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RSRack1SW1(config-if)#switchport voice vlan ?
  <1-4094>  Vlan for voice traffic
  dot1p     Priority tagged on PVID
  none      Don't tell telephone about voice vlan
  untagged  Untagged on PVID

RSRack1SW1(config-if)#switchport voice vlan dot1p

This command configures the telephone to use IEEE 802.1p priority tagging and uses VLAN 0 (the native VLAN). By default, the Cisco IP phone forwards the voice traffic with an IEEE 802.1p priority of 5.

Doc CD Navigation

• Catalyst 3560 Switch Software Configuration Guide, Rel. 12.2(46)SE
• Configuring Voice VLAN
• Voice VLAN Configuration Guidelines

Server  Client   Trans     Client

SW1 ---- SW2 ---- SW3 ----- SW4

                   |         |

                   |VLAN25   |

                   |         |

                   R5       VLAN25

Switch in the transparent mode does not take part in VTP, therefore does not send out prunning messages, except messages it receives from switch in the VTP server/client mode.

Therefore, if we enable prunning on SW1, SW2, SW4, and IF SW1 & SW2 do not have any interfaces in VLAN25, the VLAN 25 will be pruned on the trunk link between SW3 and SW4, even though SW3 may have interface on VLAN25 . Due to this behavior, the reachbility of VLAN25 between SW3 & SW4 will be broken.

To prevent VLAN25 from being pruned, we need to remove it from the Prune Eligible List.

RSRack1SW4#sh int trunk | b prune
Port        Vlans in spanning tree forwarding state and not pruned
Fa0/19      1-2,11,32,43,367

RSRack1SW4(config)#interface FastEthernet0/19
RSRack1SW4(config-if)# switchport trunk pruning vlan remove 25 

RSRack1SW4#sh run int fa0/19
interface FastEthernet0/19
 switchport trunk encapsulation dot1q
 switchport trunk native vlan 11
 switchport trunk pruning vlan 2-24,26-1001
 switchport mode dynamic desirable
end

RSRack1SW4#sh int trunk | b prune
Port        Vlans in spanning tree forwarding state and not pruned
Fa0/19      1-2,11,25,32,43,367