Difference between revisions of "Spanning tree"

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m (Instability in Switch MAC-Address table)
m (Instability in Switch MAC-Address table)
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*Switch X now thinks that PC-A is on Port 1, Port 2 and Port 3.
 
*Switch X now thinks that PC-A is on Port 1, Port 2 and Port 3.
 
[[Image:stp3.png|500px|none|thumb|Broadcast storm example]]  
 
[[Image:stp3.png|500px|none|thumb|Broadcast storm example]]  
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= The Spanning Tree protocol principle =
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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.
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[[Image:stp4.png|500px|none|thumb|Broadcast storm example]]
 
[[Category:Network]][[Category:Cisco]]
 
[[Category:Network]][[Category:Cisco]]

Revision as of 14:27, 25 March 2009

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