In PVST+, the native VLAN must be peered with the default VLAN on Extreme Networks devices, as both are the only VLANs allowed to send and receive untagged packets on the physical port.
Third-party PVST+ devices send VLAN 1 packets in a special manner. ExtremeXOS does not support PVST+ for VLAN 1. Therefore, when the switch receives a packet for VLAN 1, the packet is dropped.
When a PVST+ instance is disabled, the fact that PVST+ uses a different packet format raises an issue. If the STPD also contains ports not in PVST+ mode, the flooded packet has an incompatible format with those ports. The packet is not recognized by the devices connected to those ports.