I am trying to find a good reason why I should go through the extra effort of pruning vlans off trunks. No DTP or VTP in the network. The core has all the vlans and the IDFs only needs 2 vlans, management and an access vlan. So the only thing I can think of is if I don't prune all the vlans then extra unneed broadcast traffic will be sent to IDFs that don't need it. So here is my question. If the vlan isn't on the IDF switch will the core still send the traffic down the trunk? Does the "Vlans allowed and active in management domain" mean that is forwarding all boardcast traffic down those links?
Show int trunk on the core
6509#sh int trunk
Port Mode Encapsulation Status Native vlan
Po1 on 802.1q trunking 1
Port Vlans allowed on trunk
Port Vlans allowed and active in management domain
Port Vlans in spanning tree forwarding state and not pruned
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In answer to your questions, yes and yes.
Beside broadcast, VLAN(s) multicast could be sent down the link along with VLAN(s) unicast flooding.
If you have another switch downstream of the IDF, also trunked, it (the IDF) wouldn't forward the traffic for a VLAN(s) it didn't also recognize.
To avoid these broadcast and unicast traffic issues and also to secure the core switches from the layer 2 attacks from the IDFs (Users) cisco suggests to go with the Layer 3 links between core and MDF/IDFs.
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