2

Consider the following in the running config in Cisco IOS (switch):

spanning-tree vlan 2-20 prio 4096

If the command "no spanning-tree vlan 5 prio 4096" is issued, will only VLAN 5 be removed without any recomputation of STP topologies of the other VLANs?

4

Issuing the command no spanning-tree vlan 5 priority 4096, which is the same as spanning-tree vlan 5 priority 32768, will not affect other VLANs' instances of Per VLAN Spanning Tree Plus (PVST+).

Example:

    Eth0/1
+---+            +---+
|SW1+------------+SW2|
+---+            +---+
            Eth0/0

SW2 is hard coded as the root for VLANs 2-20. STP event debugging is enabled on SW1 to illustrate the effect of the change.

SW2(config)#spanning-tree vlan 2-20 priority 4096
SW2(config)#do show run | i prio
spanning-tree vlan 2-20 priority 4096

SW1#debug spanning-tree events
Spanning Tree event debugging is on

SW2's hard coded priority for VLAN 5 is set to default (same effect as negating the hard coded priority).

SW2(config)#spanning-tree vlan 5 priority 32768
SW2(config)#do show run | i prio
spanning-tree vlan 2-4,6-20 priority 4096

SW1#
*Jul 11 11:13:53.266: STP: VLAN0005 heard root 32773-aabb.cc00.0200 on Et0/1
*Jul 11 11:13:55.017: STP: VLAN0005 we are the spanning tree root
*Jul 11 11:13:55.025: STP: VLAN0005 Topology Change rcvd on Et0/1
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  • beautiful, thanks! Love that you had the lab handy so quick ;) – Max Jul 11 '17 at 16:49
0

By default, Cisco uses PVST (Per VLAN Spanning Tree), and that means that each VLAN has its own spanning tree. Changing something on one VLAN only affects the spanning tree for that VLAN and no others.

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