Configuring Spanning Tree Parameters Spanning Tree Operating Modes
OmniSwitch AOS Release 7 Network Configuration Guide June 2013 page 6-23
OmniSwitch PVST+ Interoperability
Native VLAN and OmniSwitch Default VLAN
Cisco uses the standard IEEE BPDU format for the native VLAN (VLAN 1) over an 802.1Q trunk. Thus,
by default the Common Spanning Tree (CST) instance of the native VLAN 1 for all Cisco switches and
the STP instance for the default VLAN of a port on an OmniSwitch interoperates and successfully creates
a loop-free topology.
802.1Q Tagged VLANs
For 802.1Q tagged VLANs, Cisco uses a proprietary frame format which differs from the standard IEEE
BPDU format used by Alcatel-Lucent per-VLAN mode, thus preventing Spanning Tree topologies for
tagged VLANs from interoperating over the 802.1Q trunk.
In order to interoperate with Cisco PVST+ mode, the current Alcatel-Lucent per-VLAN mode has an
option to recognize Cisco's proprietary PVST+ BPDUs. This allows any user port on an OmniSwitch to
send and receive PVST+ BPDUs, so that loop-free topologies for the tagged VLANs can be created
between OmniSwitch and Cisco switches.
Configuration Overview
The spantree pvst+compatibility command is used to enable or disable the PVST+ interoperability mode
globally for all switch ports and link aggregates or on a per-port/link aggregate basis. By default, PVST+
compatibility is disabled.
To globally enable or disable PVST+ interoperability, enter the following commands:
-> spantree pvst+compatibility enable
-> spantree pvst+compatibility disable
To enable or disable PVST+ interoperability for a specific port or link aggregate, use the spantree
pvst+compatibility command with the port or linkagg parameter. For example:
-> spantree pvst+compatibility port 1/3 enable
-> spantree pvst+compatibility port 2/24 disable
-> spantree pvst+compatibility linkagg 3 enable
-> spantree pvst+compatibility linkagg 10 disable
The following causes a port to exit from the enabled state:
• The link status of the port changes.
• The administrative status of the port changes.
• The PVST+ status of the port is disabled or set to auto.
To configure a port or link aggregate to automatically detect
The spantree pvst+compatibility command also provides an auto option to configure the port to handle
IEEE BPDUs initially (i.e., disable state). Once a PVST+ BPDU is received, it handles the PVST+
BPDUs and IEEE BPDUs for a Cisco native VLAN. For example:
-> spantree pvst+compatibility port 1/3 auto
-> spantree pvst+compatibility linkagg 3 auto
The following show command displays the PVST+ status.
-> show spantree mode
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