[c-nsp] VTP Pruning

[Tim Stevenson]

Right, presence of a MAC in a VLAN on a trunk is not a requirement (though 
typically it would be a logical consequence).

As far as how it works, it is just an additional TLV in a VTP summary 
advert message, which signals the pruning state of each vlan on the sending 
switch.

 From the pruning spec, the vlan pruning bit for a vlan might change when:

any port's STP state changes (forwarding to/from blocking/disabled), or
any vlan's STP status changes (active to/from inactive), or
any (static/dynamic) port assigned to a vlan, or
any port's status changes (trunk to/from non-trunk), or
a vlan's status changes on a trunk port (allowed to/from disallowed)
a trunk port's creation or deletion

Tim

At 03:30 PM 3/15/2005, Bruce Pinsky declared:
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>Dave Temkin wrote:
>| Does anyone have a pointer to a doc that shows *how* VTP Pruning works?  I
>| definitely know *what* it does, but none of the docs say how it determines
>| whether or not a VLAN is in use...
>|
>
>I haven't found a public one that I can point you at.
>
>| I had an issue years ago where even if there was an active port in a
>| VLAN, it got pruned because the CAM entry fell out after awhile.  This
>| broke things because it was in a one-way broadcast type environment
>| (market data).  The general understanding is that if a port is active in
>| that VLAN that it shouldn't be pruned....  But as I just stated, that
>| wasn't the case in the past - and I'm not sure if that was a bug or if it
>| was how it was intended to work....
>|
>
>According to the specs I've read, the pruning function is only notified of
>the following events for local VLAN access ports:
>
>- - Link state change of an access port
>- - Port state change of an access port (i.e. admin status)
>- - STP forwarding state change of a VLAN on access ports
>- - VLAN membership change of an access port
>- - Management VLAN changes
>
>So having a valid CAM entry on a port in a VLAN does not appear to be one
>of the criteria for considering that a VLAN is "active".
>
>- --
>=========
>bep
>
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Tim Stevenson, tstevens at cisco.com
Routing & Switching CCIE #5561
Technical Marketing Engineer, Catalyst 6500
Cisco Systems, http://www.cisco.com
IP Phone: 408-526-6759
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