Brocade FastIron Ethernet Switch Platform and Layer 2 Switching Configuration Guide - Manual (Page 292)

FIGURE 56 Link failure in the topology
Switch 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 no
operational Alternate port. Port3/Switch 2, which currently has a Designated port role, sends an RST
BPDU to Switch 3. The RST BPDU contains a proposal flag and a bridge ID of Switch 2 as its root
bridge ID.
When Port3/Switch 3 receives the RST BPDUs, 802.1W algorithm determines that they are inferior to
those that the port can transmit. Therefore, Port3/Switch 3 is given a new role, that of a Designated
port. Port3/Switch 3 then sends an RST BPDU with a proposal flag to Switch 2, along with the new
role information. However, the root bridge ID transmitted in the RST BPDU is still Switch 1.
When Port3/Switch 2 receives the RST BPDU, 802.1W algorithm determines that it is superior to the
RST 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 2 goes
into a forwarding state.
When Port3/Switch 3 receives the RST BPDU that Port3/Switch 2 sent, Port3/Switch 3 changes into a
forwarding state, which then completes the full convergence of the topology.
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, 802.1W 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.
Convergence at link restoration
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