When an OS10 switch that is configured in RPVST+ mode is connected to a Cisco switch running PVST+ mode, convergence happens only on VLAN1. For other VLANs, STP does not converge.
When VLAN 1 is native, the Cisco proprietary PVST+ behavior is such that it sends untagged IEEE STP BPDUs and tagged PVST+ BPDUs. For all other VLANs, Cisco sends tagged PVST+ BPDUs. When VLAN 1 is not native, untagged PVST+ BPDUs are sent for that specific VLAN. For example, if native VLAN is set to VLAN 50 then untagged PVST+ BPDUs are sent for VLAN 50.
An OS10 switch sends untagged RSTP BPDU on VLAN1 and tagged RPVST+ PBDU on other tagged VLANs, for example VLAN 50. And in this case, convergence happens for VLAN1 since Rapid-PVST+ mode is backward compatible with the legacy STP mode. However, convergence fails for other VLANs due to incompatible BPDU exchange between the OS10 switch and the peer switch (Cisco).
To support the convergence when the peer switch is running PVST+, a new feature is available with OS10.5.2.1. This new CLI needs to be enabled by the user on the interface for proper convergence with Cisco or any other vendor.
OS10(config)# no spanning-tree rapid-pvst default behavior
The command is used to enable or disable the transmission of RSTP BPDUs in VLAN 1 when the port is an untagged member other than VLAN 1. By default, OS10 sends RSTP BPDU for the untagged VLAN. The no version of this command handles the RSTP BPDU for VLAN 1 if the port is a member of VLAN 1.
A show command is introduced that displays the compatibility mode in which the port is operating. This command can be used to confirm if the port has received the STP BPDU and is running in backward compatibility mode.
OS10# show spanning-tree compatibility-mode
General log messages will be displayed when a port enters or exits the STP backward compatibility mode as shown below:
%STP_COMPATIBILITY_MODE: STP:Compatibility mode Chg Interface port-channel1 enters into STP backward compatibility mode for instance 1
%STP_COMPATIBILITY_MODE: STP:Compatibility mode Chg Interface port-channel1 exits STP backward compatibility mode for instance 1