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Cisco Catalyst switches support three types of spanning-tree protocols: PVST+, PVRST+,
and MSTP.
■ PVST+: Based on the 802.1D standard, this includes Cisco proprietary extensions,
such as BackboneFast, UplinkFast, and PortFast, which improve STP convergence
time.
■ PVRST+: Based on the 802.1w standard, this has a faster convergence than 802.1D.
■ MSTP (802.1s): Combines the best aspects of PVST+ and the IEEE standards.
To implement PVRST+, perform these steps:
Step 1 Enable PVRST+.
Step 2 Designate and configure a switch to be the root bridge.
Step 3 Designate and configure a switch to be the secondary (backup) root
bridge.
Step 4 Verify the configuration.
Table 2-13 describes the commands that you use to enable and verify PVRST+.
Table 2-13 PVRST+ Commands
Command Description
SwitchX(config)#spanning-tree mode
rapid-pvst
Sets spanning-tree mode to PVRST+
SwitchX#show spanning-tree vlan
vlan-number [detail]
Shows spanning-tree information that is VLANbased
rather than instance-based
SwitchX#debug spanning-tree pvst+ Debugs PVST+ events
SwitchX#debug spanning-tree switch
state
Debugs port state changes
Note: Like all debug commands, this command
can affect network performance.
62 Chapter 2: Medium-Sized Switched Network Construction
Example 2-7 shows how to verify the STP protocol for a given VLAN.
In this example, the statement Spanning tree enabled protocol rstp indicates that switch
X is running PVRST+, the Cisco RSTP implementation.
Switch X is the root bridge for VLAN 30. Its priority of 24606 is derived from the sum of
the assigned priority of 24576 and VLAN 30. The MAC address of switch X, which is
00d0.047b.2800, is appended to the priority, 24606, to make up the bridge ID.
As the root bridge for VLAN 30, all the interfaces of switch X are designated ports in the
forwarding state.
If all the switches in a network are enabled with the default spanning-tree settings, the
switch with the lowest MAC address becomes the root bridge. However, the default root
bridge might not be ideal because of traffic patterns, the number of forwarding interfaces,
or link types.
Before you configure STP, select a switch to be the root of the spanning tree. This switch
does not need to be the most powerful switch, but it should be the most centralized switch
on the network. All data flow across the network occurs from the perspective of this switch.
The distribution layer switches often serve as the spanning-tree root because these switches
typically do not connect to end stations. In addition, moves and changes within the network
are less likely to affect these switches.
Example 2-7 STP Protocol Verification
SwitchX#show spanning-tree vlan 30
VLAN0030
Spanning tree enabled protocol rstp
Root ID Priority 24606
Address 00d0.047b.2800
This bridge is the root
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Bridge ID Priority 24606 (priority 24576 sys-id-ext 30)
Address 00d0.047b.2800
Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec
Aging Time 300
Interface Role Sts Cost Prio.Nbr Type
-------- ----- --- --- -------- ----
Gi1/1 Desg FWD 4 128.1 P2p
Gi1/2 Desg FWD 4 128.2 P2p
Gi5/1 Desg FWD 4 128.257 P2p
Improving Performance with Spanning Tree 63
By increasing the priority (lowering the numerical value) of the preferred switch so that it
becomes the root bridge, you force spanning tree to perform a recalculation that reflects a
new topology with the preferred switch as the root.
The switch with the lowest BID becomes the root bridge for spanning tree for a VLAN. You
can use specific configuration commands to help determine which switch will become the
root bridge.
A Cisco Catalyst switch running PVST+ or PVRST+ maintains an instance of spanning
tree for each active VLAN that is configured on the switch. A unique BID is associated with
each instance. For each VLAN, the switch with the lowest BID becomes the root bridge for
that VLAN. Whenever the bridge priority changes, the BID also changes. This change
results in the recomputation of the root bridge for the VLAN.
To configure a switch to become the root bridge for a specified VLAN, use the command
spanning-tree vlan vlan-ID root primary. With this command, the switch checks the
priority of the root switches for the specified VLAN. Because of the extended system ID
support, the switch sets its own priority to 24576 for the specified VLAN if this value will
cause the switch to become the root for this VLAN. If another switch for the specified
VLAN has a priority lower than 24576, then the switch on which you are configuring the
spanning-tree vlan vlan-ID root primary command sets its own priority for the specified
VLAN to 4096 less than the lowest switch priority.
A secondary root is a switch that can become the root bridge for a VLAN if the primary
root bridge fails. To configure a switch as the secondary root bridge for the VLAN, use the
command spanning-tree vlan vlan-ID root secondary.
With this command, the switch priority is modified from the default value of 32768 to
28672. Assuming that the other bridges in the VLAN retain their default STP priority, this
switch becomes the root bridge if the primary root bridge fails. You can execute this
command on more than one switch to configure multiple backup root bridges.
Summary of Improving Performance with Spanning Tree
The following summarizes the key points that were discussed in this section.
■ A redundant switched topology includes redundantly connected switches and
EtherChannel.
CAUTION Spanning-tree commands take effect immediately, so network traffic is
interrupted while reconfiguration occurs.
64 Chapter 2: Medium-Sized Switched Network Construction
■ A redundant switched topology causes looping issues such as broadcast storms.
■ The 802.1D STP establishes a loop-free network.
■ The original STP has been enhanced by PVST+ and RSTP.
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