Allied Telesis AT-S24 - Manual (Page 158)

Spanning Tree Protocol Concepts
158
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.