28.1.6 The STP compatibility mode

(1) Overview

For a switch using Rapid PVST+, Rapid STP, or Multiple Spanning Tree, if the partner switch uses PVST+ or STP, the corresponding port runs in STP compatibility mode.

Under STP compatibility mode operation, high-speed transitions can no longer be performed on the corresponding port, requiring more time for communication to be restored.

If the partner switch is changed to a Rapid PVST+, Rapid STP, or Multiple Spanning Tree, restoration is performed from the STP compatibility mode, and high-speed transitions become possible again, but depending on the timing, the corresponding port and partner switch might continue to run in STP compatibility mode.

The STP compatibility mode recovery functionality performs forced restoration for ports running in STP compatibility mode, allowing them to perform normal high-speed transition.

(2) Recovery function

The clear spanning-tree detected-protocol operation command can be executed to perform forced restoration from STP compatibility mode. The link type of the corresponding port can be either point-to-point or shared.

(3) Automatic recovery function

If the link type of the corresponding port is point-to-point, the STP compatibility mode recovery functionality runs automatically.

If the corresponding port is a non-designated port running in STP compatibility mode, an RST BPDU or MST BPDU can be sent from the corresponding port to disable STP compatibility mode.

If the link type of the corresponding port is shared, automatic-restoration mode does not run, because it cannot run correctly.