Allied Telesis SwitchBlade x3100 Series Manual page 499

UCP with EPSR/RSTP Introduction
On Node B, the UCP protocol would also send the Upstream Port Topology Change message for the UFO
VLAN. This message is also received and forwarded to the next node until the node that receives the message
is the upstream node (A). Therefore, nodes B and A would receive the message.
Node A will then send an unsolicited (non-timer) "Upstream Port Notify" message for the UFO VLAN(s) over
both its ring ports a few times before settling back to its normal (timer) sending procedure.The other nodes
receive this message, process it as described earlier, and the result is a reconverged topology in which the
upstream/downstream direction of the interfaces are configured correctly.
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Software Reference for SwitchBlade x3100 Series Switches (Layer Two Switching)