When adding a VLAN doesn’t add a VLAN

Vendor: Cisco
Software version: 12.2(33)SXI7
Hardware: 6509-E

So this is a typical stupid question. How do you add VLANs to a trunk?

Assuming you started with a port with default configuration on it, it would be:

 interface
 switchport
 switchport mode trunk
 switchport trunk encapsulation dot1q
 switchport trunk allowed vlan
 switchport trunk native vlan

Now, I was interrupted while doing this by someone interjecting and stating categorically, that

 switchport trunk allowed vlan
 ```

Should be:

```
 switchport trunk allowed vlan add
 ```

Not really the way I would do it on a new switchport, but not wanting to hurt feelings I proceeded and saw this:

```
 TEST(config-if)#switchport trunk allowed vlan add 10,20,30
 TEST(config-if)#do show run int gi9/14
 Building configuration...
Current configuration : 279 bytes
 !
 interface GigabitEthernet9/14
 description TEST
 switchport
 switchport trunk encapsulation dot1q
 switchport mode trunk
 shutdown
 storm-control broadcast level 0.50
 storm-control multicast level 0.50
 no cdp enable
 no lldp transmit
 no lldp receive
 end
 ```

To cut a long story short, the switch takes the configuration, but doesn’t apply it. It lead to a lot of head scratching, because you’d think it should work. Switchport state when doing:

```
 show interface gi9/14 trunk
 ```

Shows a state of “other”

```
 show interface gi9/14 capabilities
 ```

was not much help either.

Any CCNP student can tell you, that this is probably due to `switchport trunk allowed vlan add ` not being applicable to ports that have all vlans trunked on them (which is the Cisco default). I'd guess as soon as `switchport mode trunk` is entered in the CLI, adding VLANs using the `switchport trunk allowed vlan add ` becomes inapplicable.

Still, it would be nice to have an error message confirming it instead of a silent failure.

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