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Tuesday, November 29, 2022

What is Switch Virtual Interface (SVI) – Configuration Example with Explanation

In computer networks, convergence is something that the routers, as well as the switches, are responsible for as both are required to make the network work. Creating a computer network is meant to allow computers in a network to exchange resources and communicate between themselves, this being the basic purpose of creating a computer network.

The communication on a local area network (LAN) is handled by switches, and the internetwork communication (i.e. between LANs or Wide Area Networks) is handled by routers.

It is understood that VLANs operate on Layer 2 networks in what is known as a “unicast domain”, which means that when a broadcast message is sent within the same Layer2 VLAN, that broadcast message will be displayed to all devices connected to this VLAN (either through the same switch or different switches).

Also, there is no need for layer 3 devices to communicate between hosts connected in the same Layer 2 VLAN because of the Layer 2 VLAN features. Unlike those devices located on the same VLAN network, devices on different VLANs are usually unable to communicate with each other unless they are rooted in some fashion.

Due to the new requirements for inter-VLAN communications, network segmentation can now become more relevant, something that can be achieved with the use of a Layer 3 switch or router.

When using a router for network segmentation, each interface on the router is essentially like a separate network segment on its own. This is known as a separate broadcast domain on a network.

Layer 3 switches can be employed to achieve this goal as long as multiple Layer 2 VLANs are set up on the switch. These VLANs are used to separate everything into multiple broadcast domains. On the switch, you should create a Layer3 interface that corresponds to each Layer2 VLAN, which will be responsible for routing traffic across the network. We call this an SVI, or a Layer3 interface.

What is Switch Virtual Interface (SVI)?

What is Switch Virtual Interface (SVI)?

As each VLAN can be considered an isolated network segment of its own, the configuration of layer 3 switches should include the ability to allow communication between VLANs.

SVIs operate with a very simple model of operation. On the switch, you will first create the Layer 2 VLAN and assign an IP address to the Layer 3 interface for the VLAN. This is similar to the process you would follow for a physical router interface.

It is important to note that the major difference is the fact that SVI Layer 3 interface is virtual rather than physical. Clients of these VLANs will also be able to access the Internet through the interface of the SVI since the switched virtual interface is their default gateway.

The standard remote management interface for layer 3 switches is created by default by the operating system, which is the SVI that is associated with logical VLAN 1 (default native VLAN). From the above, it can be deduced that if an IP address is allocated to the interface, it can be used for management purposes.

SVI configuration example

SVI configuration example

Based on the network diagram presented above, we’ll see how to create two SVIs on a layer 3 switch by using the examples presented below.

If you want this to work, then you must have a layer 3 switch, which is something that can be done.

In the example shown below, based on the data in the example, it seems as though you are already familiar with how to conduct basic switch configuration functions, such as changing the hostname, going into global configuration mode, configuring switch virtual interface, or assigning IP addresses to interfaces.


SWITCH(config)#vlan 10   <- Set up Layer2 VLAN 10 first then configure SWITCH(config)#vlan 10 I have created the SVI below for the VLAN 10 interface on Switch(config)#interface
WORKSTATIONS – SWITCH(config-if)#description
SWITCH(config-if)#ip address <- assign the IP address to the SVI

By default, layer 2 VLAN 10 is created in the first line in the following example. I have created the SVI for VLAN 10 (which is the Layer3 interface) with the following code. It is recommended that you include a description of the interface in the third line of the description, even if it is OPTIONAL. This will help the reader to understand the purpose of the SVI. Line 28 of the format inserts the IP address of the SVI configured for VLAN 10 – which looks like this in this case. In the example configuration here, we have configured two SVIs, one for a workstation network, and one for a server network.

To create an SVI for VLAN 20, you will have to use the same commands as those described above.


SWITCH(config)#vlan 20‘ – create a Layer 2 VLAN that can be shared by multiple switches
(SWITCH(config)#interface VLAN 20 *- network was set up for VLAN 20 now
it is time to describe the servers in SWITCH(config-if)#description
In SWITCH(config-if), select IP address *- assign the IP to SVI

The need for SVIs

VLANs are simply a way to separate network segments on a network, with each VLAN having its own subnet for addressing the network.

SVIs, although they can be used to achieve the same inter-VLAN communication as a router with multiple switched virtual interface, are less complex than routers with multiple interfaces.

A physical interface will be used for handling inter-VLAN connections regardless of whether the router is configured as a router-on-a-stick or as a router that uses multiple physical interfaces.

There are also sub-interfaces that can be created within a single physical interface of the router so that the IP addresses however, using SVIs to relate Layer2 and Layer3 switches does offer a number of advantages for the purpose of achieving flexible network convergence and for simplifying the management and operation of the network by making the Layer2 and Layer3 functionalities the same via the Layer3 switch (all Layer2 and Layer3 functionality is handled by the Layer3 switch).

Advantages of Switch Virtual Interface

  • A switched virtual interface (SVI) is more efficient when compared to a router-on-a-stick because everything is switched and routed by hardware rather than software.
  • From the switch to the router, there’s no need for any external links to facilitate routing.
  • We are able to get higher bandwidth using this method since we are able to use Layer 2 Ether Channels between the switch, thereby overcoming the restriction of using only one link.

Disadvantages of Switch Virtual Interface

When it comes to layer 3 switches, the only disadvantage is that they cost too much.

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