Tuesday, March 3, 2009

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

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