How does Pruning works

cisco · ‎10-21-2009

Hi,

How does a downstream switch pass on the message to upstream switch that i dont have a particular VLAN , so dont pass any info about that VLAN.

regards

Neo

Peter Paluch · ‎10-21-2009

Neo,

The VTP Pruning is made by switches periodically (every 6 seconds or so) sending the VTP Join messages. Each of these messages contains a bit map in which the bit on i-th position signifies whether the VLAN number "i" is active on that switch. If a switch sends a VTP Join message to its neighbor having the i-th bit in the bit map set to 1, then that VLAN is active. If that bit is set to 0 then the VLAN is inactive and should be pruned. Simple, even naive, but working.

Best regards,

Peter

cisco · ‎10-21-2009

Hi Peter,

any URL where i can see the details for VTP Join message.your help will be apperciated

regards

Neo

Peter Paluch · ‎10-21-2009

Hello Neo,

You may want to read this URL for some information regarding the VTP:

http://www.cisco.com/en/US/tech/tk389/tk689/technologies_tech_note09186a0080094c52.shtml

However, regarding the VTP Join messages, there is no public information about it - at least I could not find any. The info I have posted earlier comes from my own small sniffing and reverse-engineering of the VTP traffic.

Best regards,

Peter