sgtcasey over on networking-forum.com recently posed in an interesting question: what triggers VLAN pruning? Specifically, will a switch only allow pruning of a VLAN from a trunk if it has no access ports configured for that VLAN? Or is it enough to have merely no active ports?

Consider a simple trunking scenario:

Switch 1 is the VTP server, and has propagated VLANs 10, 20, and 30 to switch 2. The interfaces to which hosts A and B attach are configured as access ports in VLAN 10, and an 802.1Q trunk is formed between the two switches. By examining the trunk status on either switch we can verify that VLANs 1 and 10 are being passed while the others are pruned in both directions.

S1# show interface trunk

Port        Mode         Encapsulation  Status        Native vlan
Gi0/1       on           802.1q         trunking      1

Port      Vlans allowed on trunk
Gi0/1       1-4094

Port        Vlans allowed and active in management domain
Gi0/1       1,10,20,30

Port        Vlans in spanning tree forwarding state and not pruned
Gi0/1       1,10

Switch 2:

S2# show interface trunk
...
Port        Vlans in spanning tree forwarding state and not pruned
Fa0/1       1,10

When host B is disconnected, its interface on switch 2 becomes inactive. As switch 2 has no remaining active ports in VLAN 10, VLAN 10 becomes eligible for pruning. After roughly 30 seconds pass, we can see that switch 1 is now pruning VLAN 10 from the trunk (VLAN 10 is absent from the last line of the output):

S1# show interface trunk
...
Port        Vlans in spanning tree forwarding state and not pruned
Gi0/1       1

The VLAN remains unpruned on switch 2's end of the trunk, because it knows switch 1 still has at least one active port in VLAN 10:

S2# show interface trunk
...
Port        Vlans in spanning tree forwarding state and not pruned
Fa0/1       1,10