Allied Telesis AT-S25 - Manual (Page 158)

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Spanning Tree Protocol Operation
When STP is enabled for the first time, or when the network topology
changes due to a failure, the addition, or removal of a component, the
spanning tree algorithm automatically sets up the active topology of the
current network.
Communication
Between Bridges
Periodically, all devices running STP on a network transmit packets to
each other through the Bridge Group Address which all bridges share.
When a bridge receives a packet sent to the Bridge Group Address, the
bridge’s STP processes the packet. The packet is ignored by application
software and other LAN segments. Bridges communicate between each
other in order to determine the root bridge.
Selecting a Root
Bridge and
Designated
Bridges
During communication between bridges, one bridge is determined to
have the lowest bridge identifier. This bridge becomes the root bridge.
After the root bridge has been selected, each LAN segment looks for the
bridge that has the lowest cost relative to the root bridge. These bridges
become designated bridges.
Selecting
Designated Ports
Each designated bridge selects a designated port. This port is
responsible for forwarding packets to the root bridge.
Handling
Duplicate Paths
When the active topology of the network is determined, all packets
between any two nodes in the network use only one path. Where a
duplicate path exists, the non-designated port is put into a blocking
state.
Remapping
Network
Topology
If there is a change in the network topology due to a failure, removal, or
addition of any active components, the active topology also changes.
This may trigger a change in the state of some blocked ports.
The blocked ports do not forward packets immediately. They first pass
through two states, listening and learning, to verify that they may begin
forwarding. A port remains in each of these two states for the time
defined by the Forwarding Delay parameter. This algorithm ensures that
no temporary loops exist in the active network topology and is a
safeguard against packet forwarding during a network topology change
period.