The Purpose of SVI

In Cisco Switches, we can can make a port as an SVI and assign it IP address, but I am not sure in what circumstances we need that type of port, seeing that a Trunk port is still required between 2 switches or a switch and a router in order to allow traffic flow between vlans.
any explanation on the SVI purpose ?

Thanks
jskfanAsked:
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Jimmy Larsson, CISSP, CEHNetwork and Security consultantCommented:
If it is a pure layer2-interface, the SVI-interface (also known as VLan-interface) is only used for management purposes. To telnet/ssh/whatever to the switch and configure/control it:

interface vlan1
 ip address 192.168.1.10 255.255.255.0

If the switch is a layer3-device it is routing-capable. One way of creating layer3-interfaces then is to create multiple vlan-interface (SVI-interfaces):

int vlan1
 ip address 192.168.1.10 255.255.255.0

int vlan2
 ip address 192.168.2.10 255.255.255.0

Now you have 2 SVI-interfaces and the latyer3-switch can route between the two vlan:s.

Best regards
Kvistofta
jskfanAuthor Commented:
Let say we have SW1 with 6 vlans and SW2 with 6 other vlans that are different from the vlans in SW1.
SW1 and SW2 are L3 switches.

Now you want all vlans to talk to each other.How do you configure that ?
Do we need to connect SW1 and SW2 and config a Trunkport on each switch ?
harbor235Commented:
The purpose of a switched virtual interface is to provide layer 3 features for a vlan that is not physically
bound to a port, it is a virtual interface. Now a physical interface/port can go down and not effect the entire vlan.
It is not limited to management interfaces but can be used fro any vlan.

SVIs do not have full feature parity with physical layer 3 interfaces but they are valuable in certain design situations.

harbor235 ;}
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Ken BooneNetwork ConsultantCommented:
Well, you can connect SW1 and SW2 with a single connection between two layer interfaces.  give each interface an ip address and now you can route between the two switches.  
It would be no different than if you had 2 routers that had 7 ethernet ports on each.  Each router had 6 layer 3 interfaces that were connected to 6 vlans if you will and the 7th layer 3 interface was configured as a point to point connection to the other router.  

Then you can turn on any routing protocol and let them route between each other or configure static routing between the two switches.

The other option is to configure 1 switch as layer 3, configure all 12 vlans and put all 12 SVIs on sw1.  Configure a trunk to switch 2 but only trunk the 6 vlans that are needed on the second switch.
Craig BeckCommented:
Just to clarify, you wouldn't make a port a SVI - you would create a VLAN and then create a SVI for that VLAN, then assign ports to that VLAN.  Also, a SVI is not only for management purposes.  It is an interface, just virtual.

An SVI is a virtual interface which is attached to one or more physical ports in software.  An SVI is generally used on layer-2 devices with layer-3 capabilities such as 3560 and 6500 switches.

The easiest way to think of an SVI is if you think about a home router.  The router has an IP address and a 4-port switch.  You connect PCs to the 4-port switch on the router and they can all see the router's IP address.  This is basically the same as an SVI.

A trunk is used to transport multiple VLANs between switches or routers.  If you need to link switches together use a trunk, not an SVI.

The SVI is different to a routed port on a switch.  A routed port is usually connects two devices together in a point-to-point configuration, although you could use a routed port to connect multiple hosts via a separate switch, for example.


An example...

You have a layer-3 switch with 24 ports.
You want 12 PCs to be on VLAN1 and 12 PCs to be on VLAN2.
You want all of the PCs on VLAN1 to be able to talk to the PCs on VLAN2.

You would use 2 SVIs here and enable IP routing on the switch.  You would put 12 ports in VLAN1 and 12 ports in VLAN2.  The PCs on VLAN1 would use the IP address on SVI1 as their default gateway and the PCs on VLAN2 would use the IP address on SVI2 as their default gateway.  This would enable all of the PCs to talk to eachother via the SVIs on the switch.


Make sense?

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harbor235Commented:
One use of Layer 3 interfaces virtual or physical is to segment layer 2 domains, while you can do what you describe there are certain situations that you would not want to do that. There are lots of ways to configure a L3 switch, how you do that depends on what you are trying to do. How big do you want your layer 2 domain, where are the exit points from a layer 2 domain etc ....

So, SVIs are very useful in certain situations, for example, if you had a large number of vlans and you do not want to burn up allot of physical ports terminating them, you could trunk all the vlans over 1-8 physical ports and create hundreds if not thousands of SVIs
for as many vlans as you have.

So you see there are many uses, it depends on the application.

harbor235 ;}
jskfanAuthor Commented:
thank you Guys!
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