Allied Telesis AT-S21 - Manual (Page 84)

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When spanning tree 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.