vtp pruning weird scenario

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Nov 19th, 2008
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hi every body!

let say two switches, sw1 (vtp client) and sw2(vtp server) are connected as shown below:


sw1 has vlan 1,

sw2 has vlan 1,2

vtp pruning is enabled.

According to cisco documents, sw1 does not advertise the use of vlan2 so sw2 will prune the vlan2 on the trunk. But if use the command on sw2 " switchport trunk pruning vlan except 2" ,will sw2 forward the frame belonging to vlan 2 on the trunk because of this command or it will prune because sw1 did not advertise the use of vlan 2?

thanks a lot!

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Davy Ad Thu, 11/20/2008 - 00:23
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Hi, Sarah,

I think it should forward it , but not to SW1 ,incase you have another Sw that is on Client that has Vlan2.



sarahr202 Wed, 11/26/2008 - 18:16
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thanks for your reply!

In my example there are just two switches sw1,sw2.

Now would sw2 prune vlan2 because sw1 did not advertise vlan 2 's use or it would allow vlan2 because of the command " switchport trunk pruning except vlan 2"

thanks a lot!

wesleyterry Wed, 11/26/2008 - 22:25
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The understanding that I have is the "switchport trunk pruning vlan except 2" configuration removes vlan 2 from the eligible vlans for VTP pruning.

So VTP Pruning is on, but the configuration is telling the switch that vlan 2 is not eliglbe to be pruned. So in theory, broadcast traffic and whatnot for the vlan will trunk across the link regardless of if there is an actual client on the vlan...

sarahr202 Thu, 11/27/2008 - 08:13
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Thanks for your reply!

As sw2 did not receive any message from sw1 about vlan2 being in use on sw1, sw2 should prune vlan 2 but we manually configure the command "switchport trunk pruning except vlan2"

,the end result is sw2 will not prune vlan 2.

Am i correct ?

thanks a lot!

Jon Marshall Thu, 11/27/2008 - 08:25
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If you make vlan 2 ineligible to be pruned on the trunk link then any broadcast or unknown unicast traffic for vlan 2 will be sent across the trunk link on the connection between sw2 and sw1.




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