I have the following setup:

[phone]-~-~-~-(port-10)[Cisco 3750](port-24=trunk)----[servers]

I want the phone to talk only on vlan (say 1940), where-as Cisco switch to untag/tag its traffic, as goes to/comes from the trunk.

Basically the traffic between phone and switch will be always on a vlan, whereas traffic between trunk port and rest of the network in on no-vlan.

Phone should be able to reach any server on the up-link.

1 Answer 1


So first, what is your native VLAN ? I'll identify it by ~native~.

You need to set the port 10 as on the vlan 1940

#conf t
(config)#vlan 1940
/* description of VLAN as you want */
(config-vlan)#no shutdown
(config / config-vlan)#int f0/10
(config-if)#switchport mode access
(config-if)#switchport access vlan 1940

/* Verify */
#sh vlan brief

Now, will configure the port 24 to tag and manage packets for and from vlan 1940 as wanted :

#conf t
(config)#int f0/24
(config-if)#switchport mode trunk
(config-if)#switchport trunk native vlan ~native~
(config-if)#switchport trunk allowed vlan 1940

Finally, to tag packet, you will need to configure 802.1Q protocol.

#conf t
(config)#int f0/24.1940
(config-subif)#encapsulation dot1q 1940
/* IP CONFIG etc...*/
(config)#int f0/24
(config-if)#no shutdown

So now, your switch is configured to tag packet from vlan 1940 with the 802.1Q protocol.


Everytime you need to add a VLAN to a trunked port, you'lle need to make the last step and add the switchport trunk allowed vlan <num>

  • Thanks! I'll try it tomorrow. Native vlan is vlan-1 for now. May 10, 2016 at 22:50
  • Okay, so don't forget to change this, it's not safe to let the native as VLAN 1 May 10, 2016 at 22:51
  • ok. just tried that. First, 'int f0/10' term is not supported on 3750. Its 'gi1/0/10'. Secondly - after issuing switchport trunk allowed vlan 1940, I lost connectivity with the switch (I was connected using ssh over IP). -Can I regain access to the switch somehow? Or should I looks for serial cable.- May 11, 2016 at 12:35
  • Don't really understand, because you talked about a port 24. Can you please list all of your interface ? The problem can be resolved by specifying the correct interface May 11, 2016 at 12:38
  • I gained access again (by moving uplink cable to port 23). Nothing much is connected to the switch. This is for special setup (not in production or anything). Only 3 ports have Ethernet cable connected: 2 phones (port 10, 21) 1 uplink (now port 23). May 11, 2016 at 12:40

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.