Cisco IE 4000, Руководство по настройке программного обеспечения

Страница: 330 / 1066

326
Configuring STP
Information About Configuring STP
Spanning-Tree Timers
Spanning-Tree Configuration Guidelines
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
Configuring MSTP,
page 333
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.
Caution: 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.
Note: 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.
The switch supports PVST+, rapid PVST+, and MSTP, but only one version can be active at any time. (For example, all
VLANs run PVST+, all VLANs run rapid PVST+, or all VLANs run MSTP.) For information about the different spanning-tree
modes and how they interoperate, see
Spanning-Tree Interoperability and Backward Compatibility, page 322 .
For configuration information about UplinkFast and BackboneFast, see Information About Configuring the Optional
Spanning-Tree Features, page 353 .
Caution: Loop guard works only on point-to-point links. We recommend that each end of the link has a directly
connected device that is running STP.
Table 40 Spanning-Tree Timers
Variable Description
Hello timer Controls how often the switch broadcasts hello messages to other switches.
Forward-delay timer Controls how long each of the listening and learning states last before the interface begins
forwarding.
Maximum-age timer Controls the amount of time the switch stores protocol information received on an interface.
Transmit hold count Controls the number of BPDUs that can be sent before pausing for 1 second.