VTP pruning used in combination with allowed vlan

Question

I have a network on which most of the switches are running VTP client/server mode. Few switches are not yet in that mode - for specific customization or because they simply cannot handle it.

Thinking about implementing VTP pruning, I'm wondering how switches running VTP client/server with pruning will react when connected to a switch not having VTP.

That very nice answer here https://community.cisco.com/t5/routing/vtp-explanation-changed-modes-from-vtp-server-to-vtp-transparent/m-p/2646119/highlight/true#M248344 explains clearly the switch running VTP will prune all VLAN by default if not receiving VTP Joins message from its neighbor (that will obvisouly not going to be sent in y case).

Now, does that mean I cannot leverage the pruning option until my full network is running VTP? Or could I use VTP pruning but making sure first all my trunk links between VTP managed and non VTP managed switches are manually configured with the switchport mode allowed vlan I definitly need?

Is the switchport trunk allowed vlan supersides the VTP pruning command and force the VTP client switch to still send the VLAN trafic?

Answer 1

So, I gave several trials of VTP configurations and checked behaviors between VTP client switches and VTP transparent switches.

It appeared in my tests that VTP Client switches will not prune all the VLAN as suggested in the link I posted in my question and the theory of VTP Joins messages. It is actually doing the exact opposite, meaning pruning nothing (which at least makes the pruning option less "dangerous" by default).

That being said; I also observed non reproductible (did not happen all the time) issues where the VTP client switch kept its "prune-nothing" state even after the VTP transparent was removed from the topology...

All in all, I also find several sources recommending to simply drop VTP pruning as this technology is more and more legacy. It is having several major problems:

• Can only work on VLAN 2 to 1001 maximum where your VLAN base can grow until 4096
• VTP pruning reduces bandwidth but NOT the STP topology
• VTP pruning is not advised by Cisco on a network until all switches are in client mode (see https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3560/software/release/12-2_55_se/configuration/guide/3560_scg/swvtp.html) (got the same feeling from my tests)

Thus, VTP can be used to manage integrity of VLAN DB accross the network but you should stick with the good old allowed vlan method instead of VTP pruning one to improve your bandwidth and STP topology at the same time without the major drawbacks listed above. At least, this is the choice I made. Would be happy to hear more from you if I forgot something in this analysis.

Answer 2

Vtp pruning is used in network for reduced unwanted brocast traffic of vlans . Only choosen vlan brocast traffic will be allowed into choosen links .so that network bandwidth is optimised..and there will be no latency in network.