Spanning tree

The Spanning Tree protocol are used on Ethernet Switches to avoid broadcast storms.

Problems with Switches without Spanning Tree

Broadcast Storms

A broadcast starts when a Ethernet switch receives a broadcast from a Host and there exist a loop. See example below:

Broadcast storm example

Duplicate Ethernet Frames

Another problem with Ethernet loops is duplicate Ethernet frames. In the picture below, switch X can see PC-B mac-address on two ports and send the frame out of each port.

Duplicate Ethernet frame example

Instability in Switch MAC-Address table

Another problem with with Ethernet loops is instability in the Switches MAC-Address table See the picture below and consider:

• The MAC-Address off PC-B is timed out on both Switches.
• PC-A send a unicast packet to PC-B's MAC-Address.
• PC-B's MAC-address is unknown to Switch X which send the Frame out of all ports. (Except the originating port 3)
• Switch Y receives the Frame to PC-B on port 1 and on port 2.
• PC-B's MAC-address is unknown to Switch X which send the Frame out of all ports. (Except originating port)
• Switch X know receives Frames on port 1 and port 2 with source MAC-address o fPC-A
• Switch X now thinks that PC-A is on Port 1, Port 2 and Port 3.

Broadcast storm example

The Spanning Tree protocol principle

If the Switches has enabled the Spanning Tree Protocol - STP - the Switches discover the loop and close one of the links for traffic. The closed link will be enabled if one of the other links breaks down.

Broadcast storm example