378 BigIron RX Series Configuration Guide53-1002253-01Convergence in a complex RSTP topology13When Port2/Switch 2 receives the RST BPDUs, RSTP algorithm determines that the RST BPDUs theport received are better than those received on Port3/Switch 3; therefore, Port2/Switch 2 is giventhe role of a Root port. All the ports on Switch 2 are informed that a new Root port has beenassigned which then signals all the ports to synchronize their roles and states. Port3/Switch 2,which was the previous Root port, enters a discarding state and negotiates with other ports on thebridge to establish its new role and state, until it finally assumes the role of a Designated port.Next, the following happens:• Port3/Switch 2, the Designated port, sends an RST BPDU, with a proposal flag to Port3/Switch3.• Port2/Switch 2 also sends an RST BPDU with an agreed flag to Port2/Switch 1 and thenplaces itself into a forwarding state.When Port2/Switch 1 receives the RST BPDU with an agreed flag sent by Port2/Switch 2, it putsthat port into a forwarding state. The topology is now fully converged.When Port3/Switch 3 receives the RST BPDU that Port3/Switch 2 sent, RSTP algorithm determinesthat these RST BPDUs are superior to those that Port3/Switch 3 can transmit. Therefore,Port3/Switch 3 is given a new role, that of an Alternate port. Port3/Switch 3 immediately enters adiscarding state.Now Port3/Switch 2 does not go into a forwarding state instantly like the Root port. It waits until theforward delay timer expires twice on that port while it is still in a Designated role, before it canproceed to the forwarding state. The wait, however, does not cause a denial of service, since theessential connectivity in the topology has already been established.When fully restored, the topology is the same as that shown on Figure 54.Convergence in a complex RSTP topologyThe following is an example of a complex RSTP topology.
