3Com WX3000 Series Operation Manual page 212

Figure 1-6 The RSTP rapid transition mechanism
Figure 1-7 The MSTP rapid transition mechanism
The cooperation between MSTP and RSTP is limited in the process of rapid transition. For example,
when the upstream device adopts RSTP, the downstream device adopts MSTP and the downstream
device does not support RSTP-compatible mode, the root port on the downstream device receives no
agreement packet from the upstream device and thus sends no agreement packets to the upstream
device. As a result, the designated port of the upstream device fails to transit rapidly and can only turn to
the forwarding state after a period twice the forward delay.
Devices of some vendors adopt proprietary spanning tree protocols that are similar to RSTP in the way
to implement rapid transition on designated ports. When a device of this kind operating as the upstream
device connects with a WX3000 series device running MSTP, the upstream designated port fails to
change its state rapidly.
The rapid transition feature is developed to resolve this problem. When a WX3000 series device
running MSTP is connected in the upstream direction to a device of another vendor running proprietary
spanning tree protocols, you can enable the rapid transition feature on the ports of the WX3000 series
device operating as the downstream device. Among these ports, those operating as the root ports will
then send agreement packets to their upstream ports after they receive proposal packets from the
upstream designated ports, instead of waiting for agreement packets from the upstream device. This
enables designated ports of the upstream device to change their states rapidly.
1-42