vlans on a switch 3750

Answered Question
Dec 10th, 2007

Good day everyone I have a quick question here...I thought in order for a vlan to route you would need to define that vlan on the switch. I went on my sw1 and saw an interface configured for vlan12 but when I looked to see if it was configured on the switch it wasn't

How is this working? I can do a show IP route and it knows its next hope which goes to sw2 then on to a ASA.

I have this problem too.
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Correct Answer by Edison Ortiz about 9 years 1 month ago

The information you see in the vlan database reflects the Layer2 Vlan information.

The information you see in the running configuration interface vlan 12 reflects its Layer3 instance.

You don't need a Layer3 instance unless the switch is routing the Vlan on behalf of devices on that subnet.

You would also see vlan12 in the running config, if the switch was set to vtp mode transparent.

vtp mode server and vtp mode client does not display the layer2 information in the running-config. It's stored in the vlan.dat which can be accessed by issuing the show vlan command.

Correct Answer by Edison Ortiz about 9 years 1 month ago

How did you verify it wasn't configured in the switch ?

Please post the output from the following commands:

show ip interface | i 12

and

show vlan

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Correct Answer
Edison Ortiz Mon, 12/10/2007 - 11:22

How did you verify it wasn't configured in the switch ?

Please post the output from the following commands:

show ip interface | i 12

and

show vlan

wgranada1 Mon, 12/10/2007 - 11:57

Sorry for the dumb question it was right in front of my face. Yes I when I do a sh vlan I see it there but when I do a sh inter vlan 12 it never showed up. It was in the database but no one ever defined it on the switch. Sorry for about that and thank you Edison for your help

Correct Answer
Edison Ortiz Mon, 12/10/2007 - 12:06

The information you see in the vlan database reflects the Layer2 Vlan information.

The information you see in the running configuration interface vlan 12 reflects its Layer3 instance.

You don't need a Layer3 instance unless the switch is routing the Vlan on behalf of devices on that subnet.

You would also see vlan12 in the running config, if the switch was set to vtp mode transparent.

vtp mode server and vtp mode client does not display the layer2 information in the running-config. It's stored in the vlan.dat which can be accessed by issuing the show vlan command.

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