Catalyst Switches and SVI's

Unanswered Question
Mar 16th, 2010

Can anyone explain to me why, when you create an SVI (interface vlan10) on a Catalyst switch, the vlan interface doesn't actually come up/up until a port goes live on that switch?

This is more of a curiosity than a problem.

thanks!

I have this problem too.
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Jon Marshall Tue, 03/16/2010 - 10:38

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Can anyone explain to me why, when you create an SVI (interface vlan10) on a Catalyst switch, the vlan interface doesn't actually come up/up until a port goes live on that switch?

This is more of a curiosity than a problem.

thanks!

Because the SVI is a virtual interface but you want it to behave like a physical L3 routed interface. A physical interface will go down if the other end of the link dies but that wouldn't affect an SVI. So to emulate what the physical interface would do Cisco use something called autostate on the SVI which keeps track on whether any physical ports are active for the vlan, including trunk links.

Jon

Cisco Contracts Tue, 03/16/2010 - 11:23

Jon:

Thanks for the information.  That's exactly what I was looking for.  It makes a lot more sense thinking of it as a circuit.

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