One of the obvious advantages of automatic pruning is that you don't have to go and manually fix the prune list of all impacted trunks every time you add an interface on a switch to a new VLAN (that had no prior members on that switch).
If you have to use pruning then consider manual pruning over VTP pruning in your network. Let me list down the pros for you so that you can make an informed decision about it.
1) automatic so less administrative overhead
2) More scalable in a pure cisco network
1) More flexibility. You decide what vlans to prune and where
2) Can be used in a multi-vendor network
3) does not rely on software negotiation and does not 'fail' its pruning as its hard coded
4) All switches need not be in the same VTP domain and can be in transparent mode
Pruning is indeed considered a good practice in a sizable network with a lot of trunks on the switches.
For example, Pruning makes sense if you have lots of IP Phones and you have configured each port on the switch as a trunk port to allow the data and voice vlan. Here manual pruning would work nicely in limiting broadcast/STP traffic on all these ports and thereby server as an important enhancement in
your network health/performance. VTP pruning is not even an option here.
If you have a network with 100's of switches and each switch has a different VLAN requirement then you can use either type of pruning (provided they are all Cisco switches and are under the same domain) to reduce wasteful VLAN traffic traversing extra trunk links. Here VTP pruning may make sense as you
don't have to 'remember' what switch needs what vlan for all the 100 switches.
Does automatic or manual pruning behave differently in an environment where MST is being used ? I recall reading something about manual pruning reducing a spanning tree diameter, whereas automatic does not ?
I thought one of the advantages of manual pruning in a PVST enviroment specifically in a client server environment if you manually prune the vlans off the client supervisor will not have to allocate a instance of spanning tree for the particular vlan that gets manually pruned off thus taking some load off the supervisor . Also on some smaller switches they have limited pvst instances and this will take care of a problem like that . I know in a big pvst client/server enviroment that we manage this took quite a big load off the supervisor where as automatic pruning did nothing .
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