Configuring Spanning Tree Parameters Spanning Tree Operating Modes
OmniSwitch AOS Release 8 Network Configuration Guide December 2017 page 6-24
The following show command displays the PVST+ status.
-> show spantree mode
Spanning Tree Global Parameters
Current Running Mode : per-vlan,
Current Protocol : N/A (Per VLAN),
Path Cost Mode : 32 BIT,
Auto Vlan Containment : N/A
Cisco PVST+ mode : Enabled
Vlan Consistency check: Disabled
BPDU Processing in PVST+ Mode
An OmniSwitch port operating in PVST+ mode processes BPDUs as follows:
If the default VLAN of a port is VLAN 1 then:
• Send and receive IEEE untagged BPDUs for VLAN 1
• Don't send and receive PVST+ tagged BPDUs for VLAN 1
• Send and receive tagged PVST+ BPDUs for other tagged VLANs.
If the default VLAN of a port is not VLAN 1 then:
• Send and receive IEEE untagged BPDUs for VLAN 1
• Don't send and receive PVST+ tagged BPDUs for VLAN 1
• Send and receive untagged PVST+ BPDUs for the port's default VLAN
• Send and receive tagged PVST+ BPDUs for other tagged VLANs
Recommendations and Requirements for PVST+ Configurations
• It is mandatory that all the Cisco switches have the MAC Reduction Mode feature enabled in order to
interoperate with an OmniSwitch in PVST+ mode. This avoids any unexpected election of a root
bridge.
• You can assign the priority value only in the multiples of 4096 to be compatible with the Cisco MAC
Reduction mode; any other values result in an error message. Also, the existing per vlan priority values
are restored when changing from PVST+ mode back to per-VLAN mode. For more information on
priority, refer to “Configuring the Bridge Priority” on page 6-28.
• In a mixed OmniSwitch and Cisco environment, it is highly recommended to enable PVST+ mode on
all OmniSwitches in order to maintain the same root bridge for the topology. It is possible that the new
root bridge might be elected as a result of inconsistencies of MAC reduction mode when connecting an
OmniSwitch that does not support Cisco PVST+ mode to an OmniSwitch with the PVST+ mode
enabled. In this case, the root bridge priority must be changed manually to maintain the same root
bridge. For more information on priority, refer to “Configuring the Bridge Priority” on page 6-28.
• A Cisco switch running in PVST mode (another Cisco proprietary mode prior to 802.1q standard) is
not compatible with an OmniSwitch running in per-VLAN PVST+ mode.
• Both Cisco and OmniSwitch support two default path cost modes; long or short. It is recommended
that the same default path cost mode be configured in the same way on all switches so that the path
costs for similar interface types are consistent when connecting ports between OmniSwitch and Cisco
Switches. For more information on path cost mode, refer to “Configuring the Path Cost Mode” on
page 6-31.
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