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What is Spanning Tree Protocol?


Spanning Tree Protocol's main purpose is to prevent routing loops over redundant links. Spanning Tree runs on routers and switches. With all ports in the "spanning tree" in a forwarding state and all other ports in a blocking state. Blocked ports will not forward frames. There should only be one active path that exists beteen any pair of LAN segments (collision domains).




The first public standard for STP is IEEE 802.1d. The need for spanning tree came from the problems of broadcast storms. Broadcast storms cause broadcasts to loop around a LAN indefinitely. As a result, some links can become saturated with useless copies of the same frame. Switches flood broadcasts out all interfaces in the same VLAN (VLANs are broadcast domains), except the interface in which the frame arrived. The packet will loop until something changes - someone shuts down an interface, reloads a switch, or does something else to break the loop.


MAC table instability also occurs as a result of the looping frames. MAC table instability means that the switches' MAC address tables will keep changing the information listed for the source MAC address of the looping frame. The MAC address table could have the wrong information causing new packets to loop and possibly never reach its destination.


The third class of problem caused by not using STP in a network with redundancy is that working hosts get multiple copies of the same frame. This happens when switches flood frames sent to unknown destination unicast MAC addresses. The destination gets multiple copies of the frame







References

Odom, Wendell (2008) CCNA ICND2 Official Exam Certification Guide, Second Edition. Indianapolis: Cisco Press.