VTP pruning

Unanswered Question
Aug 28th, 2010

Hi all,

I'm studying on the 642-813 exam and i can't get a clear anwser on the floowing question about VTP pruning

(see picture attached)

The certification guide states that broadcasts and unicast floodings on a vlan are blocked on trunks to switches which doesn't have interfaces in that vlan.

The picture shows broadcast traffic send from host A to host B. Why is this broadcast traffic passed along switch 2 which doesn't have interfaces in the Red vlan?

thanx in advance

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Jon Marshall Sat, 08/28/2010 - 04:51

Dennis

Assuming host A connected to switch 4 and host B connected to switch 1 and host A sends packet to host B.

VTP pruning works in a chain so to speak. So if host A and host B are in the same vlan then switch 2 cannot prune that vlan because switch 1 and switch 4 both have hosts in that vlan. If switch 2 pruned the vlan then host A would send the packet to switch 4 but then switch 4 could not forward it on to switch 2 and therefore the hosts would not be able to communicate.

That is what i mean by a chain ie. switch 2 is an intermediary switch in the chain from switch 1 to switch 4 and both switch 1 and switch 4 have hosts in the same vlan so there will be no pruning on switch 2 for that vlan.

If switch 1 did not contain a host in the same vlan as the host on switch 4 then switch 2 could indeed prune the vlan.

Jon

DennisV99_2 Sat, 08/28/2010 - 06:10

Hi John,

Thanx for the swift reply.

It still isn't quite clear to me.

If i understand it correct vtp pruning works by a switch sending a message to a neighbouring switch if it has ports in a particular vlan. If it doesn't the neighbour switch prunes that vlan on the trunk to that switch.

In the drawing switch 2 doesn't have ports in vlan Red so to my logic switch 1 and switch 4 should prune vlan Red on the trunk to switch 2 because switch 2 does not advertise it has ports in vlan Red.

And what if there were more switches between ie switch 1 and 2 that also don't have access ports in vlan Red?

Jon Marshall Sat, 08/28/2010 - 06:52

Dennis

If there were more switches in between then they 2 would not be pruned. Think of it like this -

switch 2 cannot send a prune message to switch 4 until it knows there are no downstream switches that have members in that vlan. If switch 1 did not have any members in that vlan and because there are no downstream switches connected to switch 1 it can send a  prune message to switch 2. Switch 2 could then send a prune message to switch 4 because it too doesn't have any ports in that vlan (other than trunk ports that is).

However because switch 1 does have a member port in that vlan then it does not send a prune message to switch 2. So switch 2 knows that switch 1 has connected host in that vlan. So switch 2 does not send a prune message to switch 4 because it knows it needs to forward packets for that host to switch 1.

That is what i meant when i said it was a chain.

Jon

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Posted August 28, 2010 at 4:39 AM
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