Interoperability between Classic Spanning Tree Protocol (802.1D) and Rapid Spanning Tree (802.1w)

• Inherent fast convergence benefit of 802.1w are lost when it interacts with legacy bridges.
• Each port maintains a variable that defines the protocol to run on that segment.
• A migration delay timer of three seconds starts when the port comes up.
• Migration delay timer resets if port changes its mode of operation.
• Classic 802.1D timers (forward delay and max_age) are only used as backup and should not be necessary if point-to-point links and edge ports are properly identified and set by the administrator.

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfa.shtml