3Com WX3000 operation manual 6The Rstp rapid transition mechanism

Figure 1-6The RSTP rapid transition mechanism

Figure 1-7The 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.

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