Quick Reference Guide

PowerConnect B-Series TI24X Configuration Guide 125
53-1002269-02
Configuring STP related features
6
Next, Switch 1 is powered up (Figure 16).
FIGURE 16 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.
Switch 2
Switch 3
Switch 1
Port2
Designated
port
Port4
Designated port
Port3
Designated
port
Port3
Designated
port
Port5
Backup port
Port3
Alternate
port
Port2
Root port
Port4
Root port
Bridge priority = 1500
Bridge priority = 2000
Bridge priority = 1000