BigIron RX Series Configuration Guide 37353-1001986-01Convergence in a simple topology 13FIGURE 56 Link failure in the topologySwitch 1 sets its Port2 into a discarding state.At the same time, Switch 2 assumes the role of a root bridge since its root port failed and it has nooperational Alternate port. Port3/Switch 2, which currently has a Designated port role, sends anRST BPDU to Switch 3. The RST BPDU contains a proposal flag and a bridge ID of Switch 2 as itsroot bridge ID.When Port3/Switch 3 receives the RST BPDUs, RSTP algorithm determines that they are inferior tothose that the port can transmit. Therefore, Port3/Switch 3 is given a new role, that of aDesignated port. Port3/Switch 3 then sends an RST BPDU with a proposal flag to Switch 2, alongwith the new role information. However, the root bridge ID transmitted in the RST BPDU is stillSwitch 1.When Port3/Switch 2 receives the RST BPDU, RSTP algorithm determines that it is superior to theRST BPDU that it can transmit; therefore, Port3/Switch 2 receives a new role; that of a Root port.Port3/Switch 2 then sends an RST BPDU with an agreed flag to Port3/Switch 3. Port3/Switch 2goes into a forwarding state.When Port3/Switch 3 receives the RST BPDU that Port3/Switch 2 sent, Port3/Switch 3 changesinto a forwarding state, which then completes the full convergence of the topology.Convergence at link restorationWhen Port2/Switch 2 is restored, both Switch 2 and Switch 1 recognize the change. Port2/Switch1 starts assuming the role of a Designated port and sends an RST BPDU containing a proposal flagto Port2/Switch 2.Bridge priority = 1500Bridge priority = 2000Bridge priority = 1000Switch 2Switch 3Switch 1Port2 Port2Port3Port3Port3Port4Port4Port5
