Switch Port goes down and up after few seconds

[ittechk4u1]

Hello,
 Most of the switch ports(which is configured for data and voice together)  goes up & down after every few seconds.

031695: Sep 21 10:56:04.513 MEST: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet1/0/12, changed state to down
031696: Sep 21 10:56:06.518 MEST: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet1/0/12, changed state to up

Here si the config:

interface GigabitEthernet1/0/12
 switchport access vlan 21
 switchport trunk encapsulation dot1q
 switchport trunk native vlan 21
 switchport voice vlan 221
 switchport priority extend trust
 spanning-tree portfast trunk

Can anyone check and let me know the issue.

Thanks

[blue phoenix]

hi,

what device is connected on port Gi1/0/12?

it seems you have both access and trunk port configured in it.

[ittechk4u1]

Alcatel Telephone and then a Workstation.

[blue phoenix]

I believe that when you use a VOIP phone connected to the switch, and use that phone to connect to the workstation, you can use the voice vlan command.  Is it specifically stated in Cisco site that when using Alcatel VOIP phone, you need to configure both voice vlan and trunking at the same time on the switch port?