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3 spanning-tree protocol (stp), Spanning-tree protocol (stp), Fig. 8.8 – Siemens S223 User Manual

Page 200: Example of loop, Fig. 8.9, Principle of spanning tree protocol

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UMN:CLI User Manual

SURPASS hiD 6615 S223/S323 R1.5

200 A50010-Y3-C150-2-7619

8.3

Spanning-Tree Protocol (STP)

LAN, which is composed of double-path like token ring, has the advantage that it is pos-
sible to access in case of disconnection with one path. However, there is another problem
named Loop when you always use the double-path.

Switch A

Switch B

PC-A

PC-B

Fig. 8.8

Example of Loop

Loop is when there are more than one path between switches (SWITCH A, B), PC A
sends packet through broadcast or multicast and then the packet keeps rotating. It
causes superfluous data-transmission and network fault.

STP (Spanning-Tree Protocol) is the function to prevent Loop in LAN with more than two
paths and to utilize the double-path efficiently. It specify in IEEE 802.1d. If STP is config-
ured, there is no Loop since it chooses more effective path of them and closes the other
path. In other words, when SWITCH C in the below figure sends packet to SWITCH B,
path 1 is chosen and path 2 is blocked.

Switch B

Switch C

Switch D

Switch A

Path 1

Path 2

VLAN 1

PC-A

PC-B

Blocking

Fig. 8.9

Principle of Spanning Tree Protocol

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