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Understanding the network.pdf - Back to Home

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Figure 6.5. A bridged LAN with spanning tree enabled.<br />

Only one path can exist between each LAN; all o<strong>the</strong>r<br />

o<strong>the</strong>rs are in a blocking state.<br />

The root bridge election process and all o<strong>the</strong>r interbridge<br />

communication are accomplished by having <strong>the</strong> bridges exchange<br />

special messages known as configuration Bridge Pro<strong>to</strong>col Data Units<br />

(BPDUs). These messages are sent in a Layer 2 packet with <strong>the</strong> BPDU<br />

information contained in <strong>the</strong> data portion of <strong>the</strong> packet. The<br />

destination address of <strong>the</strong> message is set <strong>to</strong> a Layer 2 multicast<br />

address used by all bridges. BDPUs are exchanged periodically<br />

between all <strong>the</strong> connected bridges <strong>to</strong> ensure that <strong>the</strong> spanning tree<br />

remains stable. So, when spanning tree changes occur, <strong>the</strong> bridges<br />

can update <strong>the</strong>ir path calculations accordingly or reelect a root bridge<br />

if needed.

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