SWITCH 300-115 1.4 Configure and verify trunking

1.4.d Manual pruning

https://www.packet6.com/ccnp-switch-manual-pruning/

https://learningnetwork.cisco.com/thread/8047

http://etutorials.org/Networking/Lan+switching+fundamentals/Chapter+4.+Layer+2+Fundamentals/VLAN+Pruning/

from switch flg pg 66

On trunk links, it is recommended to manually prune the VLANs that are not used.
You can use VTP pruning if VTP is in use, but manual pruning (using a switch-
port trunk allowed VLAN) is a secure way of allowing only those VLANs that are
expected and allowed on the link. In addition to this, it is also a good practice to
have an unused VLAN as a native VLAN on the trunk links to prevent DTP spoof-
ing.