Reference Guide

626 | Multiple Spanning Tree Protocol
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Flush MAC Addresses after a Topology Change
FTOS has an optimized MAC address flush mechanism for RSTP, MSTP, and PVST+ that flushes
addresses only when necessary, which allows for faster convergence during topology changes. However,
you may activate the flushing mechanism defined by 802.1Q-2003 using the command
tc-flush-standard,
which flushes MAC addresses upon every topology change notification. View the enable status of this
feature using the command
show running-config spanning-tree mstp from EXEC Privilege mode.
Displaying STP Guard Configuration
To verify the STP guard configured on MSTP interfaces, enter the show spanning-tree msti
[instance-number] guard command. Refer to the Spanning Tree Protocol chapter for information on how to
configure and use the STP root guard, loop guard, and BPDU guard features.
The following text shows an example for instance 5 in an MSTP network in which:
• Root guard is enabled on a port that is in a root-inconsistent state.
• Loop guard is enabled on a port that is in a forwarding state.
• BPDU guard is enabled on a port that is shut down.
FTOS#show spanning-tree msti 5 guard
Interface
Name Instance Sts Guard type
--------- -------- --------- ----------
Gi 0/1 5 INCON(Root) Root
Gi 0/2 5 FWD Loop
Gi 0/3 5 EDS(shut) Bpdu
MSTP Sample Configurations
The running-configurations in the following examples support the topology shown in Figure 32-2. The
configurations are from FTOS systems. An S50 system using SFTOS, configured as shown in the final
example, could be substituted for an FTOS router in this sample following topology and MSTP would
function as designed.
Figure 32-2. MSTP with Three VLANs Mapped to Two Spanning Tree Instances
R1
R2
R3
1/2
3/1
3/2
1/3
2/1
2/3
root
Forwarding
Blocking