Spanning tree
From Teknologisk videncenter
The Spanning Tree protocol are used on Ethernet Switches to avoid broadcast storms.
Contents
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:
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.
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.
