1-42Figure 1-6 The RSTP rapid transition mechanismRoot port blocks other non-edge ports, changes toforwarding state and sendsAgreement to upstream deviceDownstream switchUpstream switchProposal for rapid transitionAgreementDesignated portchanges toforwarding stateRoot portDesignated portFigure 1-7 The MSTP rapid transition mechanismDownstream switchRoot port blocks othernon- edge portsRoot port changes toforwarding state andsends Agreement toupstream switchDesignated portchanges toforwarding stateUpstream switchProposal for rapid transitionAgreementRoot portDesignated portThe 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 downstreamswitch does not support RSTP-compatible mode, the root port on the downstream switch receives noagreement packet from the upstream switch and thus sends no agreement packets to the upstreamswitch. As a result, the designated port of the upstream switch fails to transit rapidly and can only turn tothe forwarding state after a period twice the forward delay.Some other manufacturers' switches adopt proprietary spanning tree protocols that are similar to RSTPin the way to implement rapid transition on designated ports. When a switch of this kind operating as theupstream switch connects with a 3Com switch 4500 running MSTP, the upstream designated port failsto change its state rapidly.The rapid transition feature is developed to resolve this problem. When a 3Com switch 4500 runningMSTP is connected in the upstream direction to another manufacturer's switch running proprietaryspanning tree protocols, you can enable the rapid transition feature on the ports of the switch 4500operating as the downstream switch. Among these ports, those operating as the root ports will thensend agreement packets to their upstream ports after they receive proposal packets from the upstreamdesignated ports, instead of waiting for agreement packets from the upstream switch. This enablesdesignated ports of the upstream switch to change their states rapidly.
