3com 5500-SI Manual De Usuario

This chapter covers the following topics:

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Spanning Tree Protocol (STP) is applied in loop networks to block some undesirable 
redundant paths with certain algorithms and prune the network into a loop-free tree, 
thereby avoiding the proliferation and infinite cycling of the packet in the loop 
network.

The fundamental of STP is that the Switches exchange a special type of protocol 
packet (which is called configuration Bridge Protocol Data Units, or BPDU, in IEEE 
802.1D) to decide the topology of the network. The configuration BPDU contains the 
information enough to ensure the Switches to compute the spanning tree. 

The configuration BPDU mainly contains the following information:

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The root ID consisting of root priority and MAC address

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The cost of the shortest path to the root 

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Designated bridge ID consisting of designated bridge priority and MAC address

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Designated port ID consisting of port priority and port number

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The age of the configuration BPDU: MessageAge

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The maximum age of the configuration BPDU: MaxAge

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Configuration BPDU interval: HelloTime

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Forward delay of the port: ForwardDelay. 

Figure 159   Designated Bridge and Designated Port

Switch A

Switch C

Switch B

CP2

BP2

CP1

BP1

AP2

AP1

LAN