Brocade FastIron Ethernet Switch Platform and Layer 2 Switching Configuration Guide - Manual (Page 295)

FIGURE 58 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 the following figure, 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 the following figure.)
Propagation of topology change
FastIron Ethernet Switch Platform and Layer 2 Switching Configuration Guide 295
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