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252 PowerConnect B-Series FCX Configuration Guide
53-1002266-01
Configuring STP related features
8
FIGURE 44 Active Layer 2 path in complex topology
Propagation of topology change
The Topology Change state machine generates and propagates the topology change notification
messages on each port. When a Root port or a Designated port goes into a forwarding state, the
Topology Change state machine on those ports send a topology change notice (TCN) to all the
bridges in the topology to propagate the topology change.
NOTE
Edge ports, Alternate ports, or Backup ports do not need to propagate a topology change.
The TCN is sent in the RST BPDU that a port sends. Ports on other bridges in the topology then
acknowledge the topology change once they receive the RST BPDU, and send the TCN to other
bridges until all the bridges are informed of the topology change.
For example, Port3/Switch 2 in Figure 45, fails. Port4/Switch 3 becomes the new Root port.
Port4/Switch 3 sends an RST BPDU with a TCN to Port4/Switch 4. To propagate the topology
change, Port4/Switch 4 then starts a TCN timer on itself, on the bridge Root port, and on other
ports on that bridge with a Designated role. Then Port3/Switch 4 sends RST BPDU with the TCN to
Port4/Switch 2. (Note the new active Layer 2 path in Figure 45.)
Bridge priority = 1000
Bridge priority = 200
Bridge priority = 60
Bridge priority = 300
Bridge priority = 400
Bridge priority = 900
Port3
Port3
Port3
Port3
Port3
Port3
Port2
Port2
Port2
Port2
Port4
Port4
Port4
Port4
Port5
Port5
Port5
Port7
Port8
Indicates the active Layer 2 path
Switch 1
Switch 3
Switch 4
Switch 6
Switch 5
Switch 2