Dell PowerConnect W-IAP Remote Configuration Guide - Page 294

8 Configuring STP related features FIGURE 44 Active Layer 2 path in complex topology Bridge priority = 200 Bridge priority = 1000 Switch 1 Port2 Port3 Port7 Port2 Switch 2 Port8 Port5 Port3 Port4 Port2 Bridge priority = 60 Switch 5 Port3 Port2 Port3 Switch 3 Port4 Bridge priority = 300 Port4 Port3 Switch 4 Port5 Bridge priority = 400 Port4 Port5 Port3 Switch 6 Bridge priority = 900 Indicates the active Layer 2 path 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.) 252 PowerConnect B-Series FCX Configuration Guide 53-1002266-01