding.

* why do we use it You use the SVI if you want to do layer 3 forwarding for the vlan on the switch. Without layer 3 forwarding each vlan on the switch would need a layer 2 connection for that vlan to some layer 3 router, which could provide the inter vlan routing.

* How to configure svi You start by configuring "interface vlan n" where n is the vlan number. Then you configure an IP address and subnet mask.

*  what are the benefits of using svi The benefits are that the switch can now perform inter vlan routing and does not need to forward traffic to another device to achieve inter vlan routing.

[mcardinal]

Richard,

First, I want to thank you for all the amazing posts I have read from you, which has been quite a few (and I am only getting started!).

Second, I want to add the use of the command "ip routing" which can be included after configuring the IP address and subnet masks of the SVIs.  This enables IP routing on the L3 switch and allows connectivity between the VLANs.

HTH

Matthew

"When you teach someone what you know, you gain a greater understanding of the subject, which increases your own retention of the knowledge."

[Richard Burts]

Matthew

First, Thank you for the kind words. It is gratifying to know that others have benefitted from and appreciate my contributions.

Second, Good catch. +5 for pointing out that without ip routing being enabled multiple SVI does not achieve the desired result.

Third, I really do like the quote in your signature. I firmly believe that it is absolutely believe that it is true.  This community is an excellent place to ask questions and to learn about networking. I hope to see you continue to be active in the community.

[moyinoluwa-adegbolu]

I also came here with the same question and there you have solved it in the simplest way possible.

Thank you so much.

[Richard Burts]

You are welcome. I am glad that my explanation was helpful for you.

[hussnisuad]

Deep and easy explanation ... Thanks dear Richad.

[Richard Burts]

@hussnisuad You quite welcome. I am glad that my explanations have been helpful.

[Joseph W. Doherty]

Maybe this will help.

First consider a router connected to a non-VLAN capable switch.  Often the router will provide a gateway IP for the devices connected to that non-VLAN switch and more non-VLAN capable switch down stream of it.  Understanding the usage and need of the router interface's  IP is important.  (If not understood so far, please let us know.)

Next consider the above router having additional interfaces also connected to more non-VLAN capable switches.  Each of those too, with IPs and acting as the gateway IP for those connected L2 domains.  The router supports routing between the different networks defined on its interfaces.

Then consider a router connected to a VLAN capable switch.  What might be done is have a router interface connected to a VLAN access port, for each VLAN, on that switch.  Basically on one switch, what's described in the prior paragraph.

The forgoing, though, would be unusual.  Instead, often the switch has a configured trunk port connected to a router port.  The trunk port can carry multiple VLANs, because frames now have attached tags to tell which VLAN they belong too.  On the router side, we define subinterfaces (which are logical interfaces!) on the one physical port.  Each subinterface has an IP associated with a VLAN tag.  Effectively, again we have what's been described above too.

Lastly, on a switch hosting VLANs (still just L2) we want it to route between those VLANs on it.  So, we need to emulate the router's IP interfaces, whether physical per interface, or logical per subinterface, for the VLANs.  To do this, on the switch, much like a router's logical subinterface, we define logical interfaces for the VLANs.  Cisco calls those SVI, because they are on the Switch, they are Virtual (or logical) and act as Interfaces.

With SVIs we no longer need a router for routing, routing can be done, using those, on a L3 switch.

As to how they are represented, on Cisco Catalyst switches (I recall) they are "interface vlan #" are configured much like a router's physical interface, e.g. "interface ethernet #".

I don't fully remember how they are defined on the Nexus series, but somewhat similar to Catalysts, I believe.