Brocade FastIron Ethernet Switch Platform and Layer 2 Switching Configuration Guide - Manual (Page 290)

FIGURE 54 Simple Layer 2 topology
The point-to-point connections between the three bridges are as follows:
• Port2/Switch 1 and Port2/Switch 2
• Port4/Switch 1 and Port4/Switch 3
• Port3/Switch 2 and Port3/Switch 3
Ports 3 and 5 on Switch 1 are physically connected together.
At start up, the ports on Switch 1 assume Designated port roles, which are in discarding state. They
begin sending RST BPDUs with proposal flags to move into a forwarding state.
When Port4/Switch 3 receives these RST BPDUs 802.1W algorithm determines that they are better
than the RST BPDUs that were previously received on Port3/Switch 3. Port4/Switch 3 is now selected
as Root port. This new assignment signals Port3/Switch 3 to begin entering the discarding state and to
assume an Alternate port role. As it goes through the transition, Port3/Switch 3 negotiates a new role
and state with its peer port, Port3/Switch 2.
Port4/Switch 3 sends an RST BPDU with an agreed flag to Port4/Switch 1. Both ports go into
forwarding states.
Port2/Switch 2 receives an RST BPDU. The 802.1W algorithm determines that these RST BPDUs that
are superior to any that any port on Switch 2 can transmit; therefore, Port2/Switch 2 assumes the role
of a Root port.
The new Root port then signals all ports on the bridge to start synchronization. Since none of the ports
are Edge ports, they all enter the discarding state and assume the role of Designated ports. Port3/
Switch 2, which previously had a Designated role with a forwarding state, starts the discarding state.
They also negotiate port roles and states with their peer ports. Port3/Switch 2 also sends an RST BPU
to Port3/Switch 3 with a proposal flag to request permission go into a forwarding state.
The Port2/Switch 2 bridge also sends an RST BPDU with an agreed flag Port2/Switch 1 that Port2 is
the new Root port. Both ports go into forwarding states.
Now, Port3/Switch 3 is currently in a discarding state and is negotiating a port role. It received RST
BPDUs from Port3/Switch 2. The 802.1W algorithm determines that the RST BPDUs Port3/Switch 3
received are superior to those it can transmit; however, they are not superior to those that are
Spanning Tree Protocol
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