Propagation Of Topology Change - Dell PowerConnect B-FCXs Configuration Manual

8
Configuring STP related features
FIGURE 44

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
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
252
Active Layer 2 path in complex topology
Bridge priority = 1000
Port2
Switch 1
Port3
Port2
Port3
Port4
Switch 3
Bridge priority = 300
Indicates the active Layer 2 path
Bridge priority = 200
Port7
Port2
Switch 2
Port3 Port4
Port3
Port4
Switch 4
Bridge priority = 400
Figure 45, fails. Port4/Switch 3 becomes the new Root port.
Figure
PowerConnect B-Series FCX Configuration Guide
Port8
Bridge priority = 60
Port2
Port5
Port3
Port3
Port4
Port5
Port5
Switch 6
Bridge priority = 900
45.)
Switch 5
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