Empty VLAN won't come up

Unanswered Question
Mar 5th, 2009

We are building a VMWare based lab with a layer 3 IOS switch (3750) to do inter-vlan routing from the various vlans within the VM chassis. There will be no switch ports specifically assigned to the VLANS, only trunked interfaces into the VM Chassis. The VLANS are not coming up and subsequently we can't do inter-vlan routing via the L3 switch. Any way to fake out the switch?

I have this problem too.
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Giuseppe Larosa Thu, 03/05/2009 - 10:16

Hello Michael,

a trunk port should be enough for making the switch happy about autostate

verify the list of permitted vlans on your trunk port to vmware you may need to update it.

also when you create a new SVI you need to do no sh

Hope to help


mlpalmer Thu, 03/05/2009 - 10:41

I neglected to issue any 'vlan' config command (such as name). Named the vlans and everything took off.

Thank you for the affirmation.

lamav Thu, 03/05/2009 - 12:15

Glad you fixed it.

Just to make sure you understand, its not the "name" command statement that made it come up, but the creation of the vlan itself in layer 2.

Without the command "vlan 10," for example, vlan 10 will not be created on the switch and will also not carry traffic across any trunk port, even if the trunk port is configured correctly. To the switch, the vlan doesnt even exist....


Giuseppe Larosa Thu, 03/05/2009 - 13:57

Hello Michael,

as Victor noted to have an STP instance running the L2 Vlan entity must exist on the switch

After the L2 Vlan broadcast domain is created, STP instance is created and starts to run.

The trunk becomes an STP forwarding port for the vlan.

And the SVI comes up.

For this reason an SVI is a semi-logical interface that is in the middle between physical L3 interfaces and loopbacks

Hope to help



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