3Com corebuilder 3500 Implementation Manual page 120

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Table of Contents

Figure 20 Hierarchy of the Root Bridge and the Designated Bridge

To designated port

To root port

Actions That Result from CBPDU Information

From the information that the CBPDUs provide:

Bridges elect a single bridge to be the root bridge. The root bridge has

the lowest bridge ID among all the bridges on the extended network.

Bridges calculate the best path between themselves and the root

Bridges elect as the designated bridge on each LAN the bridge with

the least cost path to the root bridge. The designated bridge forwards

packets between that LAN and the path to the root bridge. For this

reason, the root bridge is always the designated bridge for its

attached LANs. The port through which the designated bridge is

attached to the LAN is elected the designated port.

Bridges choose a root port that gives the best path from themselves to

the root bridge.

Bridges select ports to include in the STP topology. The ports that are

selected include the root port plus any designated ports. Data traffic is

forwarded to and from ports that have been selected in the STP

Designated bridge

To root port

To designated port