Interesting VTP Scenario

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

Best Regards,

Deepak Arora