HP ProCurve 9304M Installation And Configuration Manual page 248

Installation and Basic Configuration Guide
Convergence After a Link Failure
What happens if a link in the 802.1W topology fails?
For example, Port2/Routing Switch , which is the port that connects Routing Switch 2 to the root bridge (Routing
Switch 1), fails. Both Routing Switch 2 and Routing Switch 1 notice the topology change (Figure 8.17).
Figure 8.17 Link Failure in the Topology
Bridge priority = 1500
Bridge priority = 2000
Routing Switch 1 sets its Port2 into a discarding state.
At the same time, Routing Switch 2 assumes the role of a root bridge since its root port failed and it has no
operational Alternate port. Port3/Routing Switch 2, which currently has a Designated port role, sends an RST
BPDU to Routing Switch 3. The RST BPDU contains a proposal flag and a bridge ID of Routing Switch 2 as its
root bridge ID.
When Port3/Routing Switch 3 receives the RST BPDUs, 802.1W algorithm determines that they are inferior to
those that the port can transmit. Therefore, Port3/Routing Switch 3 is given a new role, that of a Designated port.
Port3/Routing Switch 3 then sends an RST BPDU with a proposal flag to Routing Switch 2, along with the new
role information. However, the root bridge ID transmitted in the RST BPDU is still Routing Switch 1.
When Port3/Routing Switch 2 receives the RST BPDU, 802.1W algorithm determines that it is superior to the RST
BPDU that it can transmit; therefore, Port3/Routing Switch 2 receives a new role; that of a Root port. Port3/
Routing Switch 2 then sends an RST BPDU with an agreed flag to Port3/Routing Switch 3. Port3/Routing Switch
2 goes into a forwarding state.
When Port3/Routing Switch 3 receives the RST BPDU that Port3/Routing Switch 2 sent, Port3/Routing Switch 3
changes into a forwarding state, which then completes the full convergence of the topology.
Convergence at Link Restoration
When Port2/Routing Switch 2 is restored, both Routing Switch 2 and Routing Switch 1 recognize the change.
Port2/Routing Switch 1 starts assuming the role of a Designated port and sends an RST BPDU containing a
proposal flag to Port2/Routing Switch 2.
When Port2/Routing Switch 2 receives the RST BPDUs, 802.1W algorithm determines that the RST BPDUs the
port received are better than those received on Port3/Routing Switch 3; therefore, Port2/Routing Switch 2 is given
the role of a Root port. All the ports on Routing Switch 2 are informed that a new Root port has been assigned
which then signals all the ports to synchronize their roles and states. Port3/Routing Switch 2, which was the
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Routing Switch 2
Port2
Port3
Port3
Port4
Routing Switch 3
Routing
Port3 Port5
Switch 1
Port2
Port4
Bridge priority = 1000