3com 4210 PWR 9-Port 3CR17341-91-ME User Manual

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