VTP pruning question

[mahesh18]

Hi Everyone,

Need to confirm topology below

i have layer 3 switch connected to layer 2.

Layer 3 is VTP server mode and has vlan 20.

Layer 2 switch also has vlan 20 and single trunk connection.

I enabled VTP pruning on server switch.

Layer 2 switch has only 1 port up which is trunk port to layer 3.

When i enable pruning VTP server switch shows

Port        Vlans in spanning tree forwarding state and not pruned

Fa0/12      1

It does not show vlan 20 here.

As trunk should carry all the vlans

Layer 3 switch port fa0/12 config

interface FastEthernet0/12

description Dynamic Desirable Trunk to 2950B

switchport trunk encapsulation dot1q

switchport trunk allowed vlan 1,20

switchport mode dynamic desirable

speed 100

duplex full

Is this default behaviour or not ?

Thanks

MAhesh

[Giuseppe Larosa · Accepted solution]

Hello Mahesh,

if there is no access port in Vlan 20 in up/up state on L2 switch VTP pruning is triggered and vlan 20 is actually pruned on the trunk link.

This means that unknown unicast, broadcast and multicast traffic in Vlan 20 is not sent out of the trunk port from the L3 switch side, as L2 switch has signalled via VTP that it has no users in vlan 20. And this is the VTP pruning feature in action.

However, STP is still running for Vlan 20 ( if you are using the default PVST+ STP mode).

When an user in vlan 20 is added to L2 switch the vlan 20 will be listed under ithe " STP forwarding state and not pruned" category.

At this point traffic to the new user and unknown unicast, multicast and broadcast traffic for Vlan 20 will be sent out the trunk from L3 switch side to L2 switch.

Hope to help

Giuseppe

[mahesh18]

Hi Giuseppe,

I checked on Server switch

sh spanning-tree vlan 20

VLAN0020

  Spanning tree enabled protocol rstp

  Root ID    Priority    24596

             Address     000d.28bc.fd80

             This bridge is the root

             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    24596  (priority 24576 sys-id-ext 20)

             Address     000d.28bc.fd80

             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

             Aging Time 300

Interface           Role Sts Cost      Prio.Nbr Type

------------------- ---- --- --------- -------- --------------------------------

Fa0/12              Desg FWD 19        128.12   P2p

So this shows that STP is still running on VTP server switch right????

Also i plugged pc to layer 2 switch then the sh int trunk on layer 3 switch shows

Port        Vlans in spanning tree forwarding state and not pruned

Fa0/12      1,20

thanks

MAhesh

[Giuseppe Larosa · Accepted solution]

Hello Mahesh,

the note about VTP pruning not affecting STP is very important for scalability:

relying on VTP pruning allows to minimize   flooding of traffic over the trunk links, but it does not save on the number of STP instances running on the device.

As you have seen STP is still running on both switches for vlan 20 not only on VTP server but also on VTP client.

There are some low end platforms that have limitations on the number of STP instances that can run on the box, and only the use of manual list of Vlans (switchport trunk allowed vlan )  on trunks allow to minimize the number of STP instances not VTP pruning.

Hope to help

Giuseppe

[mahesh18]

Hi Giuseppe & Kyle,

Many thanks for detaied reply back.

Now my concepts regarding Pruning are mich more  clear.

Best regards

MAhesh

[Kyle McKay · Accepted solution]

What you need to remember with VTP pruning is that the VLAN will only traverse links that it needs to. Just because you have configured a trunk to allow that VLAN does not mean it will be actively forwarding on the link anymore.

If you have users on VLAN 20 (access ports) on the L2 switch I would expect that VLAN to be running on the trunk.