Access VLANs Not Being Defined
Switches do not forward frames for VLANs that are (a) not configured or (b) configured but disabled (shutdown). This section summarizes the best ways to confirm that a switch knows that a particular VLAN exists, and if it exists, determines the state of the VLAN.
First, on the issue of whether a VLAN is defined, a VLAN can be defined to a switch in two ways: using the vlan number global configuration command, or it can be learned from another switch using VTP. This book purposefully ignores VTP as much as possible, so for this discussion, consider that the only way for a switch to know about a VLAN is to have a vlan command configured on the local switch.
Next, the show vlan command always lists all VLANs known ...
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