ZyXEL Communications ES-4024A manual How STP Works, Select a root bridge

ES-4024A 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.

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