Question about VLAN pruning...

[mmanthe]

As I understand it, VLAN pruning effectively prohibits the flooding of broadcast-type traffic (on a given VLAN) to switches without access ports in that given VLAN. so, my question is - given the topology depicted in the image I’ve included - if the HOST on CATALYST 3 sends out a broadcast, only CATALYST C should receive it (technically). But what about CATALYST A? It has no access ports in VLAN 3 - so, shouldn’t it actually prune VLAN 3 broadcasts? And if so, it would effectively never forward anything to CATALYST D (for VLAN 3), right? Given that A is the ONLY path between C and D, wouldn’t this effectively cut off D from C (in terms of the pruning of VLANs and broadcasts)?

What am I missing?

Thanks!

[Deepak Kumar]

Hi,

I am not sure that you are working with VTP or not. VTP Purning (VLAN) will work only with VTP protocol.

All unknown unicast and broadcasts in a VLAN are flooded over the entire VLAN. All switches in the network receive all broadcasts, even in situations in which few users are connected to that VLAN or no user is connected becuase  VLAN is up due to the trunk port.

Regards,

Deepak Kumar

[mmanthe]

Deepak - thanks for your reply. While I agree in part with what you’re saying, the final sentence of your answer should be:

‘VTP pruning is a feature that you use in order to eliminate or prune this unnecessary traffic.’

in other words, WITHOUT vtc pruning, switches behave the way you describe. But WITH vtc pruning, flooded/broadcasted traffic (for a given VLAN) is pruned/discarded AT the trunk port for the VLAN that is not represented by an ACCESS port (not trunk ports!) on that switch. So, I believe my question might still be valid because - in the image I included, the switch (Catalyst A) that is used as the only possible pass-through for VLAN 3 between C and D has no ACCESS ports on VLAN 3 and therefore should prune broadcast traffic for VLAN 3 at the trunk port between C and A and not forward it between C and D. I’m sure there’s a piece I’m missing - I just don’t know what it is. 

Thanks!

[Deepak Kumar]

Hi,

Thanks for clear the picture and question again. Now in your case, A Host PC which is connected to Switch C (VLAN 3) sending a broadcast packet then the same packet will receive on Switch A and Switch D. Not on the Switch B.

A clear example is here:

http://www.orbit-computer-solutions.com/vtp-pruning/

Regards,

Deepak Kumar

[Joseph W. Doherty · Accepted solution]

I believe you're correct. I.e. without any VLAN 3 access ports on switch A, with VTP pruning enabled, switch A won't be a transit for VLAN 3 between switches C and D.

I recall a couple of years ago, running down an issue caused by a similar topology. The part that made it interesting, VTP pruning didn't block a VLAN immediately like manual pruning does. When first defined, in an example as yours, switch A would transit VLAN 3 traffic between switches C and D but after a minute or so, it didn't. I believe I found a command to exclude the VLAN having this issue from VTP pruning without disabling it across all the other VTP VLANs.

[mmanthe]

Thank you for the confirmation! I was afraid that might be the case. Though this was just a theoretical endeavor/question on my part, If you happen to run across the command(s) you used to subvert your issue, it’d be a handy bit of knowledge to keep around!

thanks again!