How stp works – ZyXEL Communications ZyXEL Dimension ES-3124-4F User Manual

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ES-3100 Series Switch Support Notes

How STP Works

Spanning Tree provide a loop-free network. When a switch supported STP
recognize a loop in the network topology, it blocks one or more redundant
ports. Spanning Tree Protocol continually explore the network, so when the
network topology changes, STP automatically reconfigure switch ports to avoid
the failure by blocking certain port.
Spanning tree algorithm aware switches (bridges) exchange configuration
messages periodically. The configuration message is a multicast frame called
BPDU (Bridge Protocol Data Unit) or Hello message. According to BPDU,
these STP aware will construct a loop free network with "tree" architecture.
STP operation is listed as the following:
1. Select a root bridge
Only one switch/ bridge can be selected as the root bridge in a given network.
All other decisions in the network, such as which port is blocked and which port
is put in forwarding mode, are made regarding this root bridge. The root bridge
is the "root" of the constructed "tree".

1. One of the important field included in the BPDU is the bridge ID.

Each bridge has unique bridge ID. The root bridge is the bridge
with the lowest bridge ID in the spanning tree network.