3Com 4210 PWR Configuration Manual page 181

Configuring Rapid Transition 179
Figure 51 and Figure 52 illustrate the rapid transition mechanisms on designated
ports in RSTP and MSTP.
Figure 51 The RSTP rapid transition mechanism
Figure 52 The MSTP rapid transition mechanism
The cooperation between MSTP and RSTP is limited in the process of rapid
transition. For example, when the upstream switch adopts RSTP, the downstream
switch adopts MSTP and the downstream switch does not support
RSTP-compatible mode, the root port on the downstream switch receives no
agreement packet from the upstream switch and thus sends no agreement
packets to the upstream switch. As a result, the designated port of the upstream
switch fails to transit rapidly and can only turn to the forwarding state after a
period twice the forward delay.
Some other manufacturers' switches adopt proprietary spanning tree protocols
that are similar to RSTP in the way to implement rapid transition on designated
ports. When a switch of this kind operating as the upstream switch connects with
a 3Com series switch running MSTP, the upstream designated port fails to change
its state rapidly.
The rapid transition feature is developed to resolve this problem. When a 3Com
series switch running MSTP is connected in the upstream direction to another