VTP Pruning

[stuartmair]

We enabled VTP and started using it shortly after our new infrastructure was put in-approx 1-1/2 years ago.

However we did not enable pruning.

Would enabling in now cause any problems. We have approx 25 VLans that really at this point aren't being used to any great extent, some more than others, however we are starting to subnet our network down more and I am wondering if turning on pruning would cause any issues when first turned on?

Everything I read says it's a good idea but nothing explains what happens when first turned on.

[ibrunello]

http://www.cisco.com/univercd/cc/td/doc/product/lan/cat5000/rel_4_2/config/vlans.htm#xtocid79802

"VTP pruning takes effect several seconds after you enable it".

As far as I recall, when I enabled it on my switched network, I noticed no interruption on traffic, but a mere reduction of traffic toward access switches.

AFAIK, every switch tells its neighbors which vlans are local, and information is propagated; then unused ones are pruned.

When checking if pruning works (using the "sho int trunk" command) remember:

- if run on the access switch to the uplink toward distribution, nothing is pruned (distribution usually knows almost everything, so there is nothing to filter, from the access device perspective)

- if run on the distribution switch to the access, you should see the expected pruning (access switch often has a subset of all vlans, thus the distribution uplink prune unused ones)