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250 PowerConnect B-Series FCX Configuration Guide
53-1002266-01
Configuring STP related features
8
Convergence in a complex 802.1W topology
The following is an example of a complex 802.1W topology.
FIGURE 43 Complex 802.1W topology
In Figure 43, Switch 5 is selected as the root bridge since it is the bridge with the highest priority.
Lines in the figure show the point-to-point connection to the bridges in the topology.
Switch 5 sends an RST BPDU that contains a proposal flag to Port5/Switch 2. When handshakes
are completed in Switch 5, Port5/Switch 2 is selected as the Root port on Switch 2. All other ports
on Switch 2 are given Designated port role with discarding states.
Port5/Switch 2 then sends an RST BPDU with an agreed flag to Switch 5 to confirm that it is the
new Root port and the port enters a forwarding state. Port7 and Port8 are informed of the identity
of the new Root port. 802.1W algorithm selects Port7 as the Designated port while Port8 becomes
the Backup port.
Port3/Switch 5 sends an RST BPDU to Port3/Switch 6 with a proposal flag. When Port3/Switch 5
receives the RST BPDU, handshake mechanisms select Port3 as the Root port of Switch 6. All
other ports are given a Designated port role with discarding states. Port3/Switch 6 then sends an
RST BPDU with an agreed flag to Port3/Switch 5 to confirm that it is the Root port. The Root port
then goes into a forwarding state.
Now, Port4/Switch 6 receives RST BPDUs that are superior to what it can transmit; therefore, it is
given the Alternate port role. The port remains in discarding state.
Port5/Switch 6 receives RST BPDUs that are inferior to what it can transmit. The port is then given
a Designated port role.
Bridge priority = 1000
Bridge priority = 200
Bridge priority = 300
Bridge priority = 400
Bridge priority = 60
Bridge priority = 900
Port3
Port3
Port3
Port3
Port3
Port3
Port4
Port4
Port4
Port5 Port5
Port4
Port2
Port2
Port2
Port2
Port7
Port5
Port8
Switch 1
Switch 3
Switch 4
Switch 2
Switch 5
Switch 6