Brocade Multi-Service IronWare Switching Configuration Guide (Supporting R05.6.00) - Manual (Page 457)

Multi-Service IronWare Switching Configuration Guide 427
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Convergence in a simple topology
13
Convergence at link restoration
When Port2/Switch 2 is restored, both Switch 2 and Switch 1 recognize the change. Port2/Switch
1 starts assuming the role of a Designated port and sends an RST BPDU containing a proposal flag
to Port2/Switch 2.
When Port2/Switch 2 receives the RST BPDUs, RSTP algorithm determines that the RST BPDUs the
port received are better than those received on Port3/Switch 3; therefore, Port2/Switch 2 is given
the role of a Root port. All the ports on Switch 2 are informed that a new Root port has been
assigned 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 the
bridge 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/Switch
3.
• Port2/Switch 2 also sends an RST BPDU with an agreed flag to Port2/Switch 1 and then
places itself into a forwarding state.
When Port2/Switch 1 receives the RST BPDU with an agreed flag sent by Port2/Switch 2, it puts
that 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 determines
that 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 a
discarding state.
Now Port3/Switch 2 does not go into a forwarding state instantly like the Root port. It waits until the
forward delay timer expires twice on that port while it is still in a Designated role, before it can
proceed to the forwarding state. The wait, however, does not cause a denial of service, since the
essential connectivity in the topology has already been established.
When fully restored, the topology is the same as that shown on Figure 86 .