TravelingPacket – A blog of network musings

I run into this problem a lot. Someone has created a vlan on a cisco device by using the command –

int vlan 100

ip add 192.168.100.1 255.255.255.0

no shut

But nothing with the vlan is working after this. If you do a “show ip int br” does it shown int vlan 100 down? If so there are two causes to this.

The first is the vlan created? This is the most common problem I see.

Newer versions of firmware create the vlan when you create the interface, but older IOS did not. So even though you create the interface, you have not created the corresponding vlan yet.

You can diagnose this by – “show vlan br” – do you see the vlan?

You can create the vlan with the commands

config t

vlan 100

Cause 2 –  there are no ports/trunks in the Vlan. The Vlan interface will not come up if there are no ports with up/up status associated with that vlan. Trunk ports that have up/up status will bring up all vlans that are allowed to go through the trunk. By default that is all vlans.