Propagation of topology change, Figure 58 – Brocade BigIron RX Series Configuration Guide User Manual

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BigIron RX Series Configuration Guide

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Convergence in a complex RSTP topology

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

Figure 59

, 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’s 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 59

Switch 1

Bridge priority = 1000

Switch 2

Bridge priority = 200

Switch 5

Bridge priority = 60

Switch 3

Bridge priority = 300

Switch 4

Bridge priority = 400

Switch 6

Bridge priority = 900

Port3

Port2

Port4

Port5

Port7

Port8

Indicates the active Layer 2 path