Cisco Systems 3560 Manual De Usuario

If more VLANs are defined in the VTP than there are spanning-tree instances, you can enable PVST+ 
or rapid PVST+ on only 128 VLANs on the switch. The remaining VLANs operate with spanning tree 
disabled. However, you can map multiple VLANs to the same spanning-tree instances by using MSTP. 
For more information, see 

If 128 instances of spanning tree are already in use, you can disable spanning tree on one of the VLANs 
and then enable it on the VLAN where you want it to run. Use the no spanning-tree vlan vlan-id global 
configuration command to disable spanning tree on a specific VLAN, and use the spanning-tree vlan 
vlan-id global configuration command to enable spanning tree on the desired VLAN.

Switches that are not running spanning tree still forward BPDUs that they receive so that the other 
switches on the VLAN that have a running spanning-tree instance can break loops. Therefore, spanning 
tree must be running on enough switches to break all the loops in the network; for example, at least one 
switch on each loop in the VLAN must be running spanning tree. It is not absolutely necessary to run 
spanning tree on all switches in the VLAN. However, if you are running spanning tree only on a minimal 
set of switches, an incautious change to the network that introduces another loop into the VLAN can 
result in a broadcast storm.

If you have already used all available spanning-tree instances on your switch, adding another VLAN 
anywhere in the VTP domain creates a VLAN that is not running spanning tree on that switch. If you 
have the default allowed list on the trunk ports of that switch, the new VLAN is carried on all trunk ports. 
Depending on the topology of the network, this could create a loop in the new VLAN that will not be 
broken, particularly if there are several adjacent switches that have all run out of spanning-tree instances. 
You can prevent this possibility by setting up allowed lists on the trunk ports of switches that have used 
up their allocation of spanning-tree instances. Setting up allowed lists is not necessary in many cases and 
can make it more labor-intensive to add another VLAN to the network.

Spanning-tree commands control the configuration of VLAN spanning-tree instances. You create a 
spanning-tree instance when you assign an interface to a VLAN. The spanning-tree instance is removed 
when the last interface is moved to another VLAN. You can configure switch and port parameters before 
a spanning-tree instance is created; these parameters are applied when the spanning-tree instance is 
created.

Spanning-tree VLAN port cost (configurable on a per-VLAN basis)

1000 Mb/s: 4.

100 Mb/s: 19.

10 Mb/s: 100.

Spanning-tree timers

Hello time: 2 seconds.

Forward-delay time: 15 seconds.

Maximum-aging time: 20 seconds.

Transmit hold count: 6 BPDUs