Difference between revisions of "Spanning tree"
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= The Spanning Tree protocol principle = | = 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. | 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. | ||
| − | [[Image:stp4.png|500px|none|thumb|Broadcast storm example]] | + | [[Image:stp4.png|500px|none|thumb|Broadcast storm example]] |
| + | = BPDU: Bridge Protocol Data unit = | ||
| + | Connected Switches sends BPDU packets to each other, to make a hierarchy among them. The purpose is to build a loop free network. | ||
| + | *BPDU packets are send as [[802.1d]] [[multicast]] packets. | ||
| + | *Switches not participating in the STP sends the BPDU packets out all ports. (Broadcast) | ||
| + | *Switches participating in STP receives the packets on the [[multicast]] addresses 01-80-C2-00-00-00 and 01-80-C2-00-00-10 | ||
| + | *BPDU packets are send every 2 seconbds. | ||
[[Category:Network]][[Category:Cisco]] | [[Category:Network]][[Category:Cisco]] | ||
Revision as of 14:34, 25 March 2009
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.
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.
BPDU: Bridge Protocol Data unit
Connected Switches sends BPDU packets to each other, to make a hierarchy among them. The purpose is to build a loop free network.