Siemens S323, S223 user manual Spanning-Tree Protocol STP

UMN:CLI	User Manual
	SURPASS hiD 6615 S223/S323 R1.5

8.3Spanning-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

Fig. 8.8 Example of Loop

PC-B

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.

PC-B

VLAN 1

Switch A

Switch B	Switch D
	Blocking
	Path 1
	Path 2
PC-A	Switch C

Fig. 8.9 Principle of Spanning Tree Protocol

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